Small animal simultaneous PET/MRI: initial experiences in a 9.4 T microMRI

http://iopscience.iop.org/0031-9155/56/8/009 

Main highlights of BNL PET/MRI system:
• One-one coupling between the scintillator and photodetector: provides accurate event positioning.

• Can image both rat brain and mouse heart using the same PET system.

• High field strength MRI - 9.4 tesla: provides better spatial resolution and SNR in MR images.


Our PET system (ID = 38 mm) is based on Rat Consious Animal PET (RatCAP) detector. Its main application was to acquire PET images of an awake rat brain without administering anesthesia. Anesthesia reportedly suppresses the brain chemistry that are not visualized in PET scans. Hence the application of our PET system is novel. The rat wears the PET detector around its head and is free to move around, while PET data is being acquired. We used the same PET detector design, architecture and geometry for simultaneous PET/MRI applications, replacing all possible magnetic components. The animals, in this case, are of course anesthetized.

We are using Bruker Biospin 9.4T microMRI scanner at BNL, for all our PET/MRI studies. An in-house MRI coil was designed and built to fit inside the PET system, given its geometry.

My PhD dissertation - Evaluate PET/MRI system interactions

The main focus of my research has been to identify the electromagnetic interference (EMI) between the two; understand the source of EMI and minimize (more importantly, eliminate) these interactions to obtain uncompromised PET and MRI data simultaneously. In other words, MR images should not be influenced or contaminated during PET operation. PET information should not lose the sensitivity and retain quantitative information. The work also focuses on identifying the margins of electromagnetic compatibility during simultaneous PET/MRI operation.

Research work involves:
• Understanding PET and MR imaging physics, instrumentation, hardware, firmware, data acquisition, image processing and quality assessment, analysis and co-registration (fusion) of PET/MR images.

• Designing and conducting feasibility experiments, and performing trade-off analysis by implementing shielding and grounding topologies to evaluate the electromagnetic field interactions between PET and MRI systems to quantify the impact of one system over the other, when operated simultaneously.

• Specifically analyzing the PET signal integrity during MRI acquisition. Identifying if the interference on PET is due to gradient or radiofrequency (RF) fields. Also, the impact on the field homogeneity and signal-to-noise ratio (SNR) in MR images in the presence of PET system.

• Characterizing the PET system and detector setup in MRI environment.

• Testing custom-built MRI coils on the workbench using (network/spectrum/impedance analyzer) that fit inside the PET system. 

PET/MRI compatibility issues

The electromagnetic interactions between PET and MRI systems can be very challenging. This is because to obtain good MR images, a homogeneous magnetic field is required. The PET system comprises of electronics, metallic structures, ferromagnetic components, current carrying power cables etc. These components are not placed in the vicinity of the MRI scanner, but are integrated right at the center of the magnet where the field homogeneity is compromised leading to artifacts in MR images.

On the other hand, any powered electronics with ferromagnetic components cannot function as intended in a strong static and/or time-varying magnetic fields. Therefore, the main operation of the PET system is compromised leading to corrupt PET data.

Introduction to PET/MRI

Background: PET and MRI
Positron Emission Tomography (PET) is an imaging technique that provides metabolic/physiological/functional/pharmacological biodistribution information inside the body. With this information, one can diagnose if there is a cancerous tissue in the body and further investigate the glucose metabolism, for example. Patients undergo PET scans to monitor the tumors (in brain, breast, lungs, lymphs, abdomen, reproductive organs etc.) and is a good diagnostic tool providing high sensitivity information.

Generally, a positron emitting radionucleide/radioisotope/radiotracer is administered to the patient. For each positron that annilate with an electron in the body, two gamma photons (511 keV each) are emitted in opposite direction (near 180-degrees). The PET detectors capture these photons within a certain time window and the data is processed to obtain a 2D/3D reconstructed image. Tumor regions can be identified in the body if these radionucliedes bind to the cancerous tissue compared to a healthy ones. This is a simplistic explanation of the basic principle of PET physics. At BNL, the main research involves the design and development of novel PET systems, mainly for small animal imaging applications.

Magnetic Resonance Imaging (MRI) is another non-invasive imaging tool that provides structural/morphological/anatomical information. Even X-ray Computed Tomography (CT) does the same. Infact, PET/CT systems are currently used extensively in clinical setting, as opposed to stand-alone imaging systems. But MRI provides better soft-tissue contrast compared to CT. Different tissue contrast can be highlighted using a variety of MR imaging sequences. Furthermore, there is no additional radiation dose to the patient in MRI, unlike CT.

PET pros: High sensitivity (pico molar range); Good physiological detail.
PET cons: Poor spatial resolution and limited anatomical detail.

MRI pros: Better soft-tissue contrast compared to CT ; Good spatial resolution (few microns) and no additional radiation dose.
MRI cons: Limited or no functional detail.

So by combining PET and MRI systems, the demerits of both PET and MRI systems are negated and they mutually complement each other making them powerful tools to extract accurate diagnostic information. There are many reports and publications on PET/MRI research conducted by various research groups around the world.