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Medical > CT - Chapter 4 > Flashcards

CT - Chapter 4 Flashcards

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USMLE® Step 1 flashcards
1
Q

What is the appropriate recommendation for optimization of CCTA?

  • Arm position
  • EKG leads
  • NTG dosing
  • Breath hold commands
A
  • Arm position –> over head
  • EKG leads –> leads are placed peripherally and not over the center of the chest
  • NTG dosing –> coronary vasodilation with SL NTG 0.4 - 0.8 mg
  • Breath hold commands –> slow breath hold commands
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2
Q

What is the resultant reduction in radiation dose to the patient when reducing scan length (z-axis) by 2 cm or more?

A

30%

  • reducing scan length from
    • 16 cm to 14 cm –> 33% radiation reduction
    • 16 cm to 12 cm –> 47% radiation reduction
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3
Q

Describe CAC with use of MDCT system of 320 detector rows

A

often do not use the maximum possible number of slices

  • full coverage not used to avoid cone beam artifacts if axial scanning ( e.g. prospective ECG-triggering) is used
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4
Q

Describe the relationship:

  • Retropsective ECG-gated CT
  • CAC with use of MDCT system of 320 detector rows
A
  • can use the full available detector coverage
  • cone beam artifacts can be eliminated with special helical reconstruction techniques
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5
Q

What are the radiation doses for the studies?

  • Retrospective ECG-gated CCTA
  • Prospective ECG-gated CCTA
  • CAC
  • Annual natural background radiation
A
  • Retrospective ECG-gated CCTA –> 12-14 mSv (max - 20 mSv)
  • Prospective ECG-gated CCTA –> 2-3 mSv
  • CAC –> < 1 mSv (0.9 mSv)
  • Annual natural background radiation –> 2.4 mSv
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6
Q

What are advantages of acquiring cardiac CT images using dual energy settings (“spectral” CT) vs. standard single energy acquisition?

A
  • better tissue characterization
  • reduction of beam hardening and other artifacts
  • improved image quality
    • particularly in the setting of severe coronary calcification
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7
Q

Describe the findings

A
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8
Q

What is the preferred protocol for delayed enhancement/viability?

  • 65 year old male
  • BMI 22 kg/m2
A

100 kVp is preferred over 120 kVp

  • iodine enhancement can be visualized better due to –>
  • better low contrast resolution with 100 kVp approach
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9
Q

What is the potential disadvantage of arrhythmia rejection software?

  • Bradycardia
  • PAC’s
A

Radiation dose may be substantially increased

  • software monitors R-R cycle length for a finite time prior to scanning
  • if R wave peak occurs sooner than the average (with an arrhythmia) –>
  • switch from prospective –> retrospective (continuous) scanning
  • saves image quality but at the expense of increased radiation
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10
Q

What scan mode is required for high-pitch CCTA?

A

Prospective-ECG synchronized helical scan mode

  • scan is triggered at 60% R-R interval and
  • all images acquired during systole within 0.3s
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11
Q

What are the reduction in radiation associated with these adjustments?

  • Prospective ECG Triggering
  • Decrease tube current to 300 mA
  • Decrease tube voltage to 100 kVp
  • Breast shielding
A
  • Prospective ECG Triggering –> 70-90%
  • Decrease tube voltage to 100 kVp –> 40-50%
  • Decrease tube current to 300 mA –> 30-50%
  • Breast shielding –> 50-60% (but may compromise image quality)​
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12
Q

What is the acceptable contrast iodine concentration that can be utilized for CCTA?

A

► 300 mg/mL

​

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13
Q

What are the key strategies for dose reduction in cardiac CT?

A
  • Should not be performed in patients with high CAC
    • diagnostic accuracy is attenuated
  • Individualize scan length to minimum needed to cover the area of interest
  • ECG-based tube current modulation should be applied in all patients with NSR
  • Peak tube voltage should be 100 kVp in non-obese patients ( < 85-90kg )
  • Prospectively ECG-triggered CT should be considered in patiets with a stable and low HR
    • < 65 bpm
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14
Q

What is the recommended filter in CAC aqcuisition?

A

medium-sharp reconstruction filter kernel without edge enhancement

  • provides moderate image noise in low-dose acquisition protocols
  • helps avoid overestimation of scores and artifacts in the pericardium
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15
Q

What does high-pitch CCTA at 100 kVp allow for?

  • Pitch
  • Scan time
  • Radiation dose
A
  • Pitch –> 3.4
  • Scan time –> 0.3s
  • Radiation dose –> 1 mSv
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16
Q

What is the recommendation for CT evaluation of the LAA in a patient already undergoing CT evaluation of the pulmoary veins for A-fib ablation?

A

Acquire a second set of images limited to the LA and LAA 45-60s after the first pass images

  • first scan may demonstrate filling defect:
    • poor mixing of contrast –> resolves
    • deprived LAA function
    • thrombus
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17
Q

What is one additional requirement when performing “triple rule-out?”

A

Larger contrast volume​

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18
Q

What parameter is used to determine contrast volume necessary when performing CCTA?

A

Scan length / time

  • typically 60-80 mL are injected
  • followed by saline bolus
19
Q

What clinical scenario requires a decreased flow rate of CCTA?

A

Triple rule-out protocol

  • simultaneous imaging of the coronary arteries, pulmonary arteries and thoracic aorta requires a longer “plateau” phase of the contrast bolus
  • decrease flow rate to 4 - 4.5 cc/s is suggested
20
Q

When should oral BB be administered prior to CCTA?

A

60 minutes

21
Q

What patient-related factor is the MOST difficult for which to compensate?

  • 64-slice multidetector cardiac CT study
A

Respiration during scan

  • can create numerous non-correctible imaging artifacts
22
Q

How can these patient-related factors be overcome or compensated for?

  • Obesity with large BMI
  • Rapid HR ( > 80 bpm)
  • Normal HR with PVC’s and bigeminy
A
  • Obesity with large BMI –>
    • higher mA with a smooth reconstruction kernel
  • Rapid HR ( > 80 bpm)
    • multisegmet reconstruction or
    • using a dual source scanner with a better temporal resolution
  • Normal HR with PVC’s and bigeminy
    • End-systolic reconstructions and ECG editing
23
Q

What is the appropriate peak tube voltage?

  • MDCT protocols
  • congenital heart evaluation
  • up to 2 years of age
A

70-80 kVp

24
Q

What CT protocols should be used for cardiac acquisition in a newborn with congenital heart disease to minimize radiation exposure?

A
  • Prospectively ECG-triggered or Non-gated helical acquisitions with:
  • Submillimeter collimation (necessary)
25
Q

Define circulation time

A
  • delay time between the injection of contrast in the venous circulation and the arrival of contrast in the arterial circulation
  • influenced by Cardiac Output
26
Q

Describe the test bolus method

A
  • initial administration of a small amount of contrast with a direct measuremet of circulation time
  • followed by administration of the remainder of the contrast bolus with a similarly timed acquisition
27
Q

Describe the bolus tracking method

A
  • serial low-dose scanning of the target area
  • initiation of the scan when HU exceeds a specific number
28
Q

Describe the topogram

A
  • “scout” acquisition of chest images as the first step to coronary angiogram
  • permits accurate positioning of the scan volume
  • typical scan parameters include:
    • peak tube voltage 80 -120 kVp
    • tube current 30 mA
29
Q

What phase of the cardiac cycle is best for visualization the RCA in CCTA?

A

Low HR –> 65-70% end-diastole

or

High HR –> 40% end-systole

30
Q

Describe key features of the bolus tracking method

  • Placement
  • CT attenuation
  • Delay
A
  • Placement
    • can be placed in the ascending or descending aorta
    • should be positioned far from SVC to avoid premature triggering from artifacts
  • CT attenuation –> 100-150 HU​
    • ​will depend on placement in the ascending or descending aorta
  • Delay
    • always minimum delay depending on placement
31
Q

What is the effect on contrast and imaging when contrast bolus is delivered to obese/heavier patients?

A

Lower peak enhancement

and

Similar delay in peak enhancement

32
Q

Describe the technical problem:

  • CCTA, bolus tracking method
  • ROI triggerring at 144 HU
A

Photon starvation

  • noisy, poor quality, concentric rings emanating from the center of the image
  • scan triggered early to excessive noise (as a result of obesity, body habitus)
33
Q

What are key differences in CT protocol when assessing the Pulmonary veins?

A
  • Shorter contrast delays
    • peak opacification in pulmonary veins, not the coronary arteries
  • Dedicated timing bolus or bolus tracker focused on the the LA
  • Longer scan length
    • to include portions of the upper pulmonary veins, and sometimes aortic arch

*****Flow rates are the same

34
Q

What is the most appropriate CCTA imaging protocol?

  • 76 year old male with chest pain
  • HR 80-85 bpm (despite BB)
  • 64-detector technology
A

Retrospectively ECG-gated CT without ECG based tube current modulation

  • most appropriate to acquire good image quality
  • higher detector technology would allow for dose reduction techniques
  • prospectively ECG-triggered CT is not a good option when 64-slice technology because acquisition time typically exceeds the quiescent phases in the cardiac cycle (end systole or mid-diastole)
  • retrosepectively ECG-gated with ECG-based tube-current modulation also no recommended at this HR
35
Q

What is the formula for contrast volume?

A

Contrast volume = scan duration x injection rate

60 mL = 12 s x 5 mL/s

36
Q

What are the major advantages of a saline chaser in CCTA?

A
  • Greater arterial enhancement
  • Reduction of streak artifacts resulting from contrast in the right side of the heart
  • Enables a reduction in contrast volume by 15-20%
  • “Tighter” contrast bolus
    • maintains high target contrast density for the duration of injection
37
Q

In regards to contrast, what is still necessary, even when using a saline chaser?

A

High contrast flow rates (5-7 cc/s)

  • ​helps to maintain a tight bolus with
  • preferential coronary arterial enhancement and
  • minimal coronary venous enhancement
38
Q

What is a potential advantage of a triphasic contrast injection protocol?

  • undiluted contrast –>
  • mixed contrast / saline –>
  • saline alone
A

Improved right heart opacification

  • also helps to avoid streak artifacts in the RCA
39
Q

What scan parameter must be adjusted to take advantage of the noise reduction capabilities of iterative reconstruction and achieve dose reduction?

A

Reducing tube current

40
Q

What is the best way to optimize image quality in this patient?

  • 55 year old obese woman
  • HR 52
A

Increase tube current

  • common method to decrease image noise
41
Q

What is a common pitch for a typical CCTA using retrospectively ECG-gated 64-slice CT?

A

0.2

  • for retrospectively ECG-gated CT
  • this pitch allows for scanning at HR > 40 bpm
42
Q

What intracoronary contrast enhancement allows the optimal diagnostic accuracy of CCTA?

A

350 HU

  • visualization of the coronary artery lumen benefits from a large attenuation gradient between the lumen and the surrounding tissue
43
Q

What image acquisition protocol would best optimize contrast in the RV?

  • assessment of RV morphology
  • ICD in place
A

Longer contrast injection

  • allows opacification of both the left and right ventricles
  • other options –> tri-phasic protocol
44
Q

For helical scanning, the concept of effective mAs was introduced.

How is effective mAs calculated?

A

tube current (mA) x gantry rotation time

pitch

  • Effective mAs = mAs / pitch

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