CT - Chapter 4 Flashcards
What is the appropriate recommendation for optimization of CCTA?
- Arm position
- EKG leads
- NTG dosing
- Breath hold commands
- 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
What is the resultant reduction in radiation dose to the patient when reducing scan length (z-axis) by 2 cm or more?
30%
- reducing scan length from
- 16 cm to 14 cm –> 33% radiation reduction
- 16 cm to 12 cm –> 47% radiation reduction
Describe CAC with use of MDCT system of 320 detector rows
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
Describe the relationship:
- Retropsective ECG-gated CT
- CAC with use of MDCT system of 320 detector rows
- can use the full available detector coverage
- cone beam artifacts can be eliminated with special helical reconstruction techniques
What are the radiation doses for the studies?
- Retrospective ECG-gated CCTA
- Prospective ECG-gated CCTA
- CAC
- Annual natural background radiation
- 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
What are advantages of acquiring cardiac CT images using dual energy settings (“spectral” CT) vs. standard single energy acquisition?
- better tissue characterization
- reduction of beam hardening and other artifacts
- improved image quality
- particularly in the setting of severe coronary calcification
Describe the findings
What is the preferred protocol for delayed enhancement/viability?
- 65 year old male
- BMI 22 kg/m2
100 kVp is preferred over 120 kVp
- iodine enhancement can be visualized better due to –>
- better low contrast resolution with 100 kVp approach
What is the potential disadvantage of arrhythmia rejection software?
- Bradycardia
- PAC’s
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
What scan mode is required for high-pitch CCTA?
Prospective-ECG synchronized helical scan mode
- scan is triggered at 60% R-R interval and
- all images acquired during systole within 0.3s
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
- 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)
What is the acceptable contrast iodine concentration that can be utilized for CCTA?
► 300 mg/mL
What are the key strategies for dose reduction in cardiac CT?
- 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
What is the recommended filter in CAC aqcuisition?
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
What does high-pitch CCTA at 100 kVp allow for?
- Pitch
- Scan time
- Radiation dose
- Pitch –> 3.4
- Scan time –> 0.3s
- Radiation dose –> 1 mSv
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?
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
What is one additional requirement when performing “triple rule-out?”
Larger contrast volume
What parameter is used to determine contrast volume necessary when performing CCTA?
Scan length / time
- typically 60-80 mL are injected
- followed by saline bolus
What clinical scenario requires a decreased flow rate of CCTA?
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
When should oral BB be administered prior to CCTA?
60 minutes
What patient-related factor is the MOST difficult for which to compensate?
- 64-slice multidetector cardiac CT study
Respiration during scan
- can create numerous non-correctible imaging artifacts
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
- 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
What is the appropriate peak tube voltage?
- MDCT protocols
- congenital heart evaluation
- up to 2 years of age
70-80 kVp
What CT protocols should be used for cardiac acquisition in a newborn with congenital heart disease to minimize radiation exposure?
- Prospectively ECG-triggered or Non-gated helical acquisitions with:
- Submillimeter collimation (necessary)
Define circulation time
- delay time between the injection of contrast in the venous circulation and the arrival of contrast in the arterial circulation
- influenced by Cardiac Output
Describe the test bolus method
- 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
Describe the bolus tracking method
- serial low-dose scanning of the target area
- initiation of the scan when HU exceeds a specific number
Describe the topogram
- “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
What phase of the cardiac cycle is best for visualization the RCA in CCTA?
Low HR –> 65-70% end-diastole
or
High HR –> 40% end-systole
Describe key features of the bolus tracking method
- Placement
- CT attenuation
- Delay
- 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
What is the effect on contrast and imaging when contrast bolus is delivered to obese/heavier patients?
Lower peak enhancement
and
Similar delay in peak enhancement
Describe the technical problem:
- CCTA, bolus tracking method
- ROI triggerring at 144 HU
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)
What are key differences in CT protocol when assessing the Pulmonary veins?
- 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
What is the most appropriate CCTA imaging protocol?
- 76 year old male with chest pain
- HR 80-85 bpm (despite BB)
- 64-detector technology
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
What is the formula for contrast volume?
Contrast volume = scan duration x injection rate
60 mL = 12 s x 5 mL/s
What are the major advantages of a saline chaser in CCTA?
- 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
In regards to contrast, what is still necessary, even when using a saline chaser?
High contrast flow rates (5-7 cc/s)
- helps to maintain a tight bolus with
- preferential coronary arterial enhancement and
- minimal coronary venous enhancement
What is a potential advantage of a triphasic contrast injection protocol?
- undiluted contrast –>
- mixed contrast / saline –>
- saline alone
Improved right heart opacification
- also helps to avoid streak artifacts in the RCA
What scan parameter must be adjusted to take advantage of the noise reduction capabilities of iterative reconstruction and achieve dose reduction?
Reducing tube current
What is the best way to optimize image quality in this patient?
- 55 year old obese woman
- HR 52
Increase tube current
- common method to decrease image noise
What is a common pitch for a typical CCTA using retrospectively ECG-gated 64-slice CT?
0.2
- for retrospectively ECG-gated CT
- this pitch allows for scanning at HR > 40 bpm
What intracoronary contrast enhancement allows the optimal diagnostic accuracy of CCTA?
350 HU
- visualization of the coronary artery lumen benefits from a large attenuation gradient between the lumen and the surrounding tissue
What image acquisition protocol would best optimize contrast in the RV?
- assessment of RV morphology
- ICD in place
Longer contrast injection
- allows opacification of both the left and right ventricles
- other options –> tri-phasic protocol
For helical scanning, the concept of effective mAs was introduced.
How is effective mAs calculated?
tube current (mA) x gantry rotation time
pitch
- Effective mAs = mAs / pitch
