0.4 CT Imaging Basics

Question 1

What is CT?

Answer 1

• computer tomography
• uses X-rays to produce cross sectional images (axial slices)
• CT uses rotating X-ray beams to measure the density of tissues, and cross-sections are reconstructed using the densities

Question 2

What is the structure of a CT machine?

Answer 2

• an X-ray source is positioned opposite a detector panel, within a rotating structure called a gantry
• the attenuation coefficients detected are then reconstructed to produce a cross-sectional image

Question 3

What is multislice CT?

Answer 3

• there are multiple rows of detectors, allowing the possibility of acquiring multiple axial slices at a time
• better spatial and temporal resolution with lower radiation dose

Question 4

What is a helical CT scanner?

Answer 4

• scan is conducted in a helical direction along to examine multiple regions
• takes only a few seconds

Question 5

What is bright on CT?

Answer 5

• hyperdense structures
• dense structures (ie. metal, bone) block large number of X-rays and thus appear brighter

Question 6

What is dark on CT?

Answer 6

• hypodense structures
• regions with lower density (ie. air, fat) block less X-rays and appear darker

Question 7

What are Hounsfield-units?

Answer 7

• HU is the unit used to display attenuation coefficient relative to the density of water (0 HU)
• different densities are displayed in various shades of grey

Question 8

What is windowing in CT?

Answer 8

• it is a post-processing used to display optimal contrast for the eyes
• different window settings are used according to the tissues examined
• level is the center density of the window chosen
• width is the number of HU values within the chosen window

Question 9

What is the soft tissue window in CT?

Answer 9

L: 50
W: 400

Question 10

What is the bone window in CT?

Answer 10

L: 400
W: 1800

Question 11

What is the lung window in CT?

Answer 11

L: -600
W: 1500

Question 12

What are some possible artifacts in CT?

Answer 12

• patient-based artifact: motion artifact (more common in MRI than CT due to longer scanning time)
• physics-based artifact: beam hardening (caused by very dense objects like prosthetics or bone that filters low-energy X-ray photons)

Question 13

What are the CT contrast agents administered gastrointestinally?

Answer 13

• iodine based (ie. gastrograffin)
• water or Macrogol (hypodense compared to bowel, useful in bowel ischemia detection)

Question 14

When is gastrograffin used as a contrast agent?

Answer 14

• postsurgical bowel leakage
• GI fistulae
• suspicion of fluid collection or abscesses

Question 15

How are CT iodinated contrast agents administered gastrointestinally?

Answer 15

• per os (most common)
• per rectum
• via an enteric catheter, fistula or stoma

Question 16

When are intravenous CT contrast agents used?

Answer 16

• CT angiography
• study of vascularization
• tumor differentiation, lymphadenopathy
• inflammatory processes, abcesses
• pleural diseases

Question 17

When are intravenous CT contrast agents NOT used?

Answer 17

• foreign body visualization
• kidney stone search
• calcium scoring of coronary arteries
• lung HRCT or interstitial lung diseases
• rule out hemorrhage in stroke
• adrenal gland adenoma
• pneumothorax/pneumomediastinum
• free abdominal air/fluid
• sclerotic bone

Question 18

What is dynamic CT?

Answer 18

• typically done in the upper abdomen
• repetitive scannings after iv. contrast administration (dynamic CT/MRI)
• allows differentiation of common pathologies with different vascular characteristics

Question 19

What are the phases of contrast enhancement in dynamic CT?

Answer 19

0. non-contrast phase
1. early arterial phase
2. late arterial phase
3. hepatic phase
4. nephrogenic phase
5. delayed phase

Question 20

What is the non-contrast phase in dynamic CT?

Answer 20

• non-enhanced CT
• serves as a reference image to determine possible contrast enhancement

Question 21

What is the early arterial phase in dynamic CT?

Answer 21

• immediately after bolus tracking
• demarcation of vessels
• detection of: aortic dissection, arterial bleeding, arterial aneurysm, arterial anatomical variations, arterial stenosis, kinking, plaque

Question 22

What is the late arterial phase in dynamic CT?

Answer 22

• 15-20 seconds after bolus tracking
• enhancement of hypervascular lesions, stomach, bowel, pancreas parenchyma, and renal cortex

Question 23

What is the hepatic phase in dynamic CT?

Answer 23

• 50-60 seconds after bolus tracking
• aka. late portal phase
• enhancement of hepatic and splenic parenchyma

Question 24

What is the nephrogenic (venous) phase in dynamic CT?

Answer 24

• 70-80 seconds after bolus tracking
• enhancement of renal parenchyma, including the medulla

Question 25

What is the excretory phase in dynamic CT?

Answer 25

• 5-15 minutes after bolus tracking
• enhancement of fibrotic lesions, kidney and urinary collecting system

Question 26

What is post-contrast acute kidney injury?

Answer 26

• renal adverse reaction because iodine-based iv. contrast material can be nephrotoxic
• acute kidney injury that develops after iv. contrast administration
• in emergency situations, GFR is not measured before the CT and should be monitored after. An alternative imaging method (CEUS, MR) can be considered not using iodine-based contrast media.

Question 27

What increases the risk of post-contrast AKI?

Answer 27

• GFR < 30mL/min/1.73m2
• impaired kidney function
• multiple contrast administration in 48-72hours
• intraarterial suprarenal adminitration
• iodine is not contraindicated when GFR?30
• if GFR<30 then it should only be administered when absolutely necessary

Question 28

What are non-renal adverse effects of iv. iodine contrast agent?

Answer 28

• acute: mild urticaria to anaphylactic shock, risk of lactate acidosis
• late: skin reactions, headache
• very late: risk of thyrotoxicosis (untreated hyperthyreosis)
• miscellanous: can be given while pregnant if thyroid function of baby on mom is normal; breast feeding can continue; patients entering into radioactive iodine therapy should not have gotten iv. iodine contrast agent for at least 2 months

allergic reactions to iodine antiseptics is not the same as to iodince contrast agents

Question 29

What is ALARA?

Answer 29

• as low as reasonably achievable
• a standard CT scan has an effective dose of about 5-20mSV (sieverts), which is 100x-800x more than a PA chest radiograph
• ALARA principle aims to reduce ionizing radiation dose while maintaining image quality
• based on the clinical features appropriate imaging should be ordered, and once CT is justified the optimal settings should be considered

Question 30

What is the patient preparation related to the ionizing radiation?

Answer 30

• removal of jewelry/metal objects that might create CT image artefacts
• exclusion of pregnancy
• sedatives for non-cooperative patients or children

Question 31

What is the patient preparation related to iv. iodine based contrast agent?

Answer 31

• eGFR testing
• check for history of allergic reaction
• 3-4 hour fasting is recommended but not mandatory
• exclusion of untreated hyperthyreosis
• cessation of metformin 48h before
• absolute contraindication if there is documented previous severe allergic reaction

Question 32

What are the advantages of CT?

Answer 32

• 3D imaging
• tissues are not superimposed, not magnified
• good spatial resolution
• good soft tissue resolution (primarily after iv. contrast)
• usually available
• rapid imaging
• lung tissue visualization
• imaging of calcified plaques, kidney stones, foreign bodies, bone fragments

Question 33

What are the disadvantages of CT?

Answer 33

• high ionizing radiation
• high cost of equipment
• imaging primarily in axial plane
• soft tissue resolution is inferior to MRI’s
• bony artefacts
• contraindications: contrast allergy, kidney failure, pregnancy, children