Diagnostic imaging (SA08) Flashcards
What are the dangers associated with radiography?
- X rays are invisible
- They are painless
- Effects are ‘latent’ (not seen immediately, may only be seen years later
- Effects are cumulative; repeated low exposures are as hazardous as a single large exposure
What are the principles of radiation protection?
- X rays must only be taken if definite clinical justification
- Exposure of personnel kept to minimum
- Time - reduce need for repeat exposures
- Distance - Stand 2 meters away
- Shielding - Wear PPE, stand behind lead screen
- No dose limit should be exceeded
What is the electromagnetic spectrum?
- Radiations with similar properties
- Low to high energy
- Radio waves, Microwaves, Infared radiation, Visible light, Ultraviolet, X-rays, Gamma-rays
What do all the waves of the electromagnetic spectrum have in common?
- Travel in waves
- Travel at same velocity
- Need no medium to travel, can pass through vacuum
- Travel in straight lines
- Interact with matter through absorption or scatter
What is the difference between radiowaves and x-rays?
RADIOWAVES
- Low energy radiation
- Low frequency
- Long wavelengths
X-RAYS
- High energy radiation
- High frequency
- Short wavelengths
How are x-rays different to other waves in the electromagnetic spectrum?
- Penetrate matter visible light can’t as high energy
- Interact with photographic film, produce latent image
- Interact with phosphor in intensifying screen to produce visible light (fluorescence)
- Interact with living tissues, produce bio changes
- Energy can be captured and converted to produce electronic images
What are the 3 types of x-ray machines?
- Portable
- Mobile
- Fixed
What are portable x-ray machines?
- Wall mounted or on portable stand
- Not very powerful
- Old machines only produce 20mA
- Newer machines can produce around 60mA
- kV and mA are usually linked, a given kV delivers a set mA
- Dental and equine
What are mobile x-ray machines?
- Large transformer box and trolly
- More powerful than portable machines
- Produce up to 300mA
- mA and kV can be set independently
- Machines in practice (often x-hospital)
What are fixed x-ray machines?
- Built in the room on overhead gantry/rails
- Needs special electricity supply
- Very powerful but more costly
- Often linked to floating tables
- Used in NHS
What is the x-ray beam made up of?
- Photons (packets of energy)
- Created when electrons are slowed down/stopped
- Electrons are present in atoms of all elements
What does the structure of an atom consist of?
- Protons (positively charged)
- Neutrons (no electrical charge)
- Electrons (negatively charged)
What does it mean for an atom to be electrically neutral?
- Equal number of protons and electrons
Where in an atom are the different elements?
- Protons + neutrons (centre of nucleus)
- Electrons orbit nucleus in shells
How are the charges of atoms changed?
- Changes to amount of electrons
- More protons than electrons, positively charged, Cation
- More electrons than protons, negatively charged, Anion
- Atomic numbers are number of protons in nucleus
- Important with how x-rays interact with different types of tissue and are seen on x-rays
What is needed for the on/off switch of a radiography machine?
- Must have warning light when machine on
- Now a ‘controlled area’, only radiography personnel allowed
- Often directly linked to lights outside room
- Should be isolator to turn off machine outside room in emergencies
What is a line voltage compensator?
- Ensures incoming voltage in consistent
- Produced by an autotransformer
- Built in part of modern machines
- Older machines may have indicator light
What are kilivolts (kV)?
- High power electrical charge
- Applied to target during exposure
- Draws electrons across
- Produced by a set up transformer
- ‘Penetrating power’
What are Milliamps (mA)?
- Low power electrical charge
- Heats filament
- Releases electrons during ‘prepping’ stage
- Produced by a step down transformer
- Number of electrons (quality)
What are seconds (S)?
- Timer for exposure time
- Some machines as low as 0.01 seconds
- Most exposures made at 0.05-0.5 seconds
What are the differences in old and new machines with inputting settings?
- Some modern machines automatically input settings from species, area and depth of patient
What are the 3 exposure factors?
kV
- Controls energy of photons
- Quality/penetrating power of primary beam
- Affects contrast and density of image
mA
- Controls number of photons
- Quantity/intensity of primary beam
- Affects density of image
s
- Controls number of photons
- Quantity/intensity of primary beam
What is mAs?
- mA x s
- Control number of photons per exposure
- mA = number of electrons emitted
- s = how long emission lasts for
What will happens if mAs or kV is too high?
- Black, over exposed image
- Few shades of grey
- Everything dark grey
- Flat, lacking contrast
What happens if mAs or kV is too low?
- White, underexposed image
- x-rays not strong enough or not enough to penetrate tissues
- Tissue pale on black background
- Soot and whitewash film
What does MRI stand for?
Magnetic Resonance Imaging
What is an MRI machine?
- Typically long thin tubed shaped
- Powerful magnet running front to back
- Body part must be in exact centre of magnetic field (isocentre)
- Can take 2-D or 3-D map of body tissues
- Magnet 30,000 x stronger than earth magnetic field
- Can draw all metal objects within several yards
How is an MRI image created?
- Magnet causes nuclei of hydrogen atoms to line up in parallel formation
- Body hydrogen atoms normally point in different directions
- Radiofrequency wave produced by RF coil
- H atoms move orthogonally (Resonance)
- When turned off, move back to align with magnet
- Another RF wave is produced
- Signals produced are detected by RF coil and converted to form and image
- Speed of H atom movement creates image
What is ultrasound?
- Sound energy at higher frequency than can be heard
- Frequency determines how far waves travel through body
- Higher frequency gives better pictures but can’t travel far
- Different frequency transducer heads are available
What does ultrasound equipment consist of?
- Transducers
- TV monitor
- Control panel
How can ultrasound images be stored?
- On patient records
- Printed
- Uploaded to PACS
What are the types of ultrasound transducers?
SECTOR ARRAY
- More suitable for SA as small contact area
LINEAR ARRAY
- More suitable for large animal
How are ultrasound images produced?
- Crystals in transducer alter shape when current is applied; piezoelectric effect
- When applied to skin, waves pass through and ‘reflect’ off tissues, back to transducer
- Returning sound waves cause vibration of crystals and converted back to electric impulses to create images
What are ultrasound transducers?
Probes
What is echogenicity?
- Ability of tissue to reflect ultrasound wave
What is anechoic?
- Without echoes
- No sound waves return
- Appear black
- Fluid
What is the piezoelectric effect?
- Crystals in ultrasound transducer change shape when electrical current is applied
What is hypoechoic?
- Not many echoes
- Not many sound waves returned
- Appear dark grey
- Dense tissue
What is hyperechoic?
- Lots of echoes
- Many waves returned
- Appear bright white
- Bones/gas
What is acoustic resistance?
- When sound waves bounce back off of tissues
What does homogenous mean?
- Of uniform composition
- Smooth appearance
What does heterogenous mean?
- Not of uniform composition
- Mottled appearance
How does different tissue appear on a ultrasound?
- Fluid is black - Anechoic
- Tissues shades of grey
- Bone/gas white - Hyperechoic
What is the brightness or B mode in ultrasounds?
- Most commonly used mode
- Image of slice though organ/tissue
What is the time-motion or M mode in ultrasound?
- Vertical section through tissue
- Constantly updated
- Appears as rolling horizontal image
- Heart scans
What is the doppler mode in ultrasound?
- Used to detect movement
- Often used to detect blood flow
- Colour doppler assigns colour according to speed and direction of flow
What is contrast ultrasonography?
- Small quantities of microbubbles of sulphur hexafluoride gas
- Injected into blood stream in phospholipid capsules
- Microbubbles reflect waves
- Increased echogenicity in proportion to vascularity
- Looks at direction of blood
What other uses does ultrasonography have?
- Allows for accurate sampling
- FNA, biopsies
-Tru-cut biopsy (larger sample) - Tru-cut requires heavy sedation or GA
- Cystocentesis easier, especially if bladder small
Why may starvation be needed for an ultrasound?
- Full stomach causes increased echogenicity
- Avoid regurge in sedated animal
- Most animals won’t require sedation but stress and pain must be managed
What is the purpose of contrast studies?
- Visualise structures not normally seen on plain films
- Due to surrounding tissue similar radio-opacity
- Provides information re size, shape, position and structure
- Information re function of organ
What structures can be visualised using contrast media?
- Oesophagus
- Stomach
- Stomach & SI
- Large intestines
- Bladder
- Kidneys & Ureters
- Vagina & Urethra
- Spinal cord
- Blood vessels & heart chambers
- Internal structures of joints
What must contrast agent have the ability to do?
- Absorb a different amount of x-rays than the surrounding tissue
What are positive contrast agents?
- High atomic numbers
- Barium AN = 56
- Iodine AN = 53
- More radio-opaque than tissue
What are negative contrast agents?
- Low atomic numbers
- Air AN = 8
- More radiolucent than tissues
- Carbon dioxide sometimes used
An ideal contrast agent should:
- Show area of interest well
- Non-irritant
- Non-toxic
- Persist in area of interest
- Be totally eliminated from the body
What is Barium?
- For GI studies
- Suspension, paste, powder
- BIPS = Barium impregnated polyethylene spheres
Powder and suspension Barium?
- For stomach, small and large intestine
- Care when administering orally
- Aspiration causes granulomas
- Fatal aspiration pneumonia
- Contraindicated if perforation of oesophagus or bowels
- Leakage into thoracic or abdominal cavity causes granulomas
- Will persist indefinitely in those cavities.
