CT

Question 1

major types of CT

Answer 1

• helical- most common

- conventional, axial, step and shoot (used for high resolution scans of lungs, coronary artery)

Question 2

CT scanner

Answer 2

• x-ray tube rotates around pt and generates xray beam
• gantry motion is helical during helical and circular during conventional
• detectors record radiation and rejects scattered radiation from outside tube

Question 3

CT collimation

Answer 3

• determines thickness of section
• standard is 2.5 mm for mediastinum and 1.25 for lung parenchyma
• wider than 2.5 used in heavy pts

Question 4

CT image

Answer 4

• thousands of tiny pixels
• each pixel is -1000 to 1000 HU
• number will vary based on tissue density

Question 5

what is HU for air

Answer 5

-1000 HU

Question 6

what is HU for water

Answer 6

0

Question 7

what is HU for fat

Answer 7

-400 to -100

Question 8

what is HU for soft tissue

Answer 8

20 to 100

Question 9

what is HU for bone

Answer 9

400 to 600

Question 10

what is HU for metal

Answer 10

> 1000

Question 11

denser substances on CT

Answer 11

• absorb more xrays
• have higher CT numbers
• demonstrate increased attenuation
• display as whiter

Question 12

less dense substances on CT

Answer 12

• absorb fewer xrays
• low CT number
• demonstrate decreased attenuation
• display as blacker

Question 13

attenuation

Answer 13

• chem make up of material determines how many xrays make it through to detector
• is the stopping of xrays

Question 14

cross sectional planes in CT

Answer 14

• axial
• coronal
• sagittal

Question 15

CT limitations

Answer 15

• size of pt (500 lb max)
• claustrophobia
• pt need to hold still
• pt needs to hold breath
• water soluble contrast used for perforations

Question 16

what are possible adverse reactions to CT contrast

Answer 16

• allergy

- contrast induced nephropathy

Question 17

contrast admin for CT

Answer 17

• IV
• Enteric (PO or rectal)
• all IV is iodine-based

Question 18

positive enteric contrast for CT

Answer 18

• high attenuation
• dilute suspension of barium sulfate solution of iodinated agent
• used for most indications

Question 19

negative enteric contrast for CT

Answer 19

• water attenuation
• used to delineate bowel mucosa
• used to improve detection of active inflammation/ GI bleed
• can use plain water but only distends stomach and duodenum due to absorption

Question 20

C/I for enteric contrast

Answer 20

• pt at risk of aspiration
• high grade bowel obstructions
• no barium based contrast in pts at risk of bowel perforation

Question 21

adverse/allergic reaction prevention

Answer 21

• corticosteriod +/- antihistamine
• usually admin PO over longer duration or IV over shorter duration
• NO cross reaction with gad for MRI