PET/MRI as a MR scanner with PET: A Guide to Sequences and Tracers for Comprehensive Whole-body Imaging in Oncology
Satoru Takahashi1, Munenobu Nogami2, and Takamichi Murakami2
1Takatsuki General Hospital, Kobe, Japan, 2Kobe University Hospital, Kobe, Japan

Synopsis

Keywords: Physics & Engineering: PET/MR, Body: Body, Image acquisition: Whole Body

PET/MRI is a hybrid imaging modality that combines the benefits of PET and MRI. Compared to PET/CT, PET/MRI offers improved soft tissue contrast and avoids ionizing radiation. However, concerns such as long examination times and the challenge of lung imaging remain. To optimize PET/MRI protocols for whole-body imaging in oncology, necessary and sufficient sequences including T1-weighted, T2-weighted, and diffusion-weighted imaging will be discussed. Common tracers for whole-body imaging, such as FDG and PSMA, will be demonstrated with possible clinical scenario. PET/MRI can provide functional and metabolic information in a single study, making it a valuable tool for oncologic imaging.

Introduction and Aim

Is PET/MRI a “PET scanner” using the MRI data to create a detailed anatomical map of the body, or a “MRI scanner” using a PET tracer as a contrast agent?
Hybrid PET/MRI has been introduced after the rapid adoption of PET/CT that had changed the clinical diagnostic algorithms in oncology over the last decades. It has been expected that hybrid PET/MRI overcomes the disadvantage of PET/CT, such as the limited soft tissue contrast and the additional radiation dose from CT. There are, however, several disadvantages associated with MRI, especially long examination time, which might prevent comprehensive whole-body imaging in addition to the local evaluation.

The aim of this educational talk is to introduce the basic concept for optimizing PET/MRI protocols to the patients with specific oncologic disorders and conditions, from the perspective of whole-body MR imaging accompanying with a functional and metabolic information provided by a PET tracer.

Benefit of PET/MRI over PET/CT in whole-body imaging

Hybrid PET/MRI sytem have following potential benefits over PET/CT scanner:
  1. Improved soft tissue contrast: Thanks to superior soft tissue contrast compared to CT, MRI can more clearly discriminate and accurately localize soft tissue lesions.
  2. Reduced radiation exposure: PET/MRI can avoid the use of ionizing radiation from CT scans.
  3. Simultaneous data acquisition: Simultaneous imaging of both PET and MRI data can reduce image misregistration between the PET images and mapping images by MRI. In combination with improved soft tissue contrast with MRI, it becomes much easy to detect small lesions and accurate to localize them.
  4. Improved functional imaging: MRI can also provide functional information such as diffusion-weighted imaging and perfusion imaging. Thus, PET/MRI can provide functional and metabolic information in the single study.

Concerns of PET/MRI in whole-body oncologic imaging

  1. Respiratory gating: Respiratory-gating MR imaging, even for attenuation correction, is effective to obtain high degree temporal and spatial coregistration, which results in superior image fusion at the cost of long data acquisition duration.
  2. Multi-parametric MR data acquisition: It is time consuming to obtain multi-parametric and multiplanar MR images at each scan station. To put it another way, longer MRI scan time increases more PET data acquisition with improving image quality of PET images.
  3. Local imaging vs. whole-body imaging: Simultaneous PET and MRI acquisition, which is the most important benefit in PET/MRI, allows high quality image fusion between PET and MRI. Comprehensive local MR imaging is, therefore, preferable in oncologic study, although exploring distant metastasis with whole-body imaging would be also required in several patients’ conditions.
  4. Lung image: Assessment of lung metastasis is vital in most oncologic imaging, although pulmonary imaging is still challenging even with modern MR scanner.
  5. From a daily clinical practice point of view, total examination time should be around half to one hour in maximum.

Sequences

To take a full advantage of the benefit of PET/MRI with minimalizing and responding concerns, necessary and sufficient sequences should be acquired, although the specific sequences included would depend on the individual patient and the clinical question being asked.
  1. T1-weighted imaging: Either turbo spin echo or DIXON-based 3D gradient echo sequence. Dixon-based water only images might provide similar features to PET/CT-like images in term of visualizing “black” fat and “gray” soft tissues.
  2. T2-weighted imaging: Either turbo spin echo or single shot turbo spin echo sequences. Single shot T2-weighted imaging is mainly used for whole-body imaging, while high-resolution T2-weighted turbo spin echo imaging for local evaluation.
  3. Diffusion-weighted imaging (DWI): DWI is a widely applied in whole-body MRI for detecting malignant lesions. There is, however, an argument about the effectiveness of adding DWI on PET/MRI protocol, since lesions detected by DWI are also usually visualized by PET.

Tracers for whole-body imaging

In terms of whole-body imaging, following tracers are used for oncologic PET/MRI.
  1. 18F-Fluorodeoxyglucose (FDG): FDG is the most commonly used tracer in oncology, since it is useful for imaging a variety of cancers, including lung, colorectal, and lymphoma.
  2. 68Ga-prostate-specific membrane antigen (PSMA): As PSMA is overexpressed on the surface of prostate cancer cells, PSMA-tracers are used to detect and diagnose prostate cancer and its spread. PSMA is a promising target for theranostics.
  3. 11C-Choline or 18F-Fluorocholine (18F-FCH): Choline is a vital element for cell membrane synthesis, lipid metabolism, and neurotransmitter production. Choline labeled with either C11 or F18 has been particularly useful for imaging prostate cancer and brain tumors.
  4. 68Ga-DOTATOC: Since DOTATOC is a useful tool to visualize the expression of somatostatin receptors, the agent is applied to diagnose and detect neuroendocrine tumors and its metastasis.

Acknowledgements

I would like to express my sincere appreciation to the staff radiologists and the technicians in Department of Radiology, Kobe University Hospital for their great supports and professionalism at the hospital.

References

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Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)