1 Introduction to imaging (copy)

List the imaging modalities that use ionising radiation

Radiography, fluoroscopy, computed tomography, scintigraphy

How does ionising radiation damage cells?

It causes ionisation in atoms (electrons ejected from the atom), creating free radical atoms that lack an electron

Free radical atoms pull electrons off surrounding atoms, causing a free radical cascade that causes breaks in chemical bonds

List the three possible interactions of an x-ray with a patient

Pass through the patient to the detector

Be absorbed in the patient

Be scattered from the patient

Radiopaque

More x-rays are absorbed - whiter appearance

Radiolucent

More x-rays pass through - blacker appearance

What are the five radiographic opacities?

Gas

Fat

Soft tissue (and fluid)

Mineral

Metal

Why are gas and metal opacity contrast agents used?

They help outline organs by contrasting with surrounding structures, making it look black or white

Why are two orthogonal views required for radiography?

To create a 3D image from 2D radiographs

Try to take one from the side and one from the top

How is an image formed in radiography?

X-ray beams pass through the patient to a digital detector, recording the image

How does fluoroscopy create images?

It produces a continuous x-ray beam to create an x-ray movie

Opacity is reversed compared to radiographs

How is an ultrasound image formed?

Sound is sent into tissue and reflected back, with amplitude indicating brightness

What determines how dark/ bright the ultrasound image is?

Denser tissue = larger acoustic impedance difference = brighter dot

Acoustic impedance

The resistance the sound wave encounters through a tissue

What is computed tomography (CT)?

A method where an x-ray tube rotates around the patient and the patient move through the machine to create slices of tissue

Can be viewed in different windows

How does magnetic resonance imaging (MRI) work?

1. Radiofrequency pulse is applied → hydrogen atoms no longer spin aligned to the magnetic field

2. Radiofrequency pulse is turned off → alignment gradually returns to be aligned with the magnetic field

3. As the H+ atoms realign, they release energy, which is collected as a signal which forms the image - the time the realignment takes is different depending on which tissue the atom is in (slower in water c.f. fat)

What is scintigraphy?

A method involving injection of a radioactive substance to assess physiological processes

What is the most common imaging modality used in veterinary practice?

Radiography

What is the primary use of fluoroscopy in veterinary practice?

To evaluate collapsing trachea in small dogs

Also for evaluation of oesophageal dysfunction, orthopaedic surgery and stent placement

What is AFAST in veterinary ultrasound?

A technique for assessing abdominal fluid

What is TFAST in veterinary ultrasound?

A technique for assessing lungs, pericardial effusion, and pleural effusion

What is the primary use of ultrasound in veterinary practice?

To answer specific clinical questions, such as locating a mass

What is computed topography used for?

It is used for detailed imaging of various body structures (poor for the brain)

What imaging modality is the choice for the brain and spinal cord?

MRI

What are the advantages of low field MRI machines?

They are cheaper to buy and maintain but have poor image quality

What are the disadvantages of high field MRI machines?

They are very expensive but provide excellent image quality

What is scintigraphy used to assess?

It assesses function (kidney function, thyroid function, cardiac function)

What are the advantages of radiography?

All practices have an x-ray machine

What are the advantages of fluoroscopy?

Can see in real time during procedures

What are the advantages of ultrasound?

Movement can be seen

Function of organs can be evaluated as well as appearance

No ionising radiation

What are the advantages of CT?

Can be made in any plane

Can see different windows

What are the advantages of MRI?

Can be made in any plane

Essential for the spine

What are the advantages of scintigraphy?

Can examine function

What are the disadvantages of radiography?

Radiation risk

Only a 2D view

What are the disadvantages of fluoroscopy?

Biggest risk for radiation exposure

What are the disadvantages of ultrasound?

Requires training and practice

Can only go 10cm deep

What are the disadvantages of CT?

Can only do what can fit in it

Mostly specialists

What are the disadvantages of MRI?

Safety - any ferrous metallic object can become a projectile

ONLY specialists

What are the disadvantages of scintigraphy?

Uses gamma rays

What is the main advantage of cross-sectional imaging?

It simplifies complicated anatomy by eliminating superimposition of structures

What are the main advantages of MRI over CT?

Much better image of soft tissue structures

Best for brain and spine

What are the main advantages of CT over MRI?

Cheaper, quick

Good for all areas except the brain

What is a linear transducer used for?

Scanning things close to the surface (up to 4cm deep) e.g. horse tendons

What is a curvilinear transducer used for?

Scanning the abdomen

What is a phased array transducer used for?

Scanning the heart (echocardiography)

What are the key factors for taking good radiographs?

Sedation!!! (most important factor)

Proper positioning aids

List the four key factors for good radiology interpretation

Good quality radiographs

2 large monitors

Knowledge of normal anatomy

Systematic approach.

What does a left lateral view mean in radiography?

The left side of the patient is down

What is the dorsoventral (DV) view in radiography?

The patient is lying on their belly, and the X-ray hits the dorsal side first

What is the ventrodorsal (VD) view in radiography?

The patient is lying on their back, and the X-ray hits the ventral side first

What is the caudocranial view in radiography?

The X-ray hits the caudal surface and then the cranial surface of the limb

What is the craniocaudal view in radiography?

The X-ray hits the cranial surface and then the caudal surface of the limb

What should be done to prepare a patient for an abdominal radiograph?

The bladder should be emptied and the patient should be fasted for 12 hours

Why is a plan needed for the order of radiographs?

It saves time

When to take an x-ray for thorax?

Just before peak inspiration

When to take an x-ray for the abdomen?

Just after expiration as there is a pause, so there will be no movement blur

What is the effect of FFD (the distance between the x-ray machine and detector plate) on radiographs?

It affects exposure (greater if closer) and image detail (reduced if closer)

How many views should be taken for abdomen and thorax?

Three views: left lateral, right lateral, and ventrodorsal

What positioning aids are commonly used in radiography?

Foam, cotton wool, sandbags, tape

What do the left/right markers indicate on an abdomen/ thorax lateral radiograph?

Indicates what side is down

For example - R marker indicated right lateral radiograph (right side down)

What do the left/right markers indicate on an abdomen/ thorax VD radiograph?

Indicates what side of the patient

For example - L marker indicates that where the marker is is the left side of the patient

What do the left/right markers indicate on a limb radiograph?

Indicates whether it is the right or left limb

How should a limb marker be placed on a radiograph?

It must go on the lateral aspect of the limb

How are lateral radiographs viewed?

Cranial left and caudal right

How are VD/ DV radiographs viewed?

Left side of the patient on the right, right side of the patient on the left

How are CC radiographs viewed?

Lateral aspect of the limb (where the marker is) on the right side of the screen

List the 3 pitfalls in interpretation that are avoided by using the systematic approach

Satisfaction of search (stop after finding one lesion)

Distraction by obvious abnormalities (miss others)

Tunnel vision (preconceived idea of what will be found and stopping early)

What are roentgen signs?

Adjectives to describe abnormalities

List the roentgen signs

Size

Shape

Margins

Opacity

Number

Location

How are roentgen signs used in the systematic approach?

For each structure, ask if it is normal or abnormal

If abnormal, use roentgen signs for description

What is summation on radiographs?

When superimposed opacities add together, making overlapping structures more opaque

The structures are not in contact so their separate margins are seen

What is effacement on radiographs?

When 2 structures of the same opacity are in contact, their individual margins at the point of contact cannot be distinguished

What are the rules of effacement?

1. Same opacity

2. In contact

What is the purpose of contrast studies?

To visualise organs better by instilling a contrast agent

What are positive contrast agents?

Metal opacity - iohexol and barium (GI only) liquid

What are negative contrast agents?

Gas opacity - room air or carbon dioxide

What is the impact of collimating the beam in radiography?

It reduces the volume of tissue exposed and minimises scatter

List the 4 indications for contrast studies

Location

Rupture

Filling defect

Function

List the indication for a location contrast study and give an example

Locating an organ

Colon - pneumocolonogram

List the indication for a rupture contrast study and give an example

Is contrast leaking out? (extravasation)

Ureter - excretory urogram

List the indication for a filling defect contrast study and give an example

Is there something displacing the contrast?

Calculus in the urethra - urethrogram

List the indication for a function contrast study and give an example

Is the organ functioning normally?

Oesophagus function - oesophagram

Indications for a positive contrast cystogram

Bladder rupture

Bladder location if ultrasound not available

What contrast agent is used for a positive contrast cystogram?

Iohexol

How is iohexol instilled for a positive contrast cystogram?

Through a urinary catheter (Foley) with the balloon just inside the bladder

Indications for a positive contrast urethrogram

Filling defect causing an obstruction (calculus)

Urethra rupture

What contrast agent is used for a positive contrast urethrogram?

Iohexol

How is iohexol instilled for a positive contrast urethrogram?

Through a urinary catheter (Foley) with the balloon just inside the penis or vulva

Indications for an excretory urogram

Locate ectopic ureters

Rupture

Filling defects

What contrast agent is used for an excretory urogram?

Iohexol

How is iohexol instilled for an excretory urogram?

By intravenous injection

Indications for an upper gastrointestinal barium study

Function - the transit time of the barium

Filling defect - foreign body

What contrast agent is used for an upper gastrointestinal barium study?

Barium

How is barium instilled for an upper gastrointestinal study?

Through a stomach tube or diluted via a nasogastric tube

Indications for a pneumogastrogram

Filling defects - gastric foreign body

Location

What contrast agent is used for a pneumogastrogram?

Air

How is air instilled for a pneumogastrogram?

Stomach tube or nasogastric tube

Indications for a pneumocolonogram

Locate the colon

What contrast agent is used for a pneumocolonogram?

Air

How is air instilled for a pneumocolonogram?

Via a tube in the rectum

Indications for an oesophagram

Function of the oesophagus

Filling defect - oesophageal foreign body