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Performance Test of an LSO-APD Detector in a 7-T MRI Scanner for Simultaneous PET/MRI

Bernd J. Pichler, PhD1,2, Martin S. Judenhofer, MS1,2, Ciprian Catana, MD1, Jeffrey H. Walton, PhD3,4, Manfred Kneilling, MD5, Robert E. Nutt, BS6, Stefan B. Siegel, PhD6, Claus D. Claussen, MD2 and Simon R. Cherry, PhD1

1 Department of Biomedical Engineering, University of California, Davis, California; 2 Department of Radiology, Eberhard-Karls-University, Tuebingen, Germany; 3 Nuclear Magnetic Resonance Facility, University of California, Davis, California; 4 Biomedical Engineering Graduate Group, University of California, Davis, California; 5 Department of Dermatology, Eberhard-Karls-University, Tuebingen, Germany; and 6 Siemens Preclinical Solutions, Knoxville, Tennessee


Figure 1
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  		FIGURE 1.  (A) Functional information at highest sensitivity is provided by PET images. (B) MRI provides superior anatomic information with excellent soft-tissue contrast. (C) High-resolution animal CT also provides good anatomic information but has poorer soft-tissue contrast and requires significant radiation dose.

 

Figure 2
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  		FIGURE 2.  Top sketch shows ring consisting of compact LSO-APD detectors, which will be inserted into 7-T MRI scanner. PET detector will be mounted between RF and gradient coils to allow simultaneous PET/MRI. Bottom sketch shows longitudinal (axial) section through MRI scanner and PET insert. FOV = field of view; PCB = printed circuit board.

 

Figure 3
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  		FIGURE 3.  LSO-APD detector assembly: 10 x 10 LSO array (center right) was coupled by custom-made light guide to 3 x 3 APD array (bottom right). Signals were read by 9 hybrid charge-sensitive preamplifiers (top). Nonmagnetic 50-{Omega} coaxial cables were used to channel signals out of magnet. Sketch shows dimensions (in mm) of 3 x 3 APD array.

 

Figure 4
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  		FIGURE 4.  Representative 511-keV energy spectrum acquired with LSO-APD detector in 7-T MRI scanner while applying spin-echo sequence. Energy resolution was 18.7% (511 keV, full width at half maximum). Although energy resolution is still good and linearity is maintained between the two 22Na peaks, MRI sequence causes increased noise in low-energy region.

 

Figure 5
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  		FIGURE 5.  (A) NaCl-filled spheric phantom imaged with spin-echo sequence when no parts of PET detector or shielding materials were in MRI scanner. Measured SNR is 43.1. (B) When mouse was scanned with two 0.8-mm-thick copper cans in field of view of magnet, almost no signal could be measured. (C) SNR of 30.0 is obtained using copper cans with 0.15-mm material thickness.

 

Figure 6
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  		FIGURE 6.  Experimental setup consists of NaCl phantom (top left), RF coil (top center, with mouse), 7-T BioSpec (Bruker BioSpin) MRI scanner (right), and copper can that serves as detector shielding (bottom left).

 

Figure 7
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  		FIGURE 7.  Acquisition of position profile with LSO-APD detector outside 7-T magnet (A), inside magnet (B), and during MRI with spin-echo sequence (C). Corresponding analog signals after 200-ns shaping are shown below for detector in magnet (B) and while applying spin-echo sequence (C). Signal baseline shows slightly increased noise level (oscilloscope pictures) when spin-echo sequence is applied. Square signal in oscilloscope diagram shows trigger that initiates sampling for analog–digital conversion.

 

Figure 8
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  		FIGURE 8.  On left is MR image acquired with spin-echo sequence during which PET detector was also in MRI scanner acquiring position profiles. Although anatomic details of mouse are visible, SNR is obviously worse than in image shown in Figure 1B. On right is MR image of NaCl phantom and corresponding profiles through phantom. SNR was only 3.4.

 

Figure 9
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  		FIGURE 9.  MRI of NaCl spheric phantom during application of spin-echo sequence with LSO and APD array in field but with no electronics in place. SNR of phantom was about 45. Corresponding profiles demonstrate good homogeneity.

 




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