Agus Priatna

Impact of MR-Based Attenuation Correction on Neurologic PET Studies

Hybrid PET and MR scanners have become a reality in recent years, with the benefits of reduced radiation exposure, reduction of imaging time, and potential advantages in quantification. Appropriate attenuation correction remains a challenge. Biases in PET activity measurements were demonstrated using the current MR-based attenuation-correction technique. We aimed to investigate the impact of using a standard MR-based attenuation correction technique on the clinical and research utility of a PET/MR hybrid scanner for amyloid imaging. Methods: Florbetapir scans were obtained for 40 participants on a hybrid scanner with simultaneous MR acquisition. PET images were reconstructed using both MR- and CT-derived attenuation maps. Quantitative analysis was performed for both datasets to assess the impact of MR-based attenuation correction to absolute PET activity measurements as well as target-to-reference ratio (SUVR). Clinical assessment was also performed by a nuclear medicine physician to determine amyloid status based on the criteria in the Food and Drug Administration prescribing information for florbetapir. Results: MR-based attenuation correction led to underestimation of PET activity for most parts of the brain, with a small overestimation for deep brain regions. There was also an overestimation of SUVRs with cerebellar reference. SUVR measurements obtained from the 2 attenuation-correction methods were strongly correlated. Clinical assessment of amyloid status resulted in identical classification as positive or negative regardless of the attenuation-correction methods. Conclusion: MR-based attenuation correction causes biases in quantitative measurements. The biases may be accounted for by a linear model, although the spatial variation cannot be easily modeled. The quantitative differences, however, did not affect clinical assessment as positive or negative.

Feasibility of MRI attenuation correction in cardiac FDG-PET

Background Simultaneous acquisition PET-MRI is a new technology that has the potential to significantly impact diagnostic patient care. Cardiac imaging using PET-MRI offers high signal resolution MRI images superimposed on PET metabolic functional assessment. Specifically, 18Ffluorodeoxyglucose (FDG) PET-MR has the potential to provide both anatomic scar tissue evaluation and information regarding myocardial glucose metabolism. While early brain and soft tissue data have demonstrated that PET specific uptake values (SUVs) obtained using MRI for attenuation correction (AC) are comparable to SUVs obtained using CT AC, SUV measurements of myocardial tissue have not been compared. The objective of this pilot study is to determine the reproducibility of SUVs obtained by PET imaging using an AC µ-map comprised of a dual echo VIBE Dixon MRI sequence instead of CT. Methods

Demonstration of intermittent ischemia and stunning in hibernating myocardium

Intermittent ischemia and stunning appear to characterize hibernating myocardium. In this report, we demonstrate the variable flow abnormalities that may underlie these phenomena. A 72-year-old woman underwent a rest/vasodilator stress ECG-gated Tc-SPECT myocardial perfusion imaging (MPI). Image interpretation was consistent with a large anterior/anteroapical infarction with borderzone ischemia (Figure 1). Three days later, repeat rest/vasodilator MPI using N-ammonia and late gadolinium enhancement imaging (LGE) was performed simultaneously on a PET/MR (Biograph mMR, Siemens) as part of a research protocol to compare the accuracy of ischemia detection between SPECT and PET/MR. PET/MR MPI revealed extensive and severe anterior/anteroapical ischemia without infarction with resting and post-stress anteroapical hypokinesis (Figure 2A, B; Supplementary Movies 13). LGE PET/MR images showed no evidence of infarction in the area of ischemia detected by PET/MR (Figure 2C-E). The constellation of these findings allowed us to speculate that there was myocardial hibernation. Of note, absolute MBF calculated from the PET data demonstrated near similar resting flow values between the hibernating and normal myocardium with a paradoxical reduction in flow in the hibernating territory at stress, indicative coronary steal (Figure 3). Subsequent coronary angiography confirmed high grade LAD disease (Figure 4). This case provides evidence of the various types of flow abnormalities that characterize myocardial hibernation. They include resting hypoperfusion, impaired vasodilator capacity, and coronary steal. These flow abnormalities, either individually or collectively, can result in myocardial ischemia. However, their intermittent nature will lead to myocardial stunning (normal resting perfusion with abnormal resting wall motion). The finding of variable resting flow values based on the differences in the relative SPECT and PET/MR MPI studies is particularly noteworthy as it may impact the accuracy of SPECT and PET viability studies that are based on relative flow assessment.

Feasibility of detecting myocardial ischemia using first-pass contrast MRI and regadenoson

Summary A single injection of regadenoson can be used instead of an adenosine infusion to produce coronary vasodilatation and demonstrate myocardial ischemia during firstpass perfusion cardiac MRI. Background Cardiac stress MR perfusion imaging requires an MRI compatible infusion pump for the administration of adenosine or a non-MRI compatible pump housed in the control room or beyond the 10-Gauss line. Regadenoson is a recently FDA-approved A2A receptor agonist that can be given intravenously in a single bolus. It has been shown to provide diagnostic information regarding myocardial ischemia on SPECT-MPI. Methods