Myocardial Perfusion Imaging – Recent Advances and Applications
between 1997 and 2007 by Nandalur showed the sensitivity and specificity of PET as a diagnostic test for CAD to be 92 and 85%, respectively.48
Other studies have compared PET and SPECT head
to head with PET, showing higher sensitivity and specificity and reduced attenuation artefacts with PET.49
Sampson et al. also
demonstrated high diagnostic accuracy with PET/CT hybrid imaging.50
hyperaemia, and the LVEF reserve can be used to exclude left main or three-vessel CAD non-invasively.51
Risk stratification with PET
Other advantages of PET include the ability to quantify MBF to diagnose coronary microvascular dysfunction.52 Lastly, PET imaging can be used to assess myocardial viability by using 18F-fluorodeoxyglucose (FDG) to determine areas of myocardium capable of glucose metabolism, thus directing the need for revascularisation. Combined with PET perfusion imaging, D’Egidio et al. have shown that in patients with LVEF <35% and CAD who are being considered for revascularisation, revascularisation is superior to medical therapy in terms of cardiac death, MI and cardiac repeat hospital stay at one year when the mismatch between perfusion defect and FDG defect is >7%.53
perfusion imaging has emerged as a useful tool. Yoshinaga et al. have shown increased cardiac annual event rates in those with higher SSS and low LVEF. The annual hard event rate was 0.4, 2.3 and 7.0% in the normal, mild and moderate to severe groups, respectively.7
Cardiac Magnetic Resonance Perfusion Imaging
CMRI has the ability to assess MBF non-invasively and without radiation exposure by recording signal intensity over time characteristics of gadolinium, a paramagnetic contrast agent that
shortens T1 (longitudinal relaxation time reflecting the rate at which the tissue’s proton alignment recovers after application of radiofrequency pulse). First-pass imaging using CMRI was developed in 1990 for evaluation of perfusion in which images are acquired in pre-selected planes or slices as gadolinium traverses the vasculature and into the myocardium.54 signal increase reflects MBF.55 imaging56
The bright signal generated and the rate of Use of techniques such as parallel
allows multiple slices of data to be obtained through one
pass of gadolinium. Additionally, endocardial artefacts can be minimised with improvement in temporal resolution.57
It has been
shown that CMRI can differentiate haemodynamically relevant and non-relevant coronary stenoses better than invasive angiography and
FFR.58,59
A recent multicentre trial suggested that CMRI is a valuable alternative to SPECT for detecting CAD.61 gadolinium enhancement infarction imaging, performance of CMRI can be improved.62
Others have suggested that CMRI may be sensitive enough to detect 50% coronary stenoses by quantifying myocardial flow reserve.60
With the use of late the diagnostic
CMRI has been shown to
have sensitivity of 88%, specificity of 90% and accuracy of 89%.63 More importantly, prognostic studies with stress CMRI are consistent with the established literature for SPECT and PET in predicting cardiac events.9,64
Conventional CMRI uses a 1.5 Tesla (1.5T) magnet; however, images can be suboptimal due to the need for rapid acquisition. It has been suggested that contrast enhancement in CMRI can be improved using a stronger magnet at 3T, which results in improved image quality.65
In pig models using labelled microspheres as the
gold standard for MBF, Christian et al. recently showed that measurement of absolute MBF with first-pass CMRI is accurate at both 1.5T and 3T (3T: r=0.98, p<0.0001; 1.5T: r=0.95, p<0.0001).
EUROPEAN CARDIOLOGY
Summary
The recent advances and developments in MPI provide clinicians with multimodality tools for evaluating patients with suspected
35
In addition, PET can assess wall motion at peak
With the increase in popularity and application of CMRI, the potential complication of nephrogenic systemic fibrosis (NSF) with the use of gadolinium in patients with advanced kidney disease must be considered. NSF (first identified in 1997; first published report of 14 cases in 2000)67
is a highly debilitating and potentially life-threatening
condition characterised by progressive fibrosis involving the skin, pleura, lungs, joints, pericardium and muscle.68
NSF has only been
described in patients on dialysis or with a glomerular filtration rate (GFR) <15ml/min/1.73m2;69
therefore, the use of gadolinium should be
avoided in patients with a GFR <30ml/min/1.73m2, regardless of age, race or sex.70
When combined with vasodilator stress testing in humans, it is possible to show a lower perfusion reserve in areas with coronary stenosis.73,74
increase in MBF results in a relatively small increase of signal, thereby reducing its sensitivity compared with gadolinium-based CMRI.
Cardiac Computed Tomography
Initial cardiac CT with ECG gating used electron-beam CT (EBCT) to calculate the calcium score of coronary arteries to stratify the risk for coronary atherosclerosis.75
However, EBCT does not provide
ventricular functional information such as can be obtained with other imaging modalities. Contrast-enhanced multidetector computed tomography (CE-MDCT) is an emerging technology that examines myocardial perfusion. Using first-pass imaging (adenosine-augmented CE-MDCT), George et al. were able to demonstrate coronary flow deficit during adenosine stress in a canine model. The regional myocardial signal density showed a linear relationship compared with microsphere-derived MBF up to 8ml/g/minute.76
Furthermore,
subendocardial hypoperfusion at systole and normal perfusion at diastole seen in CE-MDCT has been suggested to be characteristic of ischaemic myocardium.77
Volumetric quantification of myocardial
perfusion using 3D data sets from MDCT allows for accurate detection of perfusion defects compared with MPS.78
Recently, Okada et al.
In addition to CTP, a comprehensive study with CTA for visualisation of coronary anatomy and CT delayed enhancement to evaluate for infarction and necrosis (similar to late gadolinium enhancement in CMRI)80
minutes later, with a similar total radiation dose to SPECT.81 have been studies on the prognostic value of CTA,82–86
can be obtained as a second scan a few While there
the prognostic
value and diagnostic accuracy of CTP are unclear, despite recent advances. Future clinical trials are needed for risk stratification to optimise patient outcome.
showed comparable detection, extent and severity of perfusion defects at rest and during stress between CT perfusion (CTP) and SPECT.79
Perfusion imaging with CMRI can be achieved without the use of exogenous contrast agents. Using water as a freely diffusible tracer and a technique called arterial spin labelling (ASL), blood flowing into a desired image slice is magnetically labelled. As the blood exchanges with tissue water and thereby changes tissue magnetisation, a change in signal is detected and a perfusion map can be calculated.71 Quantification of MBF using ASL has been shown to be possible in rabbits.72
However, an important limitation of ASL is that a large
However, signal quality is better at 3T, with a narrower 95% confidence interval.66
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