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MYOCARDIAL PERFUSION IMAGING: PRINCIPLES AND METHODS - Physics Just the Basics Webinar - Q&A

Posted By lauren small, Wednesday, July 8, 2020

This webinar took place live on June 24, 2020 and was presented by Dr. Peter Kellman and moderated by Dr. Marcus Carlsson. Over 400 individuals attended the webinar live and asked over 30 questions! Click into the comments area of this blog post to see written responses by Dr. Kellman and Dr. Carlsson to each question asked live. 

Have more questions after watching the webinar? Ask them in this thread.

To watch the webinar recording visit: http://scmr.peachnewmedia.com/store/seminar/seminar.php?seminar=159082.

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lauren small says...
Posted Wednesday, July 8, 2020
Questions & Answers

Q: Why is stress imaging done before rest? What is the interobserver reliability in post-processing of perfusion sequences (calculation of MPR)?
A: Stress imaging is almost always performed first before rest, and sometimes rest may not be performed at all. As mentioned during the webinar, by performing stress first, one maximizes the likelihood of seeing a perfusion defect because the myocardium has not previously been exposed to contrast. The question during the Webinar was whether it was technically possible to perform rest first. The answer to that is yes provided the sequence and processing are designed appropriately do get reliable estimates of the Gd concentration even when the initial Gd concentration is non-zero.

Q: Can you elaborate more about the reasons for dark rim artifacts and strategies to mitigate it?
A: There are 2 main sources of dark rim artifacts: 1) Gibb's ringing which may be mitigated by raw filtering with properly designed raw filter (recommend a truncated Gaussian with truncation approx 1.5 sd) and 2) cardiac motion during the interval of imaging which is mitigated by designing the protocol to maintain an imaging duration of 70 ms or less.

Q: When you give Adenosine for stress imaging, which criteria you use to judge if the effect of Adenosine is maximal enough to have a good stress image?
A: This is a difficult question open to some debate. It is general based on a HR rise of at least 10 bpm or a significant blood pressure drop and patients experiencing symptoms such as pressure on the chest, and it often checked whether there is a splenic "switch-off" when using adenosine (not regadenoson) although the adequacy of the vasodilator is often difficult to judge and some patients may not be sensitive to adenosine. This is one benefit of using quantitative perfusion maps, in which one may see a 2-3-fold increase in myocardial blood flow with vasodilation (up to 4-5-fold in healthy young individuals) compared to rest.


Q: Could you expand a little bit on the optimal saturation delay time? You mentioned 300ms to be optimal, you meant at stress or at rest? Is there any reference for that?
A: I was attempting to illustrate the trade-off between SNR and CNR for various saturation delays and for the specific example of contrast between 2 gd concentrations (0.2 and 1 mmol), the optimum CNR was at 300 ms with a fairly broad range. However, 1) this is only one choice of concentration for example, 2) did not consider linearity which is also important and is worse at longer saturation delays. I would not consider 300ms to be an optimal choice given all of the other factors. The presentation was meant to illustrate how the various protocol parameters are often competing. A good choice of saturation delay as measured to k-space center would be 100 -110 ms.

Q: Is the motion correction feature available on PHILIPS Ingenia scanners?
A: I do not have experience with this scanner and recommend contacting Philips for specific information

Q: What is the ideal slice thickness for perfusion imaging using SSFP or FLASH?
A: A slice thickness of 8 or 10 mm is typically used for either protocol but I am not sure if I would refer to it as ideal but rather a trade-off between SNR and potential resolution blur

Q: Does it matter how you measure the AIF? Location of the ROI in the left ventricle (base vs. apex)? Size of the ROI?
A: Arterial input function (AIF) ROI selection is typically at the base to ensure a large enough area, however additional processing is required when you are converting to Gd concentration units, and I suggest you study our reference: Kellman P, Hansen MS, Nielles-Vallespin S, Nickander J, Themudo R, Ugander M, Xue H. Myocardial perfusion cardiovascular magnetic resonance: optimized dual sequence and reconstruction for quantification. J. Cardiovasc. Magn. Reson.; 2017;19:43.

Q: Do you always do rest and stress perfusion? Or would be better to use all the contrast on stress perfusion, to increase the detection sensitivity (if we are not calculating and subtracting the stress and rest perfusion)?
A: There is still an active debate regarding the utility of rest perfusion and perfusion reserve as opposed to just using absolute stress flow. However, I would not recommend giving additional Gd dose at stress since it will potentially degrade the ability to do quantitative perfusion. I recommend a bolus of between 0.05 and 0.075 mmol/kg. If you are strictly doing a visual assessment, then perhaps a higher dose might be acceptable.


Q: When will your perfusion sequence / protocol be available to non-research Siemens sites?
A: I am not able to answer that question and suggest you ask your site representative

Q: How different is the stress perfusion protocol different from LGE imaging protocol?
A: These are distinctly different in that the perfusion is a dynamic imaging sequence used in conjunction with a bolus whereas LGE is acquired 10-20 min following Gd and is a single image

Q: What is AIF?
A: AIF stands for the arterial input function and represents the Gadolinium concentration vs time used for quantitative modeling

Q: Which parameters you use to see if the Adenosine effect is optimal to make a good stress image?
A: See the answer to Q4 above

Q: How accurate do you think MRI is today at quantifying myocardial perfusion in human patients?
A: The quantitative technique described in the webinar has been compared to quantitative perfusion with NH3-PET in patients. The two techniques showed good agreement between global myocardial perfusion both at stress (-0.1 ± 0.5 ml/min/g) and at rest (0 ± 0.2 ml/min/g) with a strong correlation (r = 0.92, p < 0.001; y = 0.94× + 0.14). Please see the reference Engblom et al. J Cardiovasc Magn Reson. 2017 Oct 19;19(1):78 for more details.

Q: How can an institution get myocardial flow mapping? Is this software upgrade widely available?
A: I am not able to answer that question and suggest you ask your site representative

Q: What is the advantage of stress CMR in ischemic heart disease in light of the results of the ISCHEMIA trial? Still room for CMR vs CT?
A: The clinical implications of the results of the ISCHEMIA trial are not yet clear. It will be interesting to see how and if guidelines are changed based on these findings. The findings that conservative medical treatment works just as well as an invasive strategy in patients with stable coronary artery disease does not answer the question whether CMR or CT should be used in patients with stable CAD. Based on the ESC guidelines stress CMR has an advantage in patients with higher likelihood of coronary artery disease and CT in patients with lower likelihood. The combined assessment of ventricular function, inducible ischemia and infarction with no ionizing radiation are the strengths of CMR.

Q: Which vendors offer myocardial flow quantification? Do parametric color-coded map images construct on the scanner or do you need postprocessing on a separate workstation via a commercial software package?
A: I am not able to answer that question and suggest you ask your site representative if your vendor has flow quantification.

Q: Is LUT to correct linearity required to be re-calibrated if scan parameters change?
A: Yes, the LUT correction must be calculated for the specific protocol. For our particular implementation, the LUT are calculated from the protocol on the fly for each scan. Kellman P, Hansen MS, Nielles-Vallespin S, Nickander J, Themudo R, Ugander M, Xue H. Myocardial perfusion cardiovascular magnetic resonance: optimized dual sequence and reconstruction for quantification. J. Cardiovasc. Magn. Reson.; 2017;19:43.

Q: Could you briefly explain the difference between pulse sequel vs Adiabatic saturation prep, and when to best use one vs the other?
A: Both techniques can be made to work, but pulse sequel approaches are preferable since they generally require a lower peak transmit B1 and may more diverse crusher gradients to avoid image artifacts.

Q: For 2RR acquisition (to obtain twice the slices), is AIF only acquire every other RR (and applied to all 6 slices) or every RR (and applied to only 3 slices)?
A: The AIF is acquired for every heartbeat

Q: Can you please repeat the name of the artifact on perfusion? subendocardial?
A: This artifact is often referred to in the literature as a dark rim artifact

Q: In the absence of quantitative perfusion technology, what are tips to detect dark rim artifact? Is it always present 360 degree circumference or can it be focal?
A: It is very difficult to be confident; these artifacts are not necessarily circumferential. Some users base true defects vs artifacts bases on persistence of the artifact in number of heart beats. These artifacts are mitigated by the sequence and reconstruction and not strictly by the quantification.


Q: Pit falls in a pediatric imaging particularly with a fast heart rate especially with post covid cardiac imaging will be pretty relevant.
A: With pediatric subjects, protocols are more challenging. We normally use 2 concatenations and acquire 2 slices on alternate beats for a total of 4 slices.

Q: Can you elaborate as to how to do perfusion in long axis views in addition to short axis view? We are currently only able to do 3 slices in short axis based on our current parameters.
A: The protocol we use acquires 3 slices at heart rates up to 120 bpm. For stress imaging where HRs may approach 120, we would acquire 6 slices with 2 concatenations where 3 slices are acquired on alternate beats. This requires good triggering and more computational power, and good saturation pulse design.

Q: What will be the expect ratio between the peak signal of 1st pass and the peak signal of 2nd pass in the AIF curve?
A: The ratio of signal strength or gd concentration between the peak of first pass bolus and the recirculation peak is highly variable depending on numerous physiological parameters. In some patients you will not see the recirculation and in others it is clearly detectable.

Q: What is the best imaging for perfusion and viability in a patient with acute MI of seven days old to decide on intervention?
A: Perfusion imaging soon after an acute MI with PCI can be challenging in the infarcted territory. The myocardium is in the healing phase and this may affect the perfusion assessment. Most often the clinical question regards other perfusion territories without acute infarction where a stenosis have been observed at the acute coronary angiography. For this a standard stress perfusion and LGE can be used to detect stress induced ischemia.

Q: There are several software’s that offer semi-quantitative methods of measuring myocardial perfusion using relative upslopes etc. What has been your experience with regards to the inter and intra observer variability with these software models (circle, medis etc.)
A: I do not have direct experience with semi quantitative methods

Q: Does using dobutamine change your approach to stress imaging?
A: We are using the same protocols for dobutamine stress perfusion as adenosine. Heart rate is often over 120 and then we use 2 concatenations and acquire 2 slices on alternate beats for a total of 4 slices. The cine portion of the dobutamine stress should be performed prior to the perfusion imaging.

Q: What sort of temporal resolution do you need for the AIF? What happens if it's too broad?
A: Temporal resolution has potentially different meanings. We sample every HB which is very important and keep the actual imaging duration to approx 40 ms to maintain a short saturation delay to remain in a regime where non-linearity may be corrected, and so that the entire AIG module does not take too much time which is needed for imaging the myocardial slices.

Q: Quantification of the defects..cut off to say that intervention will be beneficial
A: To quantify ischemia it is important to take both extent and severity of ischemia into account. Extent of ischemia can be done by calculating the number of segments affected from the 17-segment AHA model. In CMR we typically divide ischemia into transmural or subendocardial (above or below 50% of transmurality) for severity. Above 10% ischemic myocardium has been used to recommend interventional treatment and <5% medical treatment, 5-10% being the grey zone. For CMR 2 transmural segments would definitely be >10%.


Q: 1) Are you in favor of using ergometer or pharma would do the same job?
2) Could you shade the light on wideband sequences? Does the 3D LGE really bring additional value?
A: 1) There are advantages to ergometer as it is a physiological stress, takes away the need of an additional pharmaceutical and is not affected by caffeine in the body. However, there is the need to obtain expensive equipment (ergometer) and some patients cannot perform the exercise and high heart rates give less slices for perfusion. The time for the scan is in our experience in Lund shorter for a vasodilator. It is up to each site to determine which technique works best for them.

Q: What kind of segmental quantification model would you suggest? 17AHA segments or 32 or more?
A: I would recommend a minimum of 16 sectors subdivided endo/epi for 32. More angular segments (maybe 2 or even 3x) useful to avoid issue of coronary territories of actual patient not aligning with standard segmentation as shown in recent paper by Nagel, et al.

Q: What is the expected ratio between the max AIF signal during first and second pass? This could serve as a quality check, to know whether T1 saturation or T2* distortion are present.
A: This is highly variable and would not be a reliable measure of AIF quality.


Q: What would be some practical tips regarding an optimal tradeoff between temporal and spatial resolution?
A: Tradeoffs may vary depending on your criteria. I have placed a premium on maintaining imaging duration <= 70 msec to mitigate dark rim artifacts and then using accelerated imaging to acquire adequate spatial resolution

Q: Can you shed a light on stress perfusion in CHD, anamolus coronary arteries?
A: For anomalous coronary arteries vasodilation may not be appropriate as the impairment of flow may be due to systemic high pressure obstructing an artery between the aorta and pulmonary trunc. In these cases ergometer stress may be a better choice.

Q: How to ensure that the correct ROI is selected for the colour maps?
A: This is done automatically in our approach and an image is shown that show where the AIF ROI is placed.

Q: What are the recommended resolution settings for first pass perfusion sequence in a 1.5T scanner? What are the commonly used 1 beat and 2 beat acquisitions HR ranges? Are real-time cine MR of the LV to be useful after Stress and rest perfusion?
A: The spatial resolution should preferably be around 2 mm or less. We use both 1 and 2 beats depending on the number of slices wanted. A cine soon after stress can show if there is disturbed contraction that is not seen at rest. This is a sign of ischemic stunning and increases the sensitivity of the technique. The need for this may be less when quantitative perfusion is used but this remains to be evaluated.
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