PET/MRI: Where might it replace PET/CT?

Abstract

Simultaneous positron emission tomography and MRI (PET/MRI) is a technology that combines the anatomic and quantitative strengths of MR imaging with physiologic information obtained from PET. PET and computed tomography (PET/CT) performed in a single scanning session is an established technology already in widespread and accepted use worldwide. Given the higher cost and complexity of operating and interpreting the studies obtained on a PET/MRI system, there has been question as to which patients would benefit most from imaging with PET/MRI versus PET/CT. In this article, we compare PET/MRI with PET/CT, detail the applications for which PET/MRI has shown promise and discuss impediments to future adoption. It is our hope that future work will prove the benefit of PET/MRI to specific groups of patients, initially those in which PET/CT and MRI are already performed, leveraging simultaneity and allowing for greater degrees of multiparametric evaluation.

Level of evidence: 5 Technical Efficacy: Stage 5 J. Magn. Reson. Imaging 2017;46:1247-1262.

Keywords: CT; MRI; PET; PET/CT; PET/MRI.

FIGURE 1

A 67-year-old woman with known pancreas head mass (arrowhead) referred for PET/MRI to evaluate for metastatic disease. Whole body survey PET images, acquired over 2 min per table position, as well as contrast enhanced CT and MR images did not demonstrate any suspicious lesions within the liver. Dedicated PET acquisition in the liver bed position over 10 min clearly shows an FDG avid focus in the posterior hepatic lobe (arrows). This finding was confirmed to represent metastatic disease at the time of surgery.

FIGURE 2

MIP 68-GA DOTA-TOC PET images from a 71-year-old man. Uncompensated images (left) suffer from blurring along the Z-axis due to respiratory motion. Motion compensated (right) images generated by filtering temporal PET data according to MR respiratory gating show decreased respiratory artifact, improving image quality and accuracy of SUV observations.

FIGURE 3

A 69-year-old man with glioblastoma treated with surgical resection, chemoradiotherapy, and adjuvant temozolomide. Postgadolinium enhanced MRI at 6 months following completion of adjuvant therapy (left) and follow-up postgadolinium enhanced MRI at 9 months following completion of radiotherapy (center) demonstrate interval progression of enhancing and nonenhancing tumor. 18F-DOPA PET performed as part of PET/MRI at 9 months following completion of radiotherapy (right) demonstrates radiotracer uptake in regions of enhancing (arrow) and nonenhancing (arrowheads) tumor. Note physiologic putamenal uptake bilaterally.

FIGURE 4

FDG PET/MRI performed to further evaluate a tonsillar mass in a 58-year-old man. Note presence of a T2 hyperintense, enhancing lesion in the right tonsillar region (arrows), with corresponding FDG avidity. There was no FDG or MRI evidence of distant disease on whole body imaging.

FIGURE 5

PET/MRI performed for workup of a left lung mass noted in a 65-year-old woman demonstrates a large hypermetabolic left lung tumor (arrows) and extensive hepatic, adrenal, osseous, and peritoneal metastases. Subsequent biopsy of a liver mass confirmed metastatic adenocarcinoma of the lung.

FIGURE 6

A 52-year-old man with sigmoid colon cancer metastatic to the liver, status post segment V right partial hepatectomy with intra-operative ablation of a tumor in segment VIII. PET/MRI images following the procedure show two areas (arrows) of enhancement (top) and restricted diffusion (middle). These findings are quite subtle and could be interpreted as nonspecific, however, the addition of fusion FDG PET images (bottom) clearly demonstrates hypermetabolism, suggesting additional undiagnosed tumor burden.

FIGURE 7

PET/MRI images obtained in a 53-year-old woman with biopsy proven adenocarcinoma of the pancreatic body. Postcontrast T1-weighted images show a hypoenhancing tumor with abutment of the celiac axis and common hepatic arteries as well as encasement and narrowing of the splenic artery (arrowheads, top), while fused PET/T2 weighted and MIP PET images show FDG uptake attributable to the tumor (center, right). Concurrently performed MRCP demonstrates focal obliteration of the pancreatic duct in the region of the tumor, with upstream dilation (arrows, bottom).

FIGURE 8

A 71-year-old man with metastatic neuroendocrine tumor. PET/MRI exam performed with 68-Ga DOTA-TOC demonstrates intermediate T2 hyperintense lesions with mild restricted diffusion and lack of hepatocyte contrast uptake on 20-min postinjection T1-weighted images. Fused contrast enhanced/PET images demonstrate marked uptake of DOTA-TOC somatostatin analogue at the sites of MRI abnormality, compatible with metastatic disease. Note blurring of hepatic metastases on whole body MIP images, a result of uncorrected respiratory motion.

FIGURE 9

A 29-year-old woman with history of squamous carcinoma of the cervix status post radiotherapy with an enlarging potentially necrotic FDG avid pelvic mass seen on noncontrast PET/CT (left images). PET/MRI was obtained for better soft tissue delineation before an expected pelvic exenteration. PET/MRI images (center and right) more clearly demonstrate a rim enhancing cavity, without significant enhancing nodularity or reduced diffusion (bottom right) in the wall. Based on these findings, the patient was taken for an examination under anesthesia and biopsy, which returned inflammatory tissue without evidence of recurrence, saving the patient a surgical resection.

FIGURE 10

Images from a 68-year-old man with PSA of 19.1 found to have Gleason 4 + 3 prostatic carcinoma at biopsy. Combined whole body 68-Ga PSMA PET/MRI with dedicated prostate MRI performed for initial staging demonstrates an area of T2 hypointensity, diffusion restriction, and PSMA uptake in the left peripheral zone (arrows). T2 images of the pelvis demonstrate a small left internal iliac chain node (arrowheads), which appears normal in size and morphology but has PSMA uptake, suspicious for metastatic disease.

FIGURE 11

FDG PET/MRI performed in a 62-year-old woman with biopsy proven multiple myeloma. MIP image (left) shows a pattern of bone marrow hyperplasia in the proximal humeri and femora, with superimposed focal areas of hypermetabolism throughout the axial and appendicular skeleton. Coronal STIR and axial T1 images (top and middle right) show diffuse marrow replacement without focal lesions. Fused FDG PET and axial T1 image (bottom right) shows multiple foci of disease within the right iliac bone and left sacrum, which were not seen on anatomic imaging alone.

FIGURE 12

PET/MRI attenuation correction in the pelvis: MR attenuation correction (MRAC) methods in the body currently use Dixon fat/water separation techniques that neglect bone, which leads to substantial errors (>10%) for lesions within bone or soft tissues of the pelvis due to extensive surrounding bone. Preliminary results indicate that bone density can be accurately estimated from zero echo time (ZTE) MRI using a hybrid ZTE/Dixon method for pseudoCT (top left) and that MRAC is more accurate than Dixon alone compared with gold-standard CT attenuation correction maps (CTAC, bottom left). Preliminary results also indicate that these techniques can reduce average SUVmax errors of bony and soft tissue pelvic lesions to less than 5% (right).