CT Scans - What Are They and How Do I Understand Them?

Question 1

What is CT?

Answer 1

Radiographic technique for producing cross-sectional images by scanning slice of tissue from multiple directions using fan x-ray beam

Question 2

What is a voxel?

Answer 2

Small volume of tissue

Question 3

Define increased attenuation or hyperdensity

Answer 3

Whiter - more x-rays absorbed

Question 4

Define decreased attenuation or hypodensity

Answer 4

Darker - fewer x-rays absorbed

Question 5

What is the convention of displaying axial scans?

Answer 5

Looking at patient from foot of bed

Question 6

What is the convention of displaying coronal scans?

Answer 6

Looking at patient from front

Question 7

What changes how the image looks?

Answer 7

Window level and window width
Use of intravascular contrast and timing of scan
Use of other contrast
Reformats

Question 8

What is windowing?

Answer 8

Selecting range of CT densities for display to match tissue of interest

Question 9

What are commonly used contrast agents?

Answer 9

Intravascular
- Used for most body regions except
   - Most skeletal studies
   - Diffuse lung disease studies
   - Renal tract calculi
Oral/GI
- Mostly abdominal studies > to opacify stomach and bowel
Rectal
- For some studies

Question 10

How is CT colonography performed?

Answer 10

Colon distended with air/CO2 via rectal tube after colon cleansing

Question 11

How do you recognise the contrast that has been given?

Answer 11

Labelling of image may include “contrast”/”C+”/similar annotation
Blood vessels more dense than muscle/brain
Kidney more dense than muscle
- On delayed scans, pelvicalyceal systems/ureters/bladder dense

Question 12

What are non-contrast scans?

Answer 12

Scans without intravascular contrast

Question 13

What are contrast-enhanced scans?

Answer 13

Scans with intravascular contrast

Question 14

What determines what will appear opacified after commencement of intravenous contrast injection?

Answer 14

Timing of scan acquisition

Question 15

When are arterial phase scans taken?

Answer 15

Early

Question 16

When are venous phase scans taken?

Answer 16

Slightly later

Question 17

Where is the contrast in late phase scans?

Answer 17

Moving into extravascular spaces and being excreted by kidneys

Question 18

What is the commonest reformat of CT information?

Answer 18

Different planes of display

Question 19

How do urinary tract stones appear on CT compared to plain radiographs?

Answer 19

Dense on CT even though minority not dense enough to be visible on plain radiographs

Question 20

What is CT angiography (CTA) used for?

Answer 20

Show vascular anatomy

Question 21

How is CTA performed?

Answer 21

CT slices made after rapid IV injection of large bolus of contrast

Question 22

What are the applications of CTA?

Answer 22

Assessment of

• Arterial stenoses and aneurysms
• Arterial injuries/active bleeding
• Aortic dissection
• Pulmonary embolism

Question 23

How are biliary contrast agents excreted?

Answer 23

By liver, provided liver function normal

Question 24

What does CT-IV cholangiography (CT-IVC) allow you to visualise?

Answer 24

Bile ducts
Stones in ducts
Duct strictures
Bile leaks

Question 25

What is an example of a biliary contrast agent?

Answer 25

Ipodate

Question 26

What are the advantages of CT?

Answer 26

Contrast resolution 500x better than conventional radiography
Relatively non-invasive and quick
Images not degraded by
- Bowel gas
- Obesity
Imaging in transverse and other planes

Question 27

What are the disadvantages of CT?

Answer 27

Relatively high radiation dose
Artifacts can totally degrade images
- Patient motion
- Metal objects
Adverse intravascular contrast effects
Higher costs compared to some imaging modalities; eg: ultrasound