2012 IEEE Nuclear Science Symposiwn and Medical Imaging Conference Record (NSS/MIC) MI0-61

Prototype integrated system of DOI- PET and the RF­

coil specialized for simultaneous PET-MRI

measurements

Fumihiko Nishikido, Takayuki Obata, Naoko Inadama, Eiji Yoshida, Hideaki Tashima, Mikio Suga, Hideo

Murayama, Hiroshi Ito and Taiga Yamaya, Member, IEEE

Abstract-We are developing a PET system integrated with a

birdcage RF coil for PET-MRI. The integrated system is intended

to realize a highly sensitive PET system for simultaneous

measurements. In the proposed PET-MRI system, PET detectors

which consist of scintillator crystal blocks, photo sensors and

front-end circuits with four-layer DOl encoding capability are

placed close to the objective. Therefore, the proposed system can

achieve high sensitivity without degradation of spatial resolution

at the edge of the FOV due to parallax error. The photo sensors

and front-end circuits should be shielded to minimize noises from

the MRI and noise influence on the MRI imaging. Elements of

the RF coil are inserted between the crystal blocks inside of the

shielding material so as not to interfere with the RF-pulse.

At the last MIC conference, we demonstrated the possibility of

realizing the proposed PET-MRI system with a prototype PET

detector and a commercial RF-coil. The prototype PET detector

consisted of a LGSO crystal block and a 4 x 4 MPPC array. The

crystals were arranged in a 4 x 4 x 4 layer with four-layer DOl

capability. The detector and electrical circuit were packaged in

an aluminum shielding box. Since they were located inside the

MRI magnetic field, most circuit elements were made of

nonmagnetic materials.

As a next step, we constructed a new RF-coil system which can

be mounted on PET detectors between each coil element. We

carried out experiments with the prototype RF-coil and the four­

layer DOl detector. The prototype RF-coil has eight RF-coil

elements and the PET detector can be mounted on gaps between

them. Only one detector was mounted in the gap positioned at

one side of the prototype RF-coil. We evaluated the performances

from the 2D flood histogram, energy resolution and MRI images.

We observed no degradations of the performance of the PET

detector and MRI image in simultaneous measurements.

I. INTRODUCTION

WE are developing a PET system integrated with a birdcage RF coil for PET-MRI. A conceptual scheme of the

proposed PET-MRI system is shown in Fig. 1; it is intended to realize a highly sensitive PET system for simultaneous measurements.

Manuscript received November 19,2012. This work was supported in part by the U. S. Department of Commerce under Grant No. BSI23456 (sponsor acknowledgment goes here).

F. Nishikido, T. Obata, N. Inadama, E. Yoshida, H. Tashima, H. Murayama, H. Ito and T. Yamaya are with the National Institute of Radiological Sciences, Chiba 263-8555, Japan, (telephone: +81-43-206-3187, e-mail: funis@nirs. go.jp. t_obata@nirs.gojp. inadama@nirs. go.jp. rush@nirs. go.jp, mur@nirs. go.jp, ito@nirs.gojp, taiga@nirs. go.jp).

M. Suga is with Chiba University, Chiba 263-8522, Japan (telephone: +81-43-290-3688, e-mail: mikio.suga@faculty.chiba-u.jp).

PET detectors which consist of a scintillator block, photo sensors and front-end circuits with four-layer DOl encoding capability are placed close to the objective. Therefore, the proposed system can achieve high sensitivity without degradation spatial resolution at the edge of the field of view due to parallax error. The photo sensors and front-end circuits should be shielded to minimize noises from the MRI and noise influence on MRI imaging. If the shielding material is inside the RF coils, the RF pulse is blocked by the shielding material and then complete images cannot be obtained. Therefore, each RF coil element is inserted between the scintillator crystal blocks and then medially located in the shielding materials. Previously, we demonstrated the possibility of realizing the

proposed PET-MRI system with a prototype PET detector and a commercial RF-coil [1]. As a next step, we constructed a new RF-coil system on which PET detectors could be mounted in gaps between each coil element. We carried out experiments with the prototype RF-coil and a four-layer DOl detector. We evaluated the PET detector performance measured with a 3.0T MRI and image quality of the prototype RF-coil in simultaneous measurements.

Gradient coil

RF coil elem ent

Scintillator

Shielding box (MPPC and circuits)

Fig. I. A conceptual scheme of the proposed PET-MRI system

II. MATERIAL AND METHODS

A. Four-layer DOl detector for PET-MRl

The prototype PET detector consisted of an LGSO crystal

block and a 4 x 4 mUlti-pixel photon counter (MPPC) array

(SI1064-050P). The size of each crystal element was 2.9 nun

x 2.9 mm x 5.0 mm. The crystal elements were arranged in a 4

x 4 x 4 layer with reflectors. Details of the reflector

978-1-4673-2030-6/12/$3l.00 ©20 12 IEEE 2750

arrangement and the four-layer DOl encoding method were

described in ref. [2]. Readout pixels of the MPPC were 3 x 3

mm3 and they consisted of 50 �m cells. The detector and

electrical circuits were packaged in an aluminum shielding

box. Since they were located inside the MRI magnetic field,

most circuit elements were made of nonmagnetic materials.

Figure 2 shows a photograph of the prototype detector.

Specified temperature control and correction of variance of the

MPPC gains were not applied.

Fig. 2. Photograph of the prototype four-layer DOL detector for PET MR!.

B. Setup

We constructed a prototype RF-coil dedicated to the

proposed PET-MRI system. The diameter of the RF-coil was

26cm. There were eight RF-coil elements and the PET

detector could be mounted on gaps between the RF -coil

elements. Only the one detector as used in the performance

evaluation experiment was mounted in the gap at the right side

of the prototype RF-coil as shown in fig. 3.

Fig. 3. Experimental setup for simultaneous measurements of PET and MRI.

Figure 4 shows a block diagram of data acquisition system

for experiments. The MPPC output signal of each MPPC

were divided into two signals. The first signals were fed into

simple non-inverting amplifier circuits and individually

recorded by 16ch analog-to-digital converters (C009H, Hoshin

Electronics Co., LTD.) as list-mode data. The other signals fed

into sum modules and used as the trigger and gate signal.

In all measurement experiments, only the detector and read

out cable were placed in the MRI room. The other modules,

such as a power supply, shaping amplifiers and ADCs, were

placed in the operator room as shown fig.4.

MRl room op erator room

Fig. 4. Experimental setup for simultaneous measurement experiments.

C. Experiment

First, we evaluated performance of the prototype four-layer

DOl detector in simultaneous measurements with the 3.0T

MRI (Siemense, verio). In the measurements, a 22Na point-like

source was positioned in front of the DOl-PET detector. We

compared between position histograms with/without MRI

measurement.

Second, we evaluated the influence of the PET

measurement on the MRI images. We obtained MRI images

(a) without the PET detector, (b) with the PET detector turned

off and (c) with the PET detector turned on (simultaneous

measurement). The gradient echo method was applied as the

MRI protocol.

III. RESULTS

Figures 5 shows position histograms for uniform irradiation

of the 22Na point source with and without MRI measurements.

Only 511 ke V photo-peak events were indicated on these

position maps. Each spot represented interacting events in

certain crystal elements. The crystals in all four layers could

be identified in both position histograms. Comparison of both

position histograms showed no degradation of crystal

identification performance by MRI measurement. The energy

resolutions of 16.9% and 17.4% were obtained for a single

crystal element with and without MRI measurements,

respectively.

(a)Without MRI measurements (b) Simultaneous measurements Fig. 5. Position histograms for unifonn irradiation of 511 keY gamma rays with and without MRl measurements

Figures 6 show magnitude images (left) and phase images

(right) measured for an MRI phantom by the gradient echo

method. The top, middle and bottom figures were obtained in

the experiments without the PET detector, with the PET

detector turned off and with simultaneous measurements,

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respectively. Signal to noise ratios for each of the experiments

were (a)109.0, (b)88.9 and (c)87.1. These results showed that

the influence of the PET measurements was negligible in

simultaneous measurements. In addition, the influence of

presence of the PET detector on the MRI images was larger

than that of noise from the electric current of the PET detector.

(a) Without PET detector

Fig. 6. MRI magnitude images (left) and phase images (right) measured by the gradient echo method. Red boxes show the position of the prototype PET detector.

IV. DISCUSSION AND CONCLUSION

We are developing a new system with integrated PET

detectors and an RF coil for PET-MRI. This system will

realize high spatial resolution and sensitivity for the PET

scanner by using the four-layer DOl detectors. We constructed

the RF-coil system for PET-MRI on which PET detectors can

be mounted between each coil element. We carried out

experiments with the prototype RF-coil and the four-layer

DOl detector. The PET performance with the 3.0T MRI and

the image quality of the prototype RF-coil in simultaneous

measurement experiments were evaluated. As a result,

sufficient performance of the PET detector and MRI imaging

function were observed in simultaneous measurements.

In the future, we will increase the number of PET detectors

and evaluate the perfonnance in other simultaneous

measurement experiments.

REFERENCES

[I] T. Tsuda, H. Murayama, K. Kitamura, et aI., "A Four-Layer Depth of Interaction Detector Block for Small Animal PET", IEEE Trans. Nucl. Sci., vol. 51, No. 5, pp. 2537-2542, Oct., 2004

[2] F. Nishikido, A. Tachibana, T. Obata, "Feasibility study for a PET detector integrated with an RF coil for PET-MRI", 2011 IEEE NSS& M1C conf., MICI3-7, 2011, Spain

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