Synopsis

The impacts of operating a small animal PET insert in a 7T MRI system were studied. The MRI's performance was compared with and without the insert by measuring the static field, flip angle distribution, RF noise, and several imaging sequences with two RF volume coils. With the insert inside a large ¹H volume coil, the MR was limited to T1-weighted anatomical imaging, and required a surface receive coil for adequate SNR. With the insert enclosing a small ¹H/¹³C volume coil, the primary impact on MRI was up to 38% reduced SNR, and all tested MRI sequences were functional.

INTRODUCTION

MADPET4 is a small animal PET insert designed to be operated inside a 7T Agilent/GE MR901 Magnet (currently operating with Bruker AVANCE III HD electronics)¹. In this work, the impact of the PET insert on the MRI system is assessed by comparison of image quality, noise, and B0 and B1 fields, with several RF coil configurations at ¹H and ¹³C frequencies, with and without the insert in the bore or acquiring PET data. These results are important to evaluate the suitability of the PET insert design for use in combined and simultaneous PET/MRI experiments.

METHODS

The MADPET4 insert has 2640 Ce:LYSO scintillation crystals, arranged in a dual layer configuration, individually read out by silicon photomultipliers (SiPMs), providing high count rate capability and depth of interaction correction. No RF shielding and no active electronic components are located in the MR scanner bore. Instead, 1.5 m coaxial cables connect the SiPMs to the readout ASIC boards located outside the bore, which are in turn connected by 3 m coaxial cables to the digital data acquisition electronics located outside the MR scanner room Faraday cage. The insert has an inner diameter of 88 mm and outer diameter of 150 mm.

The PET insert's impact on MRI was tested with a ¹H-frequency “large” volume coil (surrounding the insert, inner diameter 150 mm), and with a dual-tuned ¹H/¹³C-frequency “small” volume coil (within the insert, inner diameter 31 mm) (both from RAPID Biomedical). MR compatibility was assessed with ¹H B0 static field mapping using the scanner's FieldMap dual gradient echo sequence, ¹H B1 mapping by varying the excitation pulse power of a gradient echo sequence, ¹H gradient echo imaging, ¹H spin echo imaging, ¹H spin echo and FID EPI imaging, ¹H and ¹³C noise scans (wide bandwidth spectra acquired without using gradients or RF excitation), and ¹³C chemical shift imaging. All tests were compared between cases without the insert, with the insert in place but inactive, and with the insert acquiring.

RESULTS

With the PET insert in the large ¹H volume coil, compared with no insert: B0 variations of +/- 1 ppm were observed on the periphery of the PET enclosed volume (Figure 1), corresponding to the positions of the cable connector on the PET insert's inner ring SiPM boards. B1 was reduced by approximately 62% (lower flip angle for a given excitation power) near the magnet and RF coil centre. In ¹H noise spectra (Figure 2), noise increased with or without PET acquiring across all measured frequencies, and additionally at various narrow frequency spikes while PET was acquiring. Combined with the already-limited RF transmit performance of this coil with rat or mouse sized subjects (for which it is not normally used), operating with the PET insert required very low transmit bandwidth for 90 and 180 degree pulses (eg. 600 Hz).

With the ¹H/¹³C small volume coil in the PET insert, compared with no insert: Negligible effects on B0 and B1 (Figure 3) were observed. In ¹H and ¹³C noise spectra (Figure 2), ¹H noise was unchanged when PET was not acquiring, but increased 30-50% across all measured frequencies and at numerous narrow frequency spikes while PET was acquiring, whereas ¹³C noise increased by more than double, regardless of whether PET was acquiring. In MR images (Figure 4) ¹H MRI SNR was reduced by 23-30%, depending on the imaging sequence used, and ¹³C CSI SNR was reduced 38% while PET was acquiring.

DISCUSSION

With the large RF volume coil: B1 effects are largely due to detuning of the coil while the PET insert is within the RF coil. SNR issues were somewhat mitigated when using a surface receive coil. Limitations on flip angle or pulse bandwidth prevented the use of inversion-prepared or spin echo sequences. Low flip angle gradient echo sequences with a surface receive coil remain useful for T1-weighted anatomical reference purposes, however. The coil detuning may be resolvable by RF shielding the cables connecting the insert to the electronics outside the bore.

With the small RF volume coil: The tested MR sequences were minimally impacted by the presence of the PET insert. There was no apparent additional distortion in gradient-sensitive EPI readout sequences. Inversion-prepared and spin echo sequences functioned normally. Increased noise could affect some SNR-limited sequences.

CONCLUSION

The MADPET4 insert can acquire PET data without substantial impact on 7T MRI when using a mouse-size RF volume coil placed within it. When enclosed by a larger RF volume coil, the insert limits MRI to T1-weighted anatomical reference imaging of rat sized or smaller subjects.

Acknowledgements

References

1. Omidvari N, Cabello J, Topping G, et al. PET performance evaluation of MADPET4: a small animal PET insert for a 7-Tesla MRI scanner. Physics in Medicine & Biology. 2017