Fluids + Imaging Flashcards
Will you see anaemia with haemorrhage? With a severe coagulopathy, what are possible ddx?
* Yes + possible hypovolaemic shock
* Anticoagulant rodenticide toxicity, hepatic failure, DIC, malabsorptive diseases
When might you see mild thrombocytopenia?
Secondary from blood loss/ increased utilisation, DIC
What is a heart murmur? Why would you see a heart murmur with anemia? What else could cause a heart murmur?
* Heart sounds produced when blood flows across one of the heart valves that is loud enough to be heard with a stethoscope– vast majority are due to turbulent blood flow brought on by high velocity blood flow
* the body has a lower than normal number of RBCs therefore the blood is thinner and flows faster than normal
* Structural abnormality (e.g. valve leaflet), fever, hyperthyroidism
In a dog with anticoagulant rodenticide toxicity, what kind of blood transfusion product would be ideal?
Fresh whole blood for the red cells, clotting factors, proteins, and platelets (lack of vitamin K reductase does not allow the activation of vitamin K and therefore lack of clotting— so fresh whole blood with stop bleeding by providing clotting factors that have been lost)
** Max is also hypovolemic and hypoxic shock from severe anemia– so supplementation of red cells is essential
What are possible complications from a transfusion?
* Acute immunological transfusion reaction- acute hemolytic reaction (type II hypersensitivity reaction), febrile non hemolytic reaction, non-hemolytic immunological reaction (type I hypersensitivity)
* delayed immunological transfusion reaction- delayed hemolytic reaction, post transfusion purpura (internal bleeding)
* acute non immunological tranfusion reactions- bacterial contamination of transfused bloods, hemolysis from damaged RBC transfusion, hyperkalaemia/hypocalcemia/hypomagnesemia after massive transfusions, hypothermia, circulatory overload
What are some things you can do to prevent transfusion reactions?
* blood typing (especially cats), cross matching major and minor (especially if second transfusion), test infusion 0.25 m/kg/h over the first 5-10 minutes, use of an in line blood filter, monitor patient parameters every 15-30 minutes during transfusion for acute reactions (demeanor, temperature, pulse rate and quality, BP, RR and character, MM colour, plasma and urine colour), ensure transfusion complete within 4-6 hours to preven bacterial growth, proper collection and storage of blood, appropriate blood donor screening
Dog with anticoagulant rodenticide poisoning, what other treatments over than an infusion?
* Vitamin K1 supplementation
* strict cage rest to prevent further hemorrhage
* oxygen supplementation
* Intravenous fluids
What are X-rays?
A form of electromagnetic radiation
EM radiations of frequencies higher than UV light have enough energy to remove an outer-shell electron from an atom and are therefore called ionising radiation (X-rays, gamma rays, and cosmic rays are ionising radiation). When ionising radiation interacts with atoms it causes the ejection of an electron and creates an ion pair which has the potential to damage cells.
** They can produce damage to molecules and induce temporary or permanent cell damage, causing problems such as mutations or cancer
What are some unusual properties of ionising radiation?
* You cannot see, smell, touch or taste it
* Penetrates all matter
* We do not develop tolerance or immunity to radiation
* There is little difference between how individuals react to a given dose
* Dose is cumulative
* Delayed effects: May take years to see the effects
What does it mean that effects of radiation are stochastic or deterministic?
* Stochastic- increased risk with increased dose– the dose of radiation you receive, a higher dose of radiation will increase the probability of a stochastic effect e.g. a genetic mutation leading to cancer– higher radiation dose or exposure will increase the chance of cancer developing (this is the one that is important in vet med)
** no threshold below which the risk of the effect will cease & effects are cumulative over a lifetime
* Deterministic- inevitable at high doses– in contrast with stochastic effects, the severity of the effect, not the probability, increases with increasing dose. Exposure below a certain threshold will not result in the effect (THERE IS A THRESHOLD). e.g. radiation induced skin burns, uncommon however in medicine
What is the direct effect of ionising radiation? Indirect effect?
* direct effects may damage DNA, protein or other macromolecules
* Indirect effects create free radicles within cytoplasm and these free radical damage macromolecules
** Cell may either repair itself or if DNA damage has occurred a mutation may result, causing cell death, cancer, or birth defects
In Victoria, the use of radiation comes under what department? And which Act? What does it state?
* Department of Health which administers The Radiation Act 2005– species that business must have a management licence to possess a radiation source and conduct a radiation practice & individuals must have a use licence
What is ARPANSA?
* A national governing body – Australian Radiation Protection and Nuclear Safety Agency– responsibility for protecting the health and safety of the people and environment from the harmful effects of ionising radiation. They publish a series of codes and standards then implement in the form of legislation.
RPS 17: Radiation Protection in Veterinary Medicine (2009)
** Also provide “Personal Radiation Monitoring Service”– monitors and records occupational exposure to radiation
What are the three basic methods of monitoring for radiation exposure?
* Thermoluminescent Dosemeter (TLD)- electrons int he crystal structure of the TLD card are excited to high energy levels as a result of irradiation and are trapped in the crystal structure. By heating the material, electrons return to their ground state and light is emitted. The amount of ligh is proportional to the radiation dose that the TLD material has received. Filters are used to allow only certain radiation to pass.
* Film badges- degree of darkening can be measured and will indicate the absorbed dose. But dose to film does not equal dose to tissue.
* pocket dosimeters– different to TLDs and film badges… pocket dosimeters can be read instantaneously– useful when high exposures are expected.
What tissues are more susceptible to radiation?
Rapidly dividing tissue
What is the recommended effective dose limit for occupational exposure? In a single year? What is the avg annual dose for veterinarians?
20 mSv per year, averaged over consecutive years
* 50 mSv in a single year
** Avg annual dose for veterinarians is around 16 microSv = 0.016 mSv
What is the ALARA principle?
* As Low As Reasonably Achievable
** Is the radiograph necessary for the patient and management of the case? Would ultrasound or a blood test answer the clinical question?
* Minimize retaking radiographs, minimize manual restraint, under no circumstances should any part of your body be within the primary x-ray beam
What are the three types of radiation encountered in medical practice?
* Useful beam
* Leakage radiation
* Scatter radiation
How can radiation exposure be minimised ?
Modifying
- Time (take good radiographs the first time around)
- Distance from the radiation source– the energy of the scatter radiation will decrease as distance from the radiation source increases, according to the inverse square law… e.g. so by doubling the distance from the radiation source, you will reduce your radiation exposure by a factor of 4. (avoid manual restraint, use ancillary positioning devices such as sand bags, stand well back)
- Shielding- concrete walls, PPE
How thick should lead PPE be?
Lead equivalent of 0.25 mm will shield scatter radiation with energy of up to 100 keV (electron volt)
What should the local radiation management plan follow?
ARPANSA RPS 17 + ALARA by observing time distance and shielding principles
How are x-rays generated? What elements are necessary for x- ray production?
* Generated in the x-ray tube by firing a high energy electron beam at a tungsten target
- A source of electrons (cathode)
- An obstacle- free path for the passage of high speed electrons
- A target (the anode) in which the electrons can interact, releasing energy in the form of x-rays
- a method of accelerating the electrons- electrical potential difference applied between the cathode and anode
What is the cathode? What is it made of?
* cathode acts as a source of electrons
* it is of a tungsten coiled filament mounted within a molybdenum focusing cup. Tungsten has a high melting point and a high atomic number. When a current is run through the cathode filament, it heats up and releases electrons.
What is the anode? What is it made of?
* when an electrical potential difference is applied across the x-ray tube, the anode has a positive charge relative to the cathode and attracts the electrons
* When electrons collide with the anode, 99% of the kinetic electron energy is converted to heat and only 1% gets converted into x-rays
Because of the heat produced anodes have a tungsten target which is resistant to heat. This is mounted on a copper base which helps conduct heat away from the target. (Either rotating– more powerful x-ray machines– or stationary– dental and portable– copper base)
What is the glass envelope?
* Contains the cathode and anode within the vacuum. The x-ray tube has a small exit window through which the x-ray photons escape. The tube is surrounded by oil which helps dissipate the heat and acts as an electrical insulator.
What is meant by quantity in regards to properties of the x-ray beam? Quality? Intensity?
Quantity= the number of x-ray photons produced
Quality- the energy of the x-ray photons, hence their penetrating power
Intensity- the amount of x radiation per unit area. The intensity is determined both by the number of photons and the energy of each photon
What does the quantity of electrons produced at the cathod filament dependent on?
The electrical current of the cathode circuit. This is an adjustable parameter that the x-ray machine operate can control, known as the Milliamperage (mA) setting. Increasing mA increases the number of electrons within the electron cloud
What does increasing mA do?
Increasing milliamperage (mA) increases the number of electrons within the electron cloud
What does the higher the kV mean? What is kVp?
* In order to accelerate the electron cloud toward the anode and produce x-rays, a potential difference or kilovoltage (kV) is applied between the anode and the cathode. The highter the kV, the greater the acceleration and thus the greater the energy of the electrons– the greater the energy of electrons– the greater the resultant x-rays which EQUALS HIGHER QUALITY X-rays
** kVp- kilovoltage peak- the maximum voltage available at the kV setting and is an adjustable paramter that the x-ray machine operator can control
What is the meant by the exposure time?
* The potential difference between the anode and cathode is only applied for a very short period of time (milliseconds)– the exposure time is an adjustable parameter that the x-ray operator can control. A long exposure time allows may electrons to impact with the anode, while a short exposure time allows fewer electrons to impact with the anode
What is the quantity of x-rays produced a function of? What determines the quality of x-rays produced?
both the mA setting and the time setting (s)
** mA determines the number of electrons produced, while time determines the number of electrons that hit the anode
** some x-ray machines allow the operator to select the mA and s independently
** kVp setting determines the energy of the electrons and therefore the energy of the resultant x-rays and thus the quality or penetrating power of the x-rays
How does the radiographer ensure the x-ray beam has the appropriate intensity?
Intensity = quality and quantity of x-ray photons
* this is so that enough x-ray photons penetrate the patient and interact with the x-ray detector to form the image
* The x-ray operator achieves this by manipulating the kVp, mA, and second settings on the machine to be appropriate for the anatomic area that is being radiographed
What is the focal spot? What does the size focal spot influence?
The area of the target with which the electrons collide… the size of the focal spot influences the detail (edge definition) of the image and also affects the heat loading capacity of the machine.
e.g. a stationary anode is limited to having a larger focal spot in order to accomodate higher temperatures. The focal spot of a rotating anode can be smaller due to the larger surface area over which electrons impact the target and superior ability of a rotating anode to dissipate heat.
Common focal spot sizes in veterinary medicine are 1-2 mm^2
What is the collimator?
The device to limit the size and shape of the primary x-ray beam
* To minimize scattered radiation, the primary beam should be confined to a size and shape that will just cover the region of diagnostic interest
What are the benefits to using the collimator to limit the x-ray beam to the area of interest?
Less scatter radiation is produced which improves image contrast and image quality and radiation safety…
What are the three main parts of the x-ray machine generator?
* The control panel- allows the operator to select appropriate setting for the voltage (kVp), tube current (mA), and the time of exposure (sec)
* The transformer assemby- step up or step down transformers to increase or decrease the voltage as required (which is needed to boil electrons from the filament and to accelerate these electrons from the cathode to the anode)
* The rectification circuit- incoming mains electricity is alternating current (AC), when x-ray exposure is made and a potential difference is applied across the x-ray tube, direct current (DC) is required. Most efficient systems produce constant positive lectrical potential, cheaper units the DC is more variable and only reaches peak voltage (kVp) for a small proportion of the time
What are the two stages that accomplish the exposure?
* Preparation- press and hold the button on the hand piece half way down; the cathode is heated by the cathode current and the anode starts rotating
* Exposure- when the system is ready, the operator fully depresses the button; this causes the potential voltage to occur for the set exposure time
What is full wave rectification?
* Converting the AC into a direct current (DC) without losing any electricity– full wave rectification creates a nearly constant electrical potential across the x-ray tube as both halves of the alternative voltage are used to produce x-rays
* A consideratlbe portion of the exposure time is still lost however while the voltage is in the valley between the two pulses
What is a three phase generator?
Present in most modern machines- produce an almost constant electrical potential difference between the anode and the cathode, which results in:
- More power available to the x-ray tube per unit time and therefore for shorter exposure
- Intensity of the x radiation generated is higher
- Radiation quality is greater because it contains less low energy x-rays
- Tube utilisation is more efficient because the target is not subjected to bombardment of low energy electrons, which only create heat
** main disadvantage is the increased cost of the machine relative to more simply rectification machines
What is the tube rating? What is it based on?
Each x-ray machine has a tube rating expressed in kilowatts– the tube rating dictates themaximum combinations of kVp, mA and time that can be safely used without over-loading the tube
Based on:
- Focal spot size
- Target angle
- Anode speed
- Electrical current
How is a radiographic image formed? What is attenuation?
Differences in attenuation between different components of the object to be radiographed. Attenuation is the proportion of x-rays that are stopped/ absorbed by a given thickness of matter
What 3 things happen when x-ray hits matter? What determines which one predominates?
- Transmission- the x-ray photon passes straight through
- Photoelectric effect- x-ray photons can be absorbed
- Compton effect- x-ray photons can be scattered
** Which one of these interactions predominates is determined by:
- energy of the x-ray beam (kVp)
- atomic number of the absorber. Remember more interactions occur with matter that has a high atomic number
- thickness and density of the part being radiographed. It makes sense that thicker or denser objects will stop more x-rays.
What is the photoelectric effect?
PE results in complete absorption of an x-ray photn by an atom and the ejection of an orbital electron, which produces a positive ion. Largely responsible for radiographic contrast. The main way x-rays are attenuated at 40-120 kV. The primary interaction between x-rays and bone.
** Probability of the PE depends on:
- Energy of the x-ray photon (must be above threshold kV)
- Atomic number of the absorber (elements with a higher atomic number are more likely to have PE absorption)
When is PE more likely to occur?
In low energy x-rays (50 to 70 kVp)
* high Z material (more in iodine, barium and lead than in oxygen, carbon or hydrogen)
** PE explains why better contrast is achieved with low kVp and why iodine and barium (used in radiographic contrast studies) stop more x-rays than organic matter
** soft tissue is made up of hydrogen, carbon, nitrogen, and oxygen– avg atomic number of 7.. vs. bone which is mostly calcium (atomic number 20)
How do you increase subject contrast?
Decrease the kVp setting
Why is PPE made from lead?
Atomic number is Z82– materials made from elements with a high atomic number are good absorbers of x-rays
What is the compton effect? Why is it not useful in diagnostic radiology? What influences the likelihood of the compton effect?
Responsible for almost all the scatter radiation produced in diagnostic radiology
No useful function in diagnostic radiation because
- Scattered radiation poses a safety threat
- Decreases contrast of the radiographic images (e.g. the difference between black and white on the radiograph is less obvious)
** Likelihood of the compton effect
- Very dense absorbers are more likely to produce compton scatter
- large volumes of irradiated tissue are more likely to produce compton scatter– thick body aprts e.g. fat dog abdomen, horse back or shoulder; or poorly collimated image will produce more scatter
- High kVp settings thuse higher energy x-ray photons are more likely to produce compton scatter
How do you absorb scatter radiation before it hits the detector?
Use a grid
They should always be used when the patient thickness is greater than 10 cm because more scatter is produced in thicker body parts
Placed between the patient and the detector- so under the patient or under the table between the patient and the cassette
How does a film screen x-ray work? What is the most commonly used x-ray films?
* x-ray film consists of a polyester base coated on both sides with a gelatin emulsion containing silver halide crystals. When visible light or x-rays interact with silver halide crystals- an invisible image is formed then processed to be made visible– by exposing the silver halide to an x-ray photon or light photon which activates the crystals which precipitate silver… which aggregates producing darkenened areas
** Most commonly used x-ray films are double emulsion- having emulsion on both sides of the base (single emulsion can be used with pocket pets)
What are cassettes?
Cassettes are used in traditional film-screen and computed radiology (can be reusable)
* rigid lightproof containers that hold both the intensifying screens and the radiographic film. Double emulsion film is used in cassettes with two screenes which therefore results in two images on either side of the film
What is screen speed?
Part of a cassette. An important characteristic of intensifying screens. Faster screens requires less radiation (lower mAs, usually achieved with shorter exposure time)– at the expense of detail however.
What is an intensifying screen? Why must the x-ray film match the intensifying screens of a cassette?
* Intensifying screens in a cassette convert x-ray photon energy into light photons, which expose the x-ray film. The use of film with intensifying screens (film-screen) is a more efficient detection system than x-ray film alone and allows a reduction in radiation exposure up to 100 times.
* X-ray films are only sensitive to one colour of light and different screen types give off different colours
What is optical density ? What is it primarily controlled by?
Measure of the blackness of film– primarily controlled by mAs
What is the relationship between optical density of a radiograph to the exposure needed to produce that density?
Sigmoidal
* Ideally, most of the radiographic image should be exposed in the linear portion of the curve. If the film is exposed at the shoulder or toe of the sigmoidal curve, the image will appear overexposed (too dark) or under exposed (too light) respectively
What is film speed?
Term used to describe how sensitive the film is to x-ray or light photons. Film that has a high speed rating is very sensitive to photons, an adequate optical density can be achieved with lower radiation exposure. The trade off for fast fil is that it has slightly lower detail/ spatial resolution compared to slower film.
What is film latitude? What is film latitude’s relationship to film contrast?
Exposure range over which acceptable optical densities are produced. Film with a wide latitude will accept a significant variation in exposure factors or processing without exhibiting any great chance in optical film density. Most x-ray film has WIDE LATITUDE making it relatively easy to avoid under or over exposure… narrow latitude film or high contrast film is less forgiving with exposure factors– FILM LATITUDE VARIES INVERSELY with FILM CONTRAST
What speed film would you want to use for cat’s manus?
Slow speed 100 to 200- higher in detail, small body part, very little movement so longer exposure times okay to use
What speed film would you use for a fat dog’s abomen?
Fast speed- 400 to 600 speed
What film speed would you want for a horse’s carpus?
400 speed- fast system. Move a lot so short exposure time
What are the steps in film processing?
- The Developer- chemical solution that converts the latent image on the film to a visible image by reducing exposed silver halide crystals to black metallic silver. ** Time and temperature dependent (chemical reaction occurs at a greater rate at higher temperatures)– usually at 20C about 3 minutes to develop
- The rings- x-ray film retains developer in the gelatin and if this were transferred to a fixer the alkaline developer would neutralize the acid fixer– so rinsing stops the developing processes and prevents carry over the fixer
- The Fixer- removes unexposed silver halide crystals. Fixation also hardens the gelatin coating so that it can be dried without damaging the film surface. Fixed fro twice the development time to ensure maximum hardening of the emulsion. An incompletely fixed film takes on a milky or cloudy appearance.
- Wash Bath- removes the remaining silver complexes and excess fixer. (20-30 minutes)
- Dry the film. Hanging.
What are the limitations to traditional film- screen?
Image can have good contrast or good latitude but not both
* And once the film has been exposed to the x-rays, the contrast and latitude can’t be adjusted
What are the three steps of Computed Radiography?
- Image acquisition- x-ray photons strike a reusable storage phosphor imaging plate (IP)– placed in a plate reader where it is scanned by a laster beam which releases the stored latent image as visible light. Then the IP plate is erased for reuse by scanning the plate with an intense white light
- Image processing- using algorithms the computer evaluates and manipulates the digital data
- Image display- the user can then apply post processing techniques such as edge enhancement, magnification, adjustment of contrast range and dynamic range
CR advantages and disadvantages?
How does direct digital radiography work?
* Direct systems use an electronic detector made of amorphous silicon or selenium or a charged couple device to capture the image. The detector is attached to the computer system via a cable. The signal is transmitted to the computer and reassembled into a digital image.
Advantages and Disadvantages of DDR
What is the major advantage of digital imagine?
Due to the wider latitude, the number of retakes due to poor exposure are reduced
BUT although digital image contrast can be manipulated by viewing software, subject contrast is still influenced by collimation, scatter and kVp settings
How does spatial resolution of digital systems compare to film-screen systems? Does this matter?
* Spatial resolution of digital systems is not as good as film-screen systems, but it is not a clinical limitation
What are the four steps to processing the digital image that occur automatically?
- Creation of a histogram- maps optical density vs. frequency which allows mathematical manipulation of the raw data- histogram is compared to an expected histogram for the anatomic area selected to be radiographed
- Data adjusted for under or over exposure- histogram shifts left or right to compensate
- Enhancement of image contrast, using a look up table
- Edge enhancement or smoothing- complex mathematical function is applied to the raw data to allow processing in the spatial frequency domain– boundaries such as bone margins can be smoothed or enhanced
What is PACS?
Archiving and distribution of digital images
What are the features of radiographic image quality?
* Optical density, radiographic contrast (subject contrast, film contrast, fog and scatter, digital image contrast), detail (spatial resolution, edge definition), technical faults
How can optical density be manipulated?
By adjusting mAs (mA or seconds) setting
How does a film with high optical density appear? Why would this occur? What should you do?
Too dark over-all due to overexposed (too much radiation reached the film– likely because mAs setting was too high) or over-developed.
** may also occur if film-focal distance is too short
** If too dark then reduce mAs +/- increase the film focal distance
Why does under exposure occur? What should you do?
* check kVp settings– have to be high enough to produce x-rays that will penetrate the patient
* most common cause of under- exposure is too low mAs setting… therefore increase the mAs and maybe increase the kVp
What is a technique chart?
Helps us determine the best x-ray machine settings (kVp, mA, seconds)
* technique charts are specific to the x-ray machine, film-screen speed, film to focal distance, whether a grid is used
** measure the thickness of the area you intend to radiograph and read off the technique chart what the ideal kVp and mAs setting should be
What are the six basic steps in making a technique chart?
What is quantum mottle?
An underexposed digital image will have a grainy appearance
Quantum mottle is caused by the discrete nature of an x-ray photon; underexposure results in an insufficient number of x-ray photons reaching the detector to fully form the image
*** think of black paint spots outlining something
What does radiographic contrast depend on? When is high contrast preferred? When is low contrast preferred?
Radiographic contrast depends on:
- Subject contrast- due to attenuation of the x-ray beam in different tissues; low kVp will produce high subject contrast as long as the kVp is high enough to penetrate the tissue (higher kVp = more compton scattering)
- Film contrast- homogeneity in size of silver halide crystals within the film emulsion, over or under exposure– technical factors, over or under deveopment will push film contrast to the toe or shoulderof the characteristic curve and produce low contrast image
- Fog and Scatter- inadvertent exposure of the x-ray film to light
High contrast is preferred for bone images; low contrast is preferred for chest and abdominal images
What are the four factors that determine the degree of attenuation of an x-ray beam as it passes through matter?
- Thickness of the absorber
- The density
- Atomic number
- Energy of the radiation
Effect of kVp, mA and exposure time on the radiographic optical density and contrast
How can you reduce the production of scatter? How can you reduce the impact scatter has on the xray image?
* Reduce production of scatter by
- Selecting lower kVP (still has to be high enugh to penetrate the patient)
- restricting the collimator field to only the area of interest
* Reduce impact of scatter on the x-ray image
- using a grid when you radiograph body parts thicker than 10 cm
- having a small air gap between the patient and the cassette
3. Reduce scatter radiation dose to the operator - PPE
- distance
What is the biggest contributor to contrast of digital images?
The image processing algorithm and windowing of the image by viewing software (still impacted by subject contrast and scatter radiation)
What are the two main components of image detail?
Spatial resolution and edge definition
What is spatial resolution? How does film screen compare to digital? How is it determined?
* determined by the x-ray detector– ability to distinguish between two adjacent contrasting objects
** gilm- screen has better spatial resolution than digital… but advantages of edge-enhancement, superior latitude and ability to zoom images outweighs the disadvantage of lower spatial resolution
** Spatial resolution of film-screen is determined by the size of the halide crystals of the film emulsion and the size of the phosphor crystals of the intensifying screen
** Spatial resolution of digital radiography is determined by the size of the detector elements and size of the pixel matrix used to store the image
What is penumbra? What determines?
Edge definition
geometric factors that determine:
- Focal spot size (small focal spot produces a shart image, think of hand and iphone)
- Focal spot to film distance (as large as possible is important to improve image sharpness and reduce magnification)
- Object to film distance (close to the film as possible to improve image sharpness and reduce magnification)
Main causes of a blurry image?How can this be minimized?
Motion of the patient or cassette
* Minimized by shortening exposure time, sedation and anaesthesia
** Cassette motion a problem in equine radiography where cassettes may be positioned by a cassette holder
* blurry images can also be caused by geometric factors taht increase the penumbra
What is distortion resulting from?
Unequal magnification of different parts of the same object
What is forshortening?
* object being radiographed must also be parallel to the film
What is parallax error?
Since the x-ray beam is divergent the periphery of radiographic images have different projections– e.g. spine radiographs
What is an artefact?
* Structure or feature not normally present in an image, but visible as a result of an external agent or action– fake out
e. g. film screen dark marks caused by pressure on the film or white marks– dust or hair int eh cassette which prevents light from the intensifying screen from reaching the film
e. g. grid cut off appears as light areas caused by misalignment or inappropriate placement of a focussed grid
e. g. milk/ opaque image- typically a problem with manual processing- incomplete fixation leaving behind silver halide crystals OR brown image– incomplete washing leaving fixer behind which oxidises over time OR linear roller marks with automatic processors with dirty rollers
e. g. digital artefacts- grainy image = underexposure; double image- double exposure with both images displayed due to wide latitude radiography; moire artefact- bands or lines from ; white lines on a CR image- dirty plate readers; misapplication of the algorithm- poor collimation and beam centering can result in mis-application of the algorithm and poor image contrast resolution; uberschwinger- excessive edge enhancement appears as a thin black rim around high denisty object such as bone margins or metal implants and may mimic bone lysis
How do contrast agents work? What are the two main categories?
Work by having different physical density and atomic number hence different radiogrpahic density than normal patient tissue.
* Positive and negative contrast agents
how do negative contrast agents appear? Examples of agents? Examples of studies? Risk?
* Negative contrast agents are gaseous and do not effectively absorb x-rays so appear radiolucent (dark)
** room air, carbon dioxide, nitrous oxide
** used to inflate organs e.g. GIT- pneumogastrogram or pneumocolon, or urinary tract- pneumocystogram
** Risk of fatal air embolism if any gas leaks into the vascular space e.g. damaged mucosa from necrotising haemorrhageic cystitis– to minimize risk of death from air embolism when performing pneumocystogram place patient in left lateral recumbency and use CO2 or NO which are more soluble than room air (would be reabsorbed in the right ventricle otherwise could become trapped in pulmonary artery or travel and occlude pulmonary circulation)
What are positive contrast agents? Examples of agents? Which agents used where?
* Positive contrast agents have a higher atomic number than soft tissue, thus are an efficient absorber of x-rays and appear more radiopaque
* iodine- Z= 53; barium Z= 56
** Barium used in the GIT– irritant so not IV– excreted by the GIT
** Iodinated contrast agents are ionic or non-ionic. Ionic agents are markedly hyperosmolar relative to blood so are associated with adverse reactions such as hypotension, nephrotoxicity and thrombosis– excreted by the kidneys, biliary tract, GIT, saliva
** All iodinated agents can trigger allergic reactions
* Iodinated agents– angiography (IV), cytography (urinary tract), myleography (spinal cord)
** Myelography should only be performed with non-ionic!!
What are the conventional hanging protocols?
* Dorsoventral OR ventrodorsal radiograph of the body- head at the top and the patient’s right on the viewers left side
* Lateral radiograph- animal standing with the head to the viewers left and dorsal (for the body) or proximal (for the limbs) to the top of the screen
* Craniocaudal or dorsopalmar/ dorsoplantar- proximal is to the top and the lateral aspect displayed to the viewers left (no matter whether you are looking at the left or right limb)
What are the five basic radiographic opacities?
Indications for ultrasound
* internal structure of organs- size, shape, margins
* dynamic evaluation of blood flow– which can provide some information on organ function e.g. echocardiography where blood flow and changes to the heart size over the cardiac cycle can be measured
* more sensitive at detecting pathology, low specificity for identifying a specific disease
* GIT e.g. heptobiliary disease, pancreatitis, vascular anamolies such as portosystemic shunts
Patient preparation?
* Hair coat clipped, skin cleaned and alcohol applied to defat the skin, ultrasound gel applied to provide good acoustic coupling (transmission of sound) to reduce artefact and allow the ultrasound probe to slide over the skin
Special considerations for echocardiography ?
Sedation will affect cardiac function, so preferably an unsedated animal
Special considerations for abdominal ultrasound? Musculoskeletal ultrasound?
* Abdominal- animals should be fasted
* musculoskeletal ultrasound- may benefit from using a stand off pad since structures are superficial and often hard
What is B-mode ultrasound imaging?
What is M-mode ultrasound?
What is Colour Doppler?
What is spectral doppler?
What is duplex doppler ultrasonography?
Simultaenous use of real time B mode imaging and spectral doppler.
What is sound? What is the relationship of wavelength and frequency at a given velocity? What is the range of frequencies of ultrasound? What is the assumed constant speed of sound for ultrasound and where is that number from?
* Sound is a pressure wave with properties of wavelength, frequency, velocity and amplitude
* For a given velocity, wavelength and frequency are inversely related e.g. high frequency = short wavelength.
* ultrasound has frequencies between 2 and 20 MHz
* Ultrasound assume constant speed of sound at 1540 m/sec (which is the average speed of sound in soft tissues)
What three things might happen when ultrasound pulses interact with the patient?
- Transmission into deeper tissues
- Refraction (bending) of sound as it passes from a tissue of one density to a tissue of a different density
- Attenuation of sound, which is a reduction in amplitude or intensity of the pressure wave
What does sound attenuation depend on? What kind of transducer do we use for superficial structures? Deep structures?
* Sound attenuation depends on the distance travelled and teh frequency of sound
* High frequency tranducer for superficial
* low frequency to image deeper structures
What is a B-mode ultrasound image composed of? What does the brightness of each x correspond to?
What is acoustic impedance?
* Sound reflection or transmission characteristics of a tissue, largely dependent on the density of the tissue and speed of sound within the tissue. Because we assume constant speed of sound in soft tissue, density is the most important component in this equation
** It is not exact value of acoustic impedence that matters but the difference in acoustic impedence between two tissues
** At boundaries between soft tissue and boen or soft tissue and air there are large differences in acoustic impedance resulting in little sound transmission and the image is displayed as a shadow– THIS IS WHY WE CLIP ANIMALS FUR, clean the skin and apply coupling gel– this removes air between the skin and transducer
WHat is the preferred imaging modality to assess bone or lung?
What does the ultrasound transducer do?
* converts electrical energy to acoustic energy and acoustic energy to electrical signals
* consist of piezoelectric crystals aligned within the footprint of the transducer and when a voltage is applied the crytal vibrates and produces a sound– each one has a unique frequency at which is vibrates and will produce sound of an inherent frequency. The tranducer transmits sound pulses for 1% of the time and listens for returning echoes 99% of the time.
** The operator can select the main frequency band that is used for imaging, selecting higher frequencies for imaging superficial structures and lower frequencies for deeper
What is the phased array?
What should you consider when selecting a transducer?
* consider frequency of the transducer and the footprint or shape of the field of view of the transducer
What is axial resolution?
Dependent on the spatial pulse length
What is lateral resolution?
how is the best spatial resolution achieved?
Imagine with a high frequency transducer and imaging within the focal zone
When are low dynamic range setting used for? When are high dynamic range setting used for? What is true for both?
Low dynamic range settings are used for echocardiography
* High dynamic range for abdominal ultrasonography
* You will have the best perception of contrast resolution when imaging with a low gain setting in a darkened room
What is nuclear scintigraphy?
Allows imaging of the distribution of radiopharmaceuticals within the body using a gamma camera. It allows imaging of both structure and function
What is the most commonly used radionuclide in vet med?
Technetium- as it decays, the nucleus is converted to a more stable state by emitting gamma radiation which is detected by a gamma camera. It has a half life of 6 hours– long enough for most imaging procedures to take place but short enoug hthat the radioactivity of the patient rapidly decays, typically reaching background levels within 24 hours… thus minimizing radiation dosage to the patient, staff and owners
Static vs. dynamic nuclear scintigraphy
* Static mode- provides anatomic information about the distribution of the radiopharmaceutical at one point in time, allowing determination of organ size, shape, position and pattern of distribution that may be specific for a disease state e.g. bone scintigraphy– shows areas of active bone turnover as seen with physes, stress fractures, infection or bone neoplasia
** dynamic mode provides functional information allowing measurement of rate of accumulation or removal or the radiopharmaceutical within an organ– example is portal scintigraphy where a series of images is acquired over a four minute period allowing mapping of the passage of pertechnetate through the portal circulation, liver and heart
What is used in bone scintigraphy?
Technetium methylene diphosphonate (MDP)- taken up by areas of active bone turnover
** bone scintigraphy is much more sensitive than radiography at detecting bone lesions but is relatively non-specific
What is thyroid scintigraphy?
Technetium pertechnetate– trapped and concentrated by thyroid follicular cells, mimicking biological behavior of iodide
** useful to evaluate hyperthyroidism– determines whether cervical masses are thyroidal or non-thyroidal in origin… differentiates between bilateral and unilateral disease, detection of ectopic thyroid tissue, and functional metastic lesions
What is portal scintigraphy used for?
Sensitive and specific for diagnosis of portosystemic shunts– even quantification of the severity of the shunt– Technetium is administered into either the colon or into the spleen where it is taken up by portal circulation
Special considerations with scintigraphy?
What is computed tomography?
Cross sectonal imaging modality based on x-rays. It gives excellent contrast resolution compared with conventional radiography. Produces high detailed cross sectional images with no superimposition of overlying structures.
** Helical scanners are the preferred type of machine– newer helical CT scanners have multiple rows of detector arrays– multi-slice CT scanners have very rapid acquisition times
What is the Hounsfield scale?
The operator can choose what part of the scale to assign the central colour gray
Indications for a CT examination
* Superimposition of structures in radiography prevents identification of lesions
* Improved soft tissue contrast is required
* lesions are suspected but not demonstrated with conventional modalities such as radiology or ultrasound
** e.g. brain and spinal imaging (MRI is best however but CT is more sensitive than MRI at detecting early brain haemorrhage with trauma), nasal disease to avoid superimposition, Tympanic bulla (changes seen with otitis media for example), musculoskeletal imagine (bone loss, complex trauma e.g. pelvic fractures provides 3D reconstructions), thoracic imaging (pulmonary metastases), abdominal imaging, foreign bodies and draining sinuses (localisation of abscesses and foreign bodies)
What is magnetic resonance imaging?
* imaging of hydrogen atoms throughout the body– measures how hydrogen atoms are influenced by radiofrequency pulses
* Most common source is water
* MR images are based on the distribution of water and the interaction of water with its surrounding molecules
* MRI exploits the property of hydrogen as a magnetic dipole– the rotating positive charge of the hydrogen atom induces a magnetic field– thus hydrogen atoms have a magnetic moment (direction and strength of their magnetic field)– with a strong magnetic field– the magnetic moments of the hydrogen atoms align with the external magnetic field
What is an MRI sequence?
What is better about MRI? Problem with MRI?
* superior contrast resolution– for soft tissue especially neural or musculoskeletal tissue… sensitive to the detection of early lesions
** takes a long time so susceptible to motion artifact from respiration or peristalsis
** most pathologies involve increased water content (tumours, inflammation, oedema, infection) and MRI is highly sensitive at detecting most diseases
Safety issues with MRI?
DDX for abdominal pain?
ischemia, inflamm, distension, traction– FB obstruction, pancreatitis, obstructive neoplasm, biliary obstruction, etc.
When does a patient need rehydration? Maintenance?
* Rehydration in response to e.g. interstitial dehydration due to increased losses from vomiting and diarrhoea, coinciding with reduced intake (inappetance)
* Maintenance– e.g. if he doesn’t eat/drink
In a 25 kg dog. TBW? ICF? ECF?
If the animal has a PCV of 50%, what is the total intravascular blood volume of the dog?
TBW = 60% of 25 kg = 15 L
ICF = 40% of 25 kg = 10 L
ECF intravascular = 1.25 L
ECF interstitial = 3.75 L
** If PCV if 50%– the other 50% is water (plasma). The IV volume of water (plasma) is 1.25 L, so that means that PCV is also 1.25 L… therefore a total blood volume of 2.5 L (2.5 L, 10% of body weight)– clinically important in resuscitative fluid therapy where one sequentially boluses portions of blood volume at a time until targeted resuscitation goals are met.
What would your fluid therapy plan be (type, amount, over what time, by which route)?
* Isotonic: LRS (Hartmann’s) or 0.9% NaCl because he is not acidemic already
* Amount:
- deficit = 25 kg * 0.5 * 1000 = 1250 mL, 52 ml/ hr + maintenance… maintenance= ((25*30) + 70)/24= 34 ml/hr. 86.2 ml/hr + ongoing losses… 28 ml/hr on going losses?? 114 ml/hr
Why do many animals appear brighter at the vets?
* Stress response, catecholamine/ cortisol release
What is Poiseulle’s Law? How does it relate clinically?
Physical law that governs the pressure drop in a gluid flowing through a long cylindical pipe. Clinically you can think about how it governs the pressure needed to push a volumetric rate of fluids e.g. in ml/hr through your tube (catheter)
** both length and radius have an effect on catheter flow but the radius effect is much greater than length. Double the length, you double the pressure it takes to get fluid through at the same rate…. Catheters have a high resistance to flow. 18 gauge are the biggest we use in peripheral veins.
What gauge through rabbit ear veins, bird wings and tiny kittens?
What gauge through cats and very small dogs?
Large cats and medium sized dogs?
Larger sized dogs (heeler upwards)?
Describe what would happen to the body water (how it would shift between compartments) in a patient in which you infuse a hypertonic fluid such as mannitol 25% or hypertonic saline 7%?
** water would shift into the IV compartment from both the interstitial and intracellular compartments– then as normal serum sodium concentration is re-established the balance between the ICF and ECF will normalize
What is normal serum osmolality in cats? Dogs? What substances contribute to serum osmolality?
290- 310 mOsm/l in dogs; 300-310 in cats
* Serum osmolality = 2 (Na+K)+ glucose+ urea (in mmol/L)
What is osmolality?
* Final osmolality is the sum of the number of mmols solutes present in the bag. Irrespective of their size or charge, it is the simple sum
What is a balanced fluid? What are others?
* LRS is balanced as it contains an electrolyte composition similar to serum
* 5% dextrose in water, 0.9% NaCl, 0.45% with 2.5% dextrose all non-balanced
Calculate how many mls 50% dextrose you would add to a 1 L bag of LRS to make a 2.5% solution? What is the resultant osmolality?
Goal is 2.5% dextrose in 1000 mL.
* to convert any percentage to mg/ml just add a zero… so 2.5% solution is 25 mg/ml
* If you want a 2.5% solution, you need 25 mg/ml and since there are only 1000 ml in the bag of LRS, you would need 25,000 mg dextrose in 1 L total solution. Since 50% dextrose - 500 mg/mL, you would add 50 mL dextrose to your bag of LRS after removing 50 ml of the LRS
** The osmolality of 50% dextrose is 2780 mOsm/L, so 2.78 mOsm/ml… since you are starting with a bag of Hartmann’s which has 272 mOsm in it, and you took out 50 mls, you are left with a bag of 245 mOsm in it (272-(50*0.272))= 253 mOsm. Then you will add back in 139 mOsm from the dextrose (2.78 mOsm/ml x 50 ml = 139 mOsm), so 258 + 139 - 397 mOsm
Before approving a veterinary radiation procedure, the veterinarian must take into account what three factors?
Need, safety for the animal, other options that do not involve radiation
Which of the following best describes the radiation effects experienced during exposures made for diagnostic radiology?
DNA damage
What ancillary positioning devices may be used?
Examples: sandbags, radioluscent pads, cassette holders, positioning troughs, other- compression bands
In Victoria, the Radiation Act 2005 states that in order to operate an x-ray machine…
must have a user license
What form of radiation x-rays take?
Ionizing electromagnetic radiation
As specified by ARPANSA (The Australian Radiation Protection and Nuclear Safety Agency), the occupational radiation effective dose limit is:
20 mSv per annum over a period of 5 years
What principles do we follow to minimise radiation exposure
Time (as few x-rays as possible), distance, and shielding
What is the radiographic constrast? Subject contrast?
Number of shades of gray in an image
* Subject contrast- different tissues absorb a different amount of rays
What is an artefact?
Something that is not real, NOT a microchip
What is the most appropriate description of the radiographic appearance of a positive contrast agent in comparison to soft tissue?
Appear more radiopaque– higher atomic number
How do you achieve the best results in regards to geometric factors?
* select the smaller focal spot when possible (as the focal spot becomes larger the penumbra is increased because blurring of the edges)
* Keep FFD as large as possible (about 100 cm)– focal spot to film distance (closer the x-ray tube is to the film, the greater the image magnification and greater the penumbra)- improve image sharpness and reduces magnification
* keep the OFD as small as possible (object to film distance)– improves image sharpness and reduces magnification
Example of positive contrast agents?
Is Helium a common example of a negative contrast agent?
* Positive contrast agents: barium and iodinated contrast agents
* Helium is NOT a negative contrast agent
Mechanism of action of a postive radiographic contrast agent?
High Atomic number is an efficient absorber of photons
What is the main route of excretion of barium? Main route of excretion of iodinated contrast agents?
* Barium is mainly excreted through faeces
* Iodinated contrast agents are mainly excreted through urine
What is the major difference between an ionic and non-ionic contrast agent?
Ionic agents have high osmolality which have adverse reactions– nauseau and vomiting.
* The iodinated anion is really the only part of the molecule that is important for radiographic purposes, both the anion and cation are osmotically active and when they dissociate in solution, tend to thave osmolalities double that of equivalent non-dissociating non-ionic compounds
Excretory urogram, what is the most appropriate contrast agent to use?
Iodinated- non-ionic agents reduce the incidence of contrast media induce nephrotoxicity (CMN) in excretory urograms
What is the most appropriate contrast agent for a myelogram?
Non-ionic iodinated– seizures, hyperthermia, prolonged recovery from anaesthesia, and intensification of pre-existing neural signs are postmyelographic effects commonly reported in dogs. Neurotoxicity of contrast media as well can occur– direct chemotoxic effects on neural tissue and their hyperosmolality
Excretory urography- to look for ectopic ureters and tumors and to evaluate kidney function
Urethrocystogram
Appropriate choice of contrast agent for performing a contrast study in a patient with suspected gastric wall mass
Barium
Appropriate medium to evaluate ureters in a young cat with abdominal trauma
Excretory urogram
Most appropriate diagnostic imaging test to evaluate kidneys with chronic renal failure?
Ultrasound
Most appropriate agent to use in a patient suspected to have an oesophageal FB and oesophageal perforation?
Non-ionic iodinated
What potential complication may occur when performing a pneumocystogram in a patient with haematuria?
Air emboli
What is an example of an adverse reaction that may occur following intravaneous injection of iodinated contrast media?
Renal failure
What is the major cause of hypotension observed following intravenous injection of iodinated contrast agent?
Osmolality
Prior to performing an excretory urogram, which of the following options lists the most appropriate tests to evaluate the risk of an adverse reaction to IV injection of an iodinated contrast agent?
Biochemistry and urinalysis
True or false: Perivascular injection of an ionic iodinated contrast agent may lead to sloughing of the skin?
True
B- mode
Which of the following statements is true of diagnostic ultrasound?
A. The ultrasound machine assumes sound travels faster in fat than in the liver
B. Diagnostic ultrasound uses sound frequencies of between 1 and 10 Hz
C. The wavelength of sound is inversely proportional to the frequency
D. Ultrasound probes continuously produce sound and receive echoes to form the B-mode image
C. The wavelength of sound is inversely proportional to the frequency
What circumstances will produce an ultrasound image with the best possible spatial resolution?
* high frequency transducer
* imaging with a low gain setting
* low frequency transducer
* imaging within the focal zone
* high dynamic range
* High frequency transducer, imaging within the focal zone
Physical properties that influence the rate at which sound is attenuated as it passes through the body?
Irregular surfaces and different tissue interfaces (NOT frequency and distance travelled– they are not physical properties)
Most appropriate use of transducer to use when imaging structures deep within the abdomen
Low frequency
Spectral doppler
Phased array
Purpose of applying gel to skin?
Better sound transmission so it can glide
In B-mode ultrasound imaging, what does the brightness of a pixel on the display indicate?
Amplitude of each returning echo, low intensity echoes are black and high intensity echoes are white
T or F: Ultrasound is very specific for identifying what disease is present?
False
