Drs. Fowler, McConathy, Khanna, Dehdashti, Benzinger,
Miller-Thomas, Parsons, Raptis, Grigsby, Laforest, Gropler, Narra,
Siegel, Kotyk, McKinstry, and Woodard practice at the
Mallinckrodt Institute of Radiology and the Alvin J. Siteman Cancer
Center, Washington University School of Medicine, St Louis, MO. Dr. Priatna is at R&D Collaborations, Siemens Healthcare, St Louis, MO.
clinical availability of hybrid positron emission tomography (PET) and
magnetic resonance imaging (MRI) systems offers medical imaging a range
of potential benefits in oncologic, neurological, and cardiovascular
applications.1-5 One obvious benefit of simultaneous
acquisition is improved image registration that allows optimal anatomic
localization of PET findings. Other advantages include lower radiation
dose in whole-body imaging compared with PET/CT (computed tomography),
shorter overall imaging times, and the ability to simultaneously observe
rapidly changing physiological and pathophysiological processes.
of the examples that follow are of patients scheduled for standard-of-
care clinical fluorodeoxyglucose (FDG)PET/CT who, with Institutional
Review Board (IRB) approval, were recruited for, and consented to
undergo, additional PET/MR imaging. The simultaneous PET/MRI studies
were acquired on a Biograph mMR system (Siemens Medical Systems,
Erlangen, Germany) recently installed in the Center for Clinical Imaging
Research (CCIR) at Washington University School of Medicine, St. Louis,
MO. In this system, the PET component of the scanner with MR-compatible
avalanche photodiodes (APDs) is present within the bore of a 3.0 tesla
(T) magnet equipped with total imaging matrix and attenuation body-array
and spine matrix coils.
Unlike PET/CT, PET/MRI uses a soft
tissue, segmentation attenuation-correction (AC) µ-map. The AC µ-map was
generated utilizing a dual-echo VIBE Dixon sequence that separates
water and fat with TE1/TE2 = 1.23 msec/2.46 msec, TR = 3.6 msec,
left-right FOV = 500 mm and anterior-posterior FOV = 300 mm. The
acquisitions were performed either in a single station for dedicated
PET/MRI or a multiple-station mode for whole-body imaging as indicated.
Depending on the application, PET images were simultaneously acquired
with the anatomical sequences from MRI, such as HASTE for whole-body
acquisition, MPRAGE for brain imaging, SPACE or HASTE for pelvic
applications, or delayed, contrast-enhanced, T1-weighted,
phase-sensitive inversion-recovery imaging for cardiac imaging.
Additional high-resolution MRI sequences were added for focused
examinations; these included high-resolution T2 TSE, diffusion-weighted
imaging (DWI) or diffusion-tensor imaging (DTI), and other sequences
depending on the applications. Recent articles detail possible
whole-body and dedicated protocols for oncologic imaging with PET/MR;
however, the optimal sequences for workflow efficiency and diagnostic
yield are yet to be determined in clinical practice.6,7
following are some examples of the clinically relevant cases acquired
for anatomically focused and whole-body examinations with the Biograph
Neuro-oncologic brain imaging
separately acquired brain MR and FDG-PET images can be fused through
various software tools, a combined examination permits streamlined
patient care and improves diagnostic specificity. The convenience and
reduced burden to the patient of a single imaging session is
particularly relevant to patients with brain tumors, who often have a
reduced functional status.
Figure 1 displays a panel of selected
simultaneous FDG-PET/MR images of a 41-year-old man with a left temporal
lobe primitive neuroectodermal tumor (PNET) treated with near-total
resection, craniospinal and resection-bed radiation therapy, and
adjuvant chemotherapy. Follow-up MRI showed a growing, enhancing mass,
and FDG-PET/MRI was performed to differentiate recurrent tumor from
radiation necrosis. The white arrow denotes an enhancing nodule with
markedly increased FDG uptake, highly suspicious for recurrent PNET.
Subsequent surgical resection demonstrated a mixture of recurrent PNET
and radiation necrosis.
Amyloid PET brain imaging
development of F-18-labeled PET tracers for imaging beta-amyloid
plaques that occur in Alzheimer’s disease (AD) has the potential to
increase the certainty of diagnosis in patients with cognitive
impairment. One of these tracers, F-18 florbetapir, recently received
FDA approval for clinical use, and several other amyloid PET tracers in
late-phase clinical trials may soon become clinically available. If
drugs that slow or reverse the development of AD dementia become
clinically available, amyloid PET imaging may be a key component of
patient selection for treatment and for monitoring response to therapy.
The anatomic, volumetric, and functional data available through MRI can
complement the biochemical information obtained through amyloid PET and
provide a comprehensive, single-session neuroimaging evaluation of
patients with cognitive impairment.
Figure 2 displays selected
negative and positive F-18 florbetapir-PET/MR images from subjects
enrolled in a research study. MRI provides excellent anatomic
correlation for the localization of F-18 florbetapir accumulation in the
white and gray matter and can also provide volumetric data for the
detection of atrophy in regions like the hippocampus, which is often
affected by AD. Clinical interpretation of F-18 florbetapir-PET relies
primarily upon assessment of gray-white differentiation, with negative
studies showing higher activity in the white matter than in the cerebral
cortex (Figure 2) and positive studies showing loss of gray-white
contrast due to the tracer binding to beta-amyloid plaques in the
cerebral cortex (Figure 2).
Head and neck oncology
head and neck PET/MRI performs well compared with PET/CT in our initial
experience. In the head and neck, PET/MRI combines the metabolic
information from FDG-PET with high spatial resolution, anatomic
localization, and soft-tissue contrast from MRI. MRI demonstrates better
soft-tissue contrast than CT, permitting detection of perineural
spread. Software fusion of PET and MRI data is challenging in the neck,
and the simultaneous acquisition of the MRI and PET data provides
Figure 3 illustrates images of a
58-year-old man with a recent diagnosis of squamous cell carcinoma of
the right buccal space and metastatic involvement of right submandibular
lymph nodes. Contrast-enhanced MRI shows excellent anatomic detail, and
diffusion-weighted images demonstrate high contrast between the lymph
nodes and adjacent fat in the submandibular space, which correlate to
the areas of increased FDG uptake.
Multiple myeloma and bone imaging
combination of functional and morphologic MRI sequences with PET
imaging is of potential value in the evaluation of osseous metastases
and primary bone neoplasms, such as multiple myeloma. DWI, T2
fat-suppressed sequences, and T1-TSE images can all depict bone lesions
that may not be evident on CT or conventional bone scintigraphy alone.
While the relationship of diffusion restriction with disease status is
complex, ADC values and appearance on DWI, in conjunction with changes
in FDG uptake, may be useful for monitoring response to therapy in
neoplasms involving the bone.
Figure 4 consists of images of a
62-year-old woman with biopsy-proven multiple myeloma presenting for
initial staging. A large, left humeral neck lesion is clearly depicted
on DWI, ADC, and T2-inversion recovery, fat-suppressed MR sequences,
with increased FDG uptake shown on the fused image. This lesion is less
conspicuous on CT alone.
PET/MRI could play a role in both cardiac ischemia and cardiac
viability assessment. Potential clinical protocols, playing on the
strengths of both modalities, include stable chest pain assessment,
performing cine MRI cardiac function assessment, N-13 ammonia or Rb-82
PET myocardial perfusion imaging, and delayed contrast-enhanced
inversion-recovery infarct imaging in a single examination. Combined FDG
and delayed contrast-enhanced inversion-recovery cardiac imaging permit
co-localized, simultaneously acquired functional and anatomic viability
assessment that, theoretically, could play a role in imaging-directed
ventricular tachycardia radiofrequency ablation or direct biventricular
pacing in dyssynchrony.
Figure 5 shows simultaneously acquired
ECG-gated PET and delayed contrast-enhanced (DCE) cardiac MR images
(simultaneous acquisition of MR 2-point Dixon also acquired for AC),
allowing for precise fusion imaging. PET data were acquired in list
mode, binned, and reconstructed into 3 phases. DCE MR images acquired in
diastole are fused with diastolic PET data to create the center image.
This patient has a normal heart.
Cervical and pelvic cancers
and MRI are well established for staging and monitoring treatment
response in patients with cervical cancer and other pelvic malignancies.
PET data provide tumor volume estimation, allowing for accurate
radiation therapy planning, as well as prognostic information related to
progression-free survival.4-7 High-resolution MRI provides
exquisite soft-tissue detail for local staging and presurgical planning.
The combination of metabolic information derived from FDG-PET with
high-resolution MRI of the pelvis shows promise in both clinical
management and potential research opportunities in correlating
functional MRI with tumor metabolism.
Given the complex anatomy
of the pelvis, multiplanar and high-resolution T2 TSE are mainstay
sequences. Using a 3-dimensional (3D) isotropic dataset, such as T2
SPACE, a single acquisition can yield high-resolution images with the
option of infinite reformatting without loss of resolution. DWI provides
improved conspicuity of lymph nodes, and ADC values for primary
cervical malignancy have been correlated with standardized uptake values
(SUVs) for FDG, permitting an additional noninvasive biomarker of
disease response. Additionally, volumetric-interpolated breath-held
examination (VIBE) can provide excellent anatomic detail of pelvic
structures. Radial VIBE allows for a free-breathing acquisition with
acceptable resolution and no motion artifact.
Figure 6 consists
of images of a 58-year-old woman with newly diagnosed cervical cancer
presenting for initial staging. Additional images from a separate
patient who underwent total abdominal hysterectomy and bilateral
salpingo-oophorectomy demonstrate the utility of T2-SPACE multiplanar
especially appealing in the pediatric population, as it is associated
with less radiation exposure than PET/CT without compromising anatomic
image quality. A recent study performed at the University of Leipzig in
Germany showed that the effective dose of a PET/MRI scan is only about
20% that of the equivalent PET/CT examination. Children with systemic
malignancies routinely evaluated with PET/CT (such as lymphoma) could
benefit from the reduced radiation exposure of PET/MRI.
acquiring PET and MRI data combines the advantages of two previously
separate advanced-imaging modalities, a tremendous advantage in young
children requiring sedation/anesthesia for imaging. The improved
soft-tissue contrast and molecular imaging (such as diffusion and
perfusion imaging) abilities of MRI permit better detection of
lymph-node and visceral disease than does CT. PET/MRI also holds great
potential in evaluating soft-tissue malignancies, such as sarcomas, in
the pediatric population.
Figure 7 presents the images of a
16-year-old boy with diffuse, large B-cell lymphoma undergoing restaging
after chemotherapy. Excellent PET and MRI coregistration helps separate
the renal lesion (solid arrowhead) from excreted FDG in the collecting
PET/MRI shows promise for
multiple clinical applications through the combination of the improved
soft-tissue contrast of MRI with lower radiation dose, and the potential
for better correlation of PET findings to anatomy given the
simultaneous acquisition. Several challenges are evident in developing
optimal protocols, including optimal MR-sequence parameters, motion
correction, and validation of the quantitative accuracy of PET with
MRI-based attenuation correction. In some cases, the combination of SUVs
measured on PET and ADC values measured on diffusion-weighted MRI may
prove more specific than subjective assessment alone in differentiating
tumor from surrounding tissue. The potential for benefit from PET/MRI
acquisition also exists in receptor-targeted oncologic imaging, dementia
assessment, and cardiac and atherosclerosis imaging.
acknowledge the invaluable assistance of Jennifer Frye, Glenn Foster,
Linda Becker, Debra Hewing, Michael Harrod, Tim Street, and Betsy
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- Kidd EA, Siegel BA, Dehdashti F, Grigsby PW. The standardized uptake
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for cervical cancer treatment response and survival. Cancer. 2007;110:1738-1744.
- Kidd EA, Grigsby PW. Intratumoral metabolic heterogeneity of cervical cancer. Clin Cancer Res. 2008; 14:5236-5241.
- Schwarz JK, Siegel BA, Dehdashti F, Grigsby PW. Association of
post-therapy positron emission tomography with tumor response and
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- Olsen JR, Esthappan J, Dewees T, et al. Tumor volume and subvolume
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