CT Scans - What Are They and How Do I Understand Them? Flashcards
What is CT?
Radiographic technique for producing cross-sectional images by scanning slice of tissue from multiple directions using fan x-ray beam
What is a voxel?
Small volume of tissue
Define increased attenuation or hyperdensity
Whiter - more x-rays absorbed
Define decreased attenuation or hypodensity
Darker - fewer x-rays absorbed
What is the convention of displaying axial scans?
Looking at patient from foot of bed
What is the convention of displaying coronal scans?
Looking at patient from front
What changes how the image looks?
Window level and window width
Use of intravascular contrast and timing of scan
Use of other contrast
Reformats
What is windowing?
Selecting range of CT densities for display to match tissue of interest
What are commonly used contrast agents?
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
How is CT colonography performed?
Colon distended with air/CO2 via rectal tube after colon cleansing
How do you recognise the contrast that has been given?
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
What are non-contrast scans?
Scans without intravascular contrast
What are contrast-enhanced scans?
Scans with intravascular contrast
What determines what will appear opacified after commencement of intravenous contrast injection?
Timing of scan acquisition
When are arterial phase scans taken?
Early
When are venous phase scans taken?
Slightly later
Where is the contrast in late phase scans?
Moving into extravascular spaces and being excreted by kidneys
What is the commonest reformat of CT information?
Different planes of display
How do urinary tract stones appear on CT compared to plain radiographs?
Dense on CT even though minority not dense enough to be visible on plain radiographs
What is CT angiography (CTA) used for?
Show vascular anatomy
How is CTA performed?
CT slices made after rapid IV injection of large bolus of contrast
What are the applications of CTA?
Assessment of
- Arterial stenoses and aneurysms
- Arterial injuries/active bleeding
- Aortic dissection
- Pulmonary embolism
How are biliary contrast agents excreted?
By liver, provided liver function normal
What does CT-IV cholangiography (CT-IVC) allow you to visualise?
Bile ducts
Stones in ducts
Duct strictures
Bile leaks
What is an example of a biliary contrast agent?
Ipodate
What are the advantages of CT?
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
What are the disadvantages of CT?
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
