Lecture 3 part 2

Question 1

Imaging modalities =

Answer 1

MRI = magnetic resonance imaging
CT = computed tomography
Scinitagraphy

Question 2

MRI - Magnetic resonance imaging

Answer 2

• A cross sectional imaging technique
• uses a strong magnet and radio waves
  images are slices through the body at different planes

Question 3

MRI continued

Answer 3

Relys on magnets and radio waves - will see different information depending on the pulse sequence used

• Detection and analysis of the radio signals emitted by the patient gives information about the chemical composition of the patient
• Appearance of tissues will vary depending on timing of ‘pulses’ (inputs of radio waves) and ‘echoes’ (signals from tissue)

Question 4

MRI continued

Answer 4

computer will reconstruct the informations received into slices
images can be acquired in any orientation in the body
Different ‘sequences’ provide information on tissue type

Question 5

Advantages of MRI

Answer 5

• Very good anatomical detail, particularly of soft tissues
• Does not use ionising radiation
• will be safe to patient and operator of equiptment
• good for imaging areas that are not very accessible to other techniques
• Particularly useful for brain and spinal cord
• Useful for accurate assessments of extent of speed of a lesion e.g. retrobulbar lesions, nasal lesions, pelvic masses

Question 6

Disadvantages of MRI

Answer 6

• Not widely available in practice
• Expensive (cost f equipments and installation, maintenance, time to do)
• Animals need to be perfectly still so usually under GA, adds to risks and costs
• Due to powerful metallic field there must be no metallic objects around

Question 7

MRI safety

Answer 7

• Is a large permeant magnet so metal objects should not be taken into MRI
• will need to use MRI compatible tablets, oxygen cylinders, anaesthetic machinery and monotoring equipment
• need to take care with implants
• pace makers are a NO

Question 8

what will happen to a microchip in a MRI?

Answer 8

No damage to the animal
The location of the chip may just cause some distortion in the image

Question 9

Standing MRI

Answer 9

Usually for horses
Used to image distal limbs- only practical for this
A ‘C’ shaped magnet is positioned around the leg whilst images are acquired
Gives good anatomical detail
Will need software to correct for motion during image acquisition - as sedated horse will likely be wobbly

Question 10

CT - Computed tomography

Answer 10

A cross sectional imaging technique
Uses ionising radiation

Patient is placed inside a ring containing x-ray detectors
A x-ray source is rotated round the patient and the ‘image’ is picked up on the detectors on the other side

Question 11

CT continued

Answer 11

A computer analyses information for all detectors and forms an ‘image’ cross sectional slices
Patient only needs to be scanned once
The representation of tissues is the same as radiography e.g. bone = white, air = black

Question 12

CT ‘windowing’

Answer 12

= Reconstructing the image with different grey scales to highlight different bits
Displays information to optimise certain tissues
- is computer manipulation

Question 13

Uses of CT

Answer 13

• Gives detailed anatomical structures - good for bone e.g. elbows
• Can still be used for soft tissue but will be less detailed in these areas as MRI
• Scanning time is shorted that MRI
• Availability is increasing

Question 14

CT of elbows

Answer 14

Can take a transverse plane image which will avoid superimposition that would otherwise happen on a radiograph
will be able to see coronoid process

Question 15

CT of nasal/ aural lesions

Answer 15

CT is more sensitive for detections of changes to the nasal turbinates and bullae

Question 16

CT of lymph nodes

Answer 16

Lyphnodes are routinely seen on CT
- On radiographs they are only visible is they are enlarged

Question 17

CT of lung masses and metastases

Answer 17

CT is better and more sensitive for detection of lug metastases - will detected them earlier than a radiograph

Question 18

3D CT reconstruction

Answer 18

reconstruction of many CT places to 3D model - can be rotated and viewed in all aspects
possible to 3D print bones from this data
Facilitates surgical planning

Question 19

Disadvantages of CT

Answer 19

• Not widely available in practice
• Expensive compared to radiography
• Scan time is several seconds - quicker than MRI but slower than x-ray
• Uses ionising radiation - has higher patient doses than radiography
• Animal would need deep sedation or GA

Question 20

Scintigraphy

Answer 20

Uses a radioisotope - commonly technetium 99^m
is a gamma ray isotope

This will be bound to another substance that will determine where in the body it will localise
e.g. hydroxymethylene diphoshonate (HDP) which binds to bone

increased binding with increased metabolism - so increased metabolism of bone will mean that more will bind there

Question 21

How does scintigraphy work?

Answer 21

The bound isotope is injected into the patient, usually by IV
Time is allowed for the isotope to reach the tissue of interest
A gamma camera is used to detect the pattern of gamma radiation emitted from the patient

Question 22

Main veterinary uses of scintigraphy

Answer 22

• Skeletal injury in horses, especially ones difficult to radiography e.g. pelvis, spine
• LA and SA where it is difficult to localise a source of lameness
• For ectopic thyroid tissue (thyroid tissue further down neck or in thorax) in hyperthyroid cats

Question 23

Advantages of scintigraphy

Answer 23

• Increasing in availability in large equine centres
• Has an element of functional assessment as the uptake of the bound isotope depends on metabolism so can assess the function and not just the structure

Question 24

Disadvantages of scintigraphy

Answer 24

• Uses ionising radiation- risks to patient and operator
• patient will remain radioactive for sometime afterwards (48hrs)
• Need to minimise contact time with patient
• need to consider disposal of bedding and feces as will be radioactive
• Poor anatomical detail so can be difficult to interpret images - but can see different density
• Additional legislation apply: environmental agency due to disposal of isotopes