CT - Chapter 5

Question 1

What are thin-slab MIP most useful for evaluation of?

Answer 1

Smaller branches

• longer segments of smaller caliber vessels

Question 2

How many seconds will be required for a sequential (“step and shoot”) CT scan?

• collimation = 64 x 1 mm
• scan length = 22 cm
• HR = 80 bpm

Answer 2

5 seconds

• scan will require 4 acquisitions
  
  • 6.4 cm x 4 = 25.6cm (enough to cover 22 cm scan length)
• At HR 80 bpm:
  
  • data will be acquired every second heart cycle
  • to allow for the table to move to the next position –>
  • 7 heart cycles to complete the scan (1, 3, 5, 7)
• Each cycle takes:
  
  • 1 / (80 / 60) = 0.75 s
• 7 cycles x 0.75s = 5.25 s

Question 3

Describe maximum intensity projection (MIP)

Answer 3

• Pros:
  
  • takes a projection of a slab of images and projects onto the imaging plane those that have the highest density
  • Effect of enhancing borders between contrast-filled and non-contrast filled structures
• Cons:
  
  • enhances other high-density objects (metallic stent struts, calcified plaques) –> lumen visualization is difficult when present

Question 4

What is the main disadvantage of curved MPR images?

Answer 4

Potential for misinterpretation of stenosis resulting from an off-axis centerline placement

• off-axis centerline placement –> “pseudostenosis”

Question 5

What algorithm is most commonly used in reconstruction of CT images?

Answer 5

Filtered back-projection (FBP)

• iterative reconstruction holds promise as an improved method, but is not yet widely utilized

Question 6

At what portion of the cardiac cycle is there typically the least coronary motion?

Answer 6

Diastasis

• least cardiac, and hence coronary, motion during diastasis
• occurs during mid-to-late diastole, prior to atrial contraction

Question 7

What is the contrast to noise ratio (CNR) considered non-diagnostic for a CT examination?

Answer 7

« 1

• CNR is considered an image quality measure reflecting the detectability of the anatomy of interest relative to the noise of the image
• CNR:
  
  • « 1 –> non-diagnostic
  • 2-4 –> tolerable
  • ► 4 –> diagnostic

Question 8

What reconstruction increment (slice interval) and slice thickness, respectively, will result in isovolumetric voxel dimensions?

• FOV: 20 x 20 cm
• Matrix: 500 x 500

Answer 8

Voxel dimension (z-axis) = 0.4 mm; slice thickness is irrelevant

• FOV / Matrix = voxel size in z-axis dimension
• 200 mm / 500 = 0.4 mm

Question 9

What cardiac phase is least likely to show motion artifacts in the RCA?

• Retrospectively gated MDCT
• HR = 60 bpm

Answer 9

80%

• Low HR = diastasis = 80% RR interval
  
  • phase between early rapid filling and atrial contraction
• High HR = 40% RR interval

Question 10

What is the temporal resolution?

• 64-slice scanner
• Rotation time = 400 ms
• reconstructed using a bi-segmental reconstruction algorithm

Answer 10

100-200 ms

• single segment partial san algorithm –> 200 ms temporal resolution
• bisegmental reconstruction (combining data from two consecutive heart cycles) –> improves temporal resolution to 100-200 ms

Question 11

What is the difference in pitch?

• Single-segment reconstruction
• Multi-segment reconstruction

Answer 11

• Single-segment reconstruction –> Higher pitch
• Multi-segment reconstruction –> Lower pitch
  • ​to allow for sufficient overlap
  • to combine data from multiple cardiac cycles

**** Pitch does not apply to step and shoot imaging

Question 12

What is the main disadvantage of MPR?

Answer 12

• only short segments can be analyzed
  
  • due to the non-linear pathway of the coronary arteries
• should not be used alone in the grading of plaque stenosis
  
  • ​due to partial averaging effect

Question 13

Describe the findings and technique used to improve the quality of the image

Answer 13

• Misregistration artifact
  
  • ​caused by PAC / PVC
• ECG-editing and reconstruction​

Question 14

Where is the temporal resolution optimized within a CT dataset acquired in a cranio-caudal direction?

Answer 14

Circumferential center of the scan

• most effective at the center of the scan where the smallest rotation is sufficient to acquire the necessary projections for image reconstruction

Question 15

Which phase in the cardiac cycle is generally optimal for image reconstruction?

Answer 15

Mid-diastole (diastasis)

• rapid early passive and late active filling of the ventricle more likely cause artifacts than the mid-diastolic phase
• heart is relatively motionless during this phase

Question 16

Describe the findings:

Answer 16

Normal inflow pattern

• normal inflow pattern of contrast in the right heart acquired during consecutive heart cycles

Question 17

In which circumstance can multi-cycle reconstruction improve CT image quality?

Answer 17

HR > 80 bpm

• multi-cycle reconstruction improves temporal resolution and image quality by reducing acquisition time

Question 18

Describe features of sharp reconstruction kernels / filters

Answer 18

• used in the evaluation of highly calcified coronary arteries and stents
• increase:
  
  • spatial resolution
  • image noise
• decrease:
  
  • contrast resolution

*****no effect on radiation exposure