Radio Principles Theory 11a & b Flashcards
DIAGNOSTIC ULTRASOUND/ BONE DENSITOMETRY
What are the indications for ultrasound?
Pregnancy: Discrepancy in pregnancy dating To determine foetal age Suspected multiple pregnancies High-risk pregnancy Suspected foetal abnormalities Suspected foetal death Structural problem with uterus To locate the placenta Placental abnormalities Determine ectopic pregnancy and other abnormalities of pregnancy Abnormal bleeding Suspected ovarian tumour/fibroids
Soft tissue pathology: -Abdomen -Pelvis -Neck -Breast -Testes/scrotum Limited use for musculoskeletal imaging Shallow structures e.g. rotator cuff
Ultrasound equipment
Sound waves with frequency above human hearing transmitted into tissue
- reflected differently of different density tissues
- reflections are recieved and changes into images which closely resemble an antarctic blizzard.
Equipment has 2 main components:
- transducer
- display
Tell me about the transducer
-converts one form of energy into another
(electrical signal into sound waves, and sound waves into electrical signals)
Key component
Piezoelectric crystal
-thin layer (typically 0.5mm)
-emit electricity when deformed (squeezed or physically strained).
Transduce (sound heard)
- transmitter and reciever in one
- contains piezoelectric crystals
- piezoelectric effect is the property of a crystal to vibrate in the presence of energy.
- can be used to create waves or detect them.
- electrciity put into the crystals makes them give off small amountf of electricity, which can be detected and convertede into image.
Tell me about the Principles of Sounds Transmission.
- Ultrasound waves require a medium for transmission to occur
- because theyre sound (in space no one can hear you scream)
Relevant characteristics of the trasnmitting medium:
- compressability
- density
Velocity depends on the medium the sound passes through:
- constant for any particular medium
- independant of frequency
- inversly related to the compressability and density of the medium (in all liquids, density and compressiibility are inversly proportional)
Relative speeds:
-slowest in gases
-intermediate in liquids (mostly body tissues behave like liquids, so have similar speeds of transmission)
-rapidly in solids
A table shows velocity slowest to fastest: air, fat, water, brain, kidney, muscle, bone.
U/S equipment
Coupling gel
- sound waves dont travel well through low-density material (eg air)
- gel is applied to transducer to keep continuous higher-density contact with patient.
Principles of SOund Transmission
The frequency of US waves is between?
1 MHz and 20MHz (1-20 million cycles/second)
- the higher the frequency the greater the absorption within tissue
- thus lower frequency transducers must be used for abdominal imaging coz more tissue is transversed.
- US pulses are longitudinal waves (as apposed to transverse)
- once generated, the wave continues in its original direction untull it is either:
- reflected
- refracted (scattered)
- absorbed.
Producing the Image
There is variation in the percentage of reflection as they pass between the different tissue interfaces. % of reflection depends on what?
Variation in the percentage of reflection as they pass between different tissue interfaces
-The percentage of reflection depends on:
the tissue’s acoustic impedance
the sound beam’s angle of incidence
The acoustic impedance of a material is the product of its density and the velocity of sound in that material
- Since each of these factors is a constant for any given material, the acoustic impedance of different materials is a known amount
Acoustic Impedance
Amount of reflection is determined by
Practical application. In what kind of enviro would you get more reflexion?
Amount of reflection is determined by the difference in the impedances of 2 tissues
- The greater the difference, the greater the % reflected
The difference between most body tissues is small; however, there is a large difference between soft tissues and air or bone
Practical application:
A soft tissue – air interface would reflect almost the entire beam and produce a hyperechoic (bright) line on the display representing this interface.
Between soft tissue interfaces, little reflection would occur; thus, these areas would appear more hypoechoic (darker) than at the interfaces.
Depending on the homogeneity of tissue, the degree of darkness will vary
e.g., passing through the liver, there is a mixture of water, vascular, and fatty tissue, so the liver is less hypoechoic (brighter) than the lumen of the gall bladder, which is only filled with bile.
Angle of Incidence
The amount of reflection is determined by the angle of incidence between the sound beam and the reflecting surface (the angles of incidence and reflection are equal and opposite – as with light).
- The higher the angle of incidence, the less the amount of sound which will be reflected back to the transducer.
- At an angle of incidence above 3° from perpendicular with the tissue surface, almost no echo will be detected by the transducer.
Refraction
When the sound beam strikes the surface a different tissue medium at an angle, there is a change in direction of the beam
This is necessitated by a change in wavelength (i.e., since the frequency stays constant, and the speed at which sound travels through different media is constant for each medium, the wavelength must change: V = vλ)
Refraction results in spatial distortion artifacts (i.e., structures are projected in the wrong location) and loss of resolution
Absorption
- Absorption occurs as friction opposes the sound waves, with the loss of energy from the beam being converted to heat (ie, absorption = conversion of ultrasonic energy to thermal energy).
- Absorption is determined by: the frequency of the sound wave; the viscosity of the tissues; and, the relaxation time of the medium.
- As the frequency increases, the amount of absorption increases. As such, thicker body parts require a lower frequency transducer.
- Viscosity reduces particle freedom within the tissues, and increases friction. Liquids have less viscosity than soft tissues, which have less viscosity than bone.
- Relaxation time is the time needed for a molecule to return to its original position after displacement (which is how the sound waves propagate). Relaxation time is constant for any given tissue.
- If a sound wave hits a molecule that is still relaxing (ie, moving towards it), it uses up more energy to move the molecule forward again as compared to a molecule that has already relaxed
Doppler Ultrasound
- Doppler effect (Christian Doppler, 1843) is the change in frequency of a sound from a moving object
- Specially designed U/S units record the difference in frequency of its transmitted and received signals when reflected off moving objects
- Used to measure blood flow (cardiac scans)
- Some types give colour pictures, some B/W, others allow audio monitoring for detection of flow
BONE DENSITOMETRY
Bone mineral density analysis type ?
Plain films very insensitive (30-50% bone loss)
Types
Ultrasound measures the heel
DEXA (Dual Energy X-ray Absorptiometry) measures the spine, hip or total body
SXA (single Energy X-ray Absorptiometry) measures the wrist or heel
PDXA (Peripheral Dual Energy X-ray Absorptiometry) measures the wrist, heel or finger
RA (Radiographic Absorptiometry) uses an X-ray of the hand and a small metal wedge to calculate bone density
DPA (Dual Photon Absorptiometry) measures the spine, hip or total body
SPA (Single Photon Absorptiometry) measures the wrist
QCT (Quantitative Computed Tomography) measures spine or hip
What are the most common types
Most common types:
Ultrasound
-Not the most accurate (so far)
-No ionising radiation
Quantitative computed tomography (QCT)
- Very accurate
- Highest dose
- Regular CT scanner with special software
Dual energy x-ray absorptiometry (DEXA)
- Most accurate
- Two different energies of x-rays are passed through patient and measured for attenuation
- radiation exposure approximately 1/10th that of a standard chest x-ray.
BMD results
Results are graphs or numbers, not images.
BMD expressed as a relationship to two norms:
-The expected BMD for the patient’s age and sex (Z-score)
-The expected BMD “young normal” adults of the same sex (T-score)
-The difference between the patient’s score and the norm is expressed in standard deviations (SD) above or below the mean
Usually, 1 SD equals 10 to 20% of the bone density value
Interpreting bone density result :
Normal:
BMD is within 1 SD of a “young normal” adult (T-score at -1.0 and above)
Low bone mass (osteopaenia):
BMD is between 1 and 2.5 SD below that of a “young normal” adult (T-score between -1 and -2.5)
Osteoporosis:
BMD is 2.5 SD or more below that of a “young normal” adult (T-score at or below -2.5).
Women in this group who have already experienced one or more fractures are deemed to have severe or “established” osteoporosis
11B
FLOUROSCOPY/ CINERADIOGRAPHY
why do we use it?
- The fluoroscope allows for dynamic imaging assessment, and is still the main means by which we can look at real-time internal motion and motility of the body systems.
- Common assessments include GI contrast studies, cineangiography, fluoroscopically-guided catheter insertion and instrumentation procedures, and spinal motion studies.
How do these flouroscopic tubes produce their X-rays?
Same as plain film, but there are some different components.
The flurouroscopic Tube:
X-ray tube capable of continuous loe- energy x-ray output.
-could be 10 or more seconds per exposure
-timer also present to shut the system off after a finite period of tim (eg 3 mins) to prevent excessive exposure to the patient
Tube is located under the table
-x-ray beam through the patient towards the image intensifier above the table.
- a collimator is located on the tube to reduce exposure to a small area of interest
- A separate standard x-ray tube is also available to take “overhead” images with cassettes under the table top
FLOUROSCOPY/
Image intensifier
Red goggles? By allowing only one wavelength of light through you essentially allow yourself to see in the dark once you get into dark.
The intensifier is house in an evacuated glass tube, which contains ladi da da
How it works?
x-ray photons passing through the patient interact with the input phosphor and are converted into visible light.
The general conversion range in modern fluoroscopes is 50 – 300 (ie, about a 5,000 – 30,000x gain in brightness compared to a non-intensified image).
FLOUROSCOPY/
Image Intensifier:
Image brightness can also be controlled by kvp and Ma Saettings independently from the basic characteristics of the intensifier.
Image brightness can also be controlled by kVp and mA settings independently from the basic characteristics of the intensifier.
NB: higher kV and mA result in brighter areas on the image – which is opposite what we see with plain film imaging – as we are getting a positive image (eg, air is bright,
bone is dark).
To clean up scatter, a grid is
placed on the undersurface of
the intensifier facing the patient.
FLOUROSCOPY/
The Imaging table/ couch
Contains the fluoroscope tube, grid, and overhead imaging cassette holder.
Fluoroscopic tables are rotating, which allows the patient to be imaged in any position from fully upright to fully recumbent.
FLOUROSCOPY/
The Video monitor
- “Real time” imaging, so is best viewed in video format
- The output phosphor of the intensifier is directly or indirectly connected to a video camera
- The camera converts the light image from the output phosphor into an electrical signal, which is sent to the video monitor.
- Monitors can then be placed anywhere, to allow multiple people to view the procedure without having to be near the x-ray tube.
- As it is video, it can be recorded for later review.
The Cine and/ or spot Cameras
- A special apparatus is inserted between to the intensifier and the camera tube which allows video or photo-spot cameras to directly record the images.
- The photo-spot camera is like a video camera, but only records one frame per exposure.
- It receives its image from the output phosphor, which may be recorded on 70-105mm film.
- It is preferred over x-ray spot film imaging as it is quickly performed & gives less dose to the patient.
- If these are not in place, video can be recorded via the video monitor system, and spot images can be recorded on x-ray film
Scintigraphy/ Radionuclide Imaging
A way of getting images using radioactive material. X-rays arent a source of radiation.
WIth this a radioactive substance is injected into you then recorded to make diagnostic image.
- Gamma rays, not X-rays
- Physiology as well as anatomy
- ‘emission’ rather than ‘transmission’ radiation
- as such the radiation source and detection systems are significantly different.
- the emitted radiation from the body is captured by a gamma camera
- very complicated system with a variety of designs available
- 2 main components
- collimator
- detector
Scintigraphy/ Radionuclide Imaging
Collimator
- Gamma rays emitted from the patient go in every direction
- Device placed between the detector and patient to selectively allow gamma-rays traveling perpendicular to the detector to reach the detector.
The collimator behaves and looks in many ways like a grid used for x-rays, except that instead of long, parallel lead strips, there are ~ 3000 – 10000 various-shaped openings (usually round or hexagonal) across the plate, separated by lead septa (think of a honeycomb).
Scintigraphy/ Radionuclide Imaging
Detector
Detectors are composed primarily of a NaI crystal, a photomultiplier tube (PMT) array, and a pulse-height analyzer (PHA).
If the incident gamma-ray interacts with the crystal and results in a photoelectric event, the resultant photoelectron will be absorbed by the crystal and produce several thousand light photons.
Those light photons that reach the PMT array at a specified angle will interact with the PMT and be converted into electrical signal, from which the image is ultimately created.
- Importantly, the further the source of light is from the PMT, the less likely the PMT is to see the light
- For this reason, bone scans are generally taken AP and PA, with the structures closest to the camera appearing darker – from higher light uptake – than those further away.
The PHA is designed to assess the energy of all the light photons reaching the PMTs, and will discard those of lower energy likely to be resulting from scatter. In this manner, the image resolution is improved.
Bone Scans
Although any body tissue can be scanned with this technique, “bone scanning” is most relevant to us.
In this instance, nuclear imaging is used to evaluate pathological bone metabolism, which is assessed via perfusion / hyperaemia and/or active bone formation.
The radionuclides used most frequently with bone imaging are various technetium 99m phosphonates.
In single-phase (delayed) bone scanning, the patient is injected with a technetium-99m phosphonate compound and then asked to ingest up to 1 litre of fluids before scanning (to expedite excretion of unused radionuclide).
Spot images or a whole-body scan are obtained ~2 hours later. This procedure is used when looking for conditions such as bone malignancies.
When looking for an inflammatory process or an occult fracture, 2- or 3-phase bone scans are used.
For these examinations, the patient is injected while under view of the gamma camera and dynamic perfusion scans of the region of interest are obtained using a typical frame rate of 1 image in 15 sec.
