Brian J Lee1,2, Ronald D Watkins2, Alexander M Grant2,3, Chen-Ming Chang2,4, and Craig S Levin2,3,5,6
1Mechanical Engineering, Stanford University, Stanford, CA, United States, 2Radiology, Stanford University, Stanford, CA, United States, 3Bioengineering, Stanford University, Stanford, CA, United States, 4Applied Physics, Stanford University, Stanford, CA, United States, 5Physics, Stanford University, Stanford, CA, United States, 6Electrical Engineering, Stanford University, Stanford, CA, United States
Synopsis
Simultaneous acquisition of MRI and PET
shows great promise for disease characterization as it enables concurrent
collection of complementary molecular and anatomical information.
To overcome the limited dissemination of integrated
PET/MRI systems due to the high cost, we have developed an RF-penetrable
PET insert integrated with custom RF coils that can be inserted into any
existing MRI system for simultaneous PET/MRI acquisition. The RF-penetrable PET/RF-receiver
insert enables the use of the built-in body coil for more uniform transmit RF
field, fast acquisition using parallel imaging, and high SNR MR and PET images.Purpose
Integrated
positron emission tomography (PET) and magnetic resonance imaging (MRI) holds
great promise for disease characterization as it enables the simultaneous
measurement of complementary molecular and anatomical information on disease. Due to the extremely high cost of integrated PET/MRI systems, the clinical acceptance is limited by the dissemination which leads to a restricted amount of installations. To accommodate this problem, the concept of an insertable and removable PET insert
is attractive (1-3). While some groups are developing PET inserts with dedicated RF transmit/receive
(TX/RX) coils placed inside the PET ring, we have developed an RF-penetrable
PET insert (4, 5)
integrated with a custom RX-only coil that enables the use of the built-in body
coil for more uniform transmit field, parallel imaging, and high SNR images.
Methods
The RF-penetrable
PET system consists of 16 PET detector modules arranged in a 32 cm inner
diameter ring pattern with 1 mm inter-module gaps. Each PET detector module
consists of scintillation crystals, photodetectors, and analog optical links in
a shielded copper Faraday cage. We electrically float the PET system with respect to the
MR system using electro-optical signal transmission and battery power. RF
transmits from the body coil through the gaps and ends of the PET ring with
various RX configurations.
We
have developed custom birdcage coils for 3T MRI that fit inside the PET. The
birdcage coils are composed of two end rings and 16 copper rungs taped on an
acrylic tube (28 cm inner diameter).
PET/RF
coil configurations listed in Table 1 were tested inside the MRI with an agar
phantom (17 cm diameter) placed at the isocenter.
The B1
field map was acquired using the double angle method to evaluate the transmit
field uniformity (6). Signal-to-noise ratio (SNR) of MR images was
acquired with gradient echo (GRE) and fast spin echo (FSE) (Table 2).
SNR and
uniformity of the acquired B1 field maps and MR images were quantitatively
analyzed based on the recommendation of the American Association of Physicists
in Medicine (7).
Results and Discussions
The
magnitude, SNR and uniformity of the B1 maps and MR images are
listed in Table 3 and Figure 2.
When
the RF field transmits from the body coil through the PET insert (D), the B1
field uniformity was 67.1%, which is lower than that acquired without PET (A,
B) or TX/RX coil (C). However, simulations have shown that increasing the
inter-module gaps or decreasing the shielding cage heights can improve the field
uniformity when transmitted from the body coil (8).
Images
acquired with the body coil as a transmitter/receiver (A) showed the worst SNR
due to the RF attenuation during the receiving process. The GE 8-channel array coil
with no PET insert present (B) was tested as a reference coil and showed the
highest SNR (Table 3).
The PET
scintillation crystals are located at the axial edge of the Faraday cage and
part of the custom birdcage coil extends out past the Faraday cage (Figure 1).
This led to axially non-uniform B1 field and FSE image with PET +
TX/RX coil (C) showed a phase-related artifact.
When
the RF field penetrates through the PET insert (C, D), the transmit gain was
increased to compensate for the RF attenuation. The SNR was 31.4% higher when
imaged with a TX/RX coil inside the PET (C) compared to the image taken with a
RX-only coil (D). However, an RX-only array coil is under development and
expected to have better MR performance with parallel imaging capability.
Conclusion
We
have shown that the SNR of the GRE MR image acquired with the custom PET/MR
receive coil is 40% higher than that acquired with the body coil. By using an
RF-penetrable PET insert, the MRI body coil can be used to transmit RF pulses in
conjunction with a sensitive RX-only coil for uniform transmit RF field and higher SNR images.
Acknowledgements
This work was partially funded by
NIH-NIBIB grant R01EB01946501.References
1. Wu, et al. PET
Performance Evaluation of an MR-Compatible PET Insert. IEEE TNS. 2009
2. Pichler, et al.
Performance Test of an LSO-APD Detector in a 7-T MRI Scanner for Simultaneous
PET/MRI. JNM. 2006
3. Jung, et al.
Development of brain PET using GAPD arrays. Med.
Phys. 2012
4. Chang, et al.
Preliminary PET performance evaluation of an RF field-penetrable brain-sized
PET insert for simultaneous PET/MR imaging. JNM.
2015
5. Olcott, et al. Prototype positron emission
tomography insert with electro-optical signal transmission for simultaneous
operation with MRI. PMB. 2015
6. Insko, et al. Mapping
of the Radiofrequency Field. Magnetic
Resonance. 1993
7. Price, et al. Quality
assurance methods and phantoms for magnetic resonance imaging. Med Phys. 1990
8. Lee, et al. Studies of
electromagnetic interference of PET detector insert for simultaneous PET/MRI. IEEE NSS/MIC, 2013