International Circulation:  Cardiac CT offers a better， cost-effective approach to diagnose heart diseases especially for the patients with chest pain. Do you think it is also useful for myocardial perfusion imaging?

Dr. Ruehm:  I think there is information about myocardial perfusion available which has been underutilized in the past.   Coronary CTA for is certainly able to provide information about  myocardial  perfusion in addition to the display of coronary artery morphology. 

International Circulation:  Compared to MRI， PET or SPECT， what is the major advantage and shortcoming for cardiac CT in myocardial perfusion imaging?

Dr. Ruehm:  Comparing these imaging modalities， the major advantage of coronary CTA is that it is currently the only non-invasive technology available which allows the combined  display of coronary artery anatomy， morphology as well as myocardial perfusion.  Modalities such as PET， SPECT or MRI typically provide secondary information which reflect potential stenotic disease of the epicardial oronary arteries.  At this point coronary CTA does not appear to be a first-line imaging modality for the assessment of coronary artery and myocardial perfusion per se.  It is a first-line modality for the assessment of coronary artery stenotic disease.  Alternative technologies like PET or MRI are limited or unable to display coronary artery anatomy.  Ideally， if an assessment of  myocardial perfusion is requested， you would also like to get additional information about coronary artery anatomy and stenotic diseaseTherein lies the unique advantage of  coronary CTA that you don’t have with competing modalities that typically don’t produce as decent of teh coronary artery anatomy or pathology.

International Circulation:  In your talk you mentioned taking repeated images during multiple cardiac cycles when performing coronary CTA.  Has any evidence been found that increased exposure to radiation during cardiac CT increases the risk of developing cancer? What is your opinion about it?

Dr. Ruehm:  Absolutely， increased radiation is associated with increased rates of cancer.  Therefore I would not recommend the use of repeated CT data acquisitions as part of a routine coronary CTA protocol. Serial myocardial imaging is mandatory for the quantification of myocardial blood flow in  in absolute numbers， It has been shown in the literature that this is feasible with CT.  In reality， however， it is rarely done and usually not required in clinical routine. As I mentioned in my presentation， you may not need to acquire multiple data sets for the assessment of  myocardial perfusion，  if you obtain the imaging data during an early arterial phase of contrast administration， as typically done in routine coronary CT angiography. As an advantage， the semiquantatitaive information on myocardial perfusion can nicely be correlated with morphologic information of the coronary arteries as part of a single imaging study
International Circulation:  What is the status of cardiac CT for myocardial perfusion imaging in clinic practice?

Dr. Ruehm:  Right now the perfusion information is underutilized.  People rarely look at it but it is available on the data sets.  I think we really need to increase the awareness that there is additional information available.  Currently it is not ordered as a first-line imaging modality for the assessment of myocardial perfusion.  The “gold standard” is currently PET， which doesn’t provide the whole picture. It allows the quantification of myocardial perfusion abnormalities but does not provide morphologic information of the coronary arteries itself. 

 This is a potential limitation.  For example， it has been shown that， from a predictive perspective，  the critical stenoses are not necessarily those that cause a high grade stenosis.  It appears that  stenoses which are less severe causing  less than 50% narrowing， are often the stenoses at higher risk of sudden occlusion of a vessel and  therefore causing acute myocardial infarction. More important appears the  composition of the plaque from a predictive point of view. . In this context a   vulnerable plaque or a plaque that is prone to rupture causing a sudden occlusion of the vessel cannot be identified with PET.  Whereas a PET scan can rule out myocardial perfusion abnormalities，  an area of decreased the patient can still be at high risk for coronary artery disease due to a vulnerable plaque.  This is information about the vessel wall is potentially available by coronary CTA.

International Circulation:  Besides awareness， are there other limitations with coronary CTA that is preventing it from replacing PET as the “gold standard”？

Dr. Ruehm:  Due to radiation concerns it appears not appropriate to  acquire CT data in a dynamic fashion， which means the acquisition of multiple CT data sets over time following the administration of intravenous contrast agent.  Therefore CT is limited in the quantification of myocardial perfusion expressed in absolute numbers. Nevertheless， perfusion defects can easily be depicted by visual inspection. They typically appear as subendocardial areas with decreased contrast uptake. A further potential disadvantage of cardiac CT is that the data is  typically not acquired during stressYou may remember a graph I showed as part of my  presentation. This graph illustrated that an 80~90% narrowing is typically required  for a stenosis to generate perfusion abnormalities visualized by traditional nuclear perfusion studies.  In order to detect stenotic disease of lower severity， Nuclear Medicine Imaging typically requires the acquisition of data at rest and stress.    Technically  CT imaging can be combined in combination with pharmacologically induced stress， similarly to Nuclear MedicineThe problem is that CT is often performed by a radiologist and radiologists are not always comfortable giving these stress agents. In addition when you consider doing a pharmacologic stress study in every patient refrred for coronary CTA， the test is likely to  lose this appeal of being a non-invasive imaging modality.  And it is  more cumbersome for the patient， for the technician， and for the radiologist to obtain all imaging studies with a combinedstress est.  Overall it  may not really be necessary in all patients.  It really depends on the indication for the study.   If it is just a screening technique and theer is no  high grade stenosis visualized the perfusion information obtained with pharmacological stress is usually not required.  However， if perfusion information at stress is requested as part of the coronary CTA， it can certainly be obtained.  However， currently the utiliza