PMT, CRYSTAL, POWER SUPPLY AND DAQ OF TRANS-CAM GAMMA CAMERA MELALUI PMT, CRYSTAL, BEKALAN KUASA DAN DAQ KAMERA GAMMA TRANSCAM

Nor Arymaswati Abdullah, Glam Hadzir Patai Mohamad Dr, Mohd Ashhar Hj Khalid Dr, and Ramzah Mohamad

Pusat Instrumentasi dan Automasi, Bahagian Sokongan TeknikalMalaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor, Malaysia

arymas@nuclearmalaysia.gov.my

Abstract

Gamma camera is one of the commonly used equipment in medical especially diagnostic scanning to generate radionuclide image. A gamma camera of TransCam model manufactured by ADAC Laboratories was bought by Malaysian Nuclear Agency in 1992. The aim of this paper is to describe the hitch of the components in the TransCam gamma camera. The most of analogue and digital electronics were tested and measured including develop new ±15V power supply board as well as to validate all 37 photomultiplier tubes functionality, scintillation crystal and data acquisition card status.

Abstrak

Kamera gamma adalah salah satu peralatan yang biasa digunakan dalam perubatan terutamnya imbasan diagnostik untuk menghasilkan imej radionuklid. Kamera gamma model TransCam yang dibuat oleh ADAC Laboratories ini telah dibeli oleh Agensi Nuklear Malaysia pada tahun 1992. Tujuan kertas kerja ini adalah untuk menggambarkan masalah-masalah komponen dalam kamera gamma TransCam sehingga ia tidak boleh berfungsi. Yang paling utama elektronik analog dan digital telah diuji dan diukur termasuklah membangunkan semula papan bekalan kuasa ±15V yang baru serta mengesahkan kefungsian ke semua 37 photomultiplier tiub, kristal sintilasi dan status papan pemerolehan data.

BACKGROUND

Gamma camera is commonly utilized in nuclear medicine to trace the location and produce the image of the organs that the cells area absorbs the sugar with the intensity proportional to the metabolic activities of tissues. The sugar is marked with meta-stable radioactive atoms likeTc99. Normally, the sugar is injected in the blood flow in proximity of the area under study. The gamma photons energy emitted by the body after injection striking the crystal and generates blue light (scintillations) whose intensity is proportional to the energy loss of the gamma photons. The light photons are detected and breed in the array of PMT tubes. From the PMT tube the pulse signals are generated and amplified in the preamplifiers. The Anger type of a modern gamma camera consists of a camera-head, a gantry, an electronic processing unit for position and energy determination, counting system to measure the gamma rays activity and a computer for data acquisition as well as image reconstruction. The camera-head is closed in metal. All sides except the face are shielded by minimum of 3mm lead and the crystal is covered with a layer of 1mm (normally) aluminum to prevent external light from reaching the PMT’s and charged particles from reaching the crystal. A gantry consists of camera bow, arm system and column. The gantry renders to the user the possibility of different movements. The bow bearing on the camera enables the user to swivel the camera around the vertical axis, parallel to the collimator surface and to adjust the tilt of the latter axis. The arm system enables the horizontal and vertical movement. A collimator which is attach in front of camera head, be used to select the direction of the photons incident to the camera and will determine the geometrical field of view (FOW). It also essentially determines the spatial resolution and the sensitivity of the system. A range of collimators is required to image different photon energies and to achieve sufficient compromise between spatial resolution and sensitivity. The most commonly used are the parallel-hole, converging, diverging and pinhole collimators. These types exist as low- or middle-energy collimators depending on the required thickness of absorber.

CRYSTAL

PMT1 Preamp1

PMT2

High Voltage

Preamp2

PMT37 Preamp37

Summing amplifier X+

Summing amplifier Y+

Summing amplifier X-

Summing amplifier Z+

Summing amplifier Z-

Summing amplifier Y-

-+

-+

X Y Z X Y Z X

Y Z X Y Z

Network resistor

15

6

72

13

45

9

13

17

8

1819

10

16

11

20

14

29

2122

12

2728

23

24

25

26

37

38

34

3536

3231

30

THE TRANSCAM SYSTEM

The TransCam gamma camera was bought by Malaysian Nuclear Agency in 1990s. The device was utilized by Medical Technology Division in research of studying in medical imaging technology. The TransCam system consists of four basic subsystems: the detector, the acquisition/ correction, the gantry control and the computer system for data acquisition, counting system and image construction as well as other controlling system such as AGC and motion control. There is also a rack attached to the body of TransCam. The camera-head contains the detector assembly, consisting of a scintillation crystal, photomultiplier tubes and light guides, preamplifier, Automatic Gain Control (AGC) and a collimator, shielded with lead to reduce the detection of background radiation of the crystal. In the ideal system, the TransCam was equipped with 24V battery, which sources the 950Volt power supply used by the photomultiplier tubes (PMT) in the detector head. The vertical movements of the detector arm system were using motorized system. The horizontal movements are done by the detector head by hand, while pressing the brake switch. The detector assembly is located in the camera head and consist of 6.5mm NaI(TI) crystal, a glass light guide, an array of 37 PMTs and two printed circuit board ie. motherboard holding the preamplifiers and summing network as well as gain control (called AGC) board for the preamp. Figure 1 shows the detector block diagram.

Figure 1 The detector block diagram

The signals generated are amplified and converted to three different signals represent position (two signals) and intensity (one signal). The position is determined by the resistor network with different value, referring to the physical placement of the PMT. Table 1 shows the value of those weighting resistor. The intensity is determined by constant value of resistor, 4.99kOhm for each PMT.

Table 1 Value of weighting resistor

Figure 2 Physical placement of PMTs

PMT X Y1 0 02 0 15k3 15k 30k14 15k 30k15 0 15k6 15k 30k17 15k 30k18 0 7k509 15k 10k

10 7k50 15k11 7k50 012 7k50 15k13 15k 10k14 0 7k5015 15k 10k16 7k50 15k17 7k50 018 7k50 15k19 15k 10k

PMT X Y20 0 4k9921 15k 6k0422 7k50 7k5023 4k99 10k24 4k99 30k125 4k99 30k126 4k99 10k27 7k50 7k5028 15k 6k0429 15k 4k9930 15k 6k0431 7k50 7k5032 4k99 10k33 4k99 30k134 4k99 30k135 4k99 10k136 7k50 7k5037 15k 6k04

The detector is equipped with a package of one ¼’’ (or 6.5mm) and diameter of 365.8mm NaI(TI) scintillation crystal and one light guide of glass with an optical window of 15”(or 381mm) and all assembled in an aluminum ring. On the light guide of the crystal are placed 37 hexagonal PMTs. The physical placements of the tubes are showed in Figure 2 above.

THE METHODOLOGY

The TransCam consists of several parts as mentioned above but the remaining parts of the gamma camera were either preserved in its original shape like the camera-head, rack and the gantry. The testing was done only for the main remain components. There are power supplies, PMTs, preamplifier, crystal cleaning and part of the counting system. From the observation, AGC board could not supply the required voltage or gain to the preamplifier. Therefore these was affecting the preamplifier and summing circuit. Thus, the required gain voltage was set to the minimum of 3.8Volt by using external power supply.

A. Power supply

The TransCam rack is divided into six type module: nine data cards module, ±12Volt module, ±15Volt module, +5Volt/ ±12Volt module, high voltage power supply module and motor control indicator module. The entire module was tested one by one except for data cards and motor control indicator module. Both ±15Volt and ±12Volts module are used to supply the power to AGC module where at certain stage it was stepped down to ±8Volt and ±6Volt. The high voltage module is resourced from a battery of 24Volt to generate 950Volt. However, the ±15Volt module was broken down, and be replaced with new own build module. Figure 3 shown the ±15Volt module.

Figure 3 ±15Volt module (left) and physical placement of PMTs (right)

B. PMT

The AGC board controls the preamplifiers in such a manner that the output from each preamplifier has the same amplitude when a point source is placed on the crystal surface in the center of the corresponding PMT. The outputs of preamplifier are symmetrical. The pulse measured between out+ and out- has a peak to peak of 150mV with a Tc99/1mCu point source placed on the detector. However, Tc99 point source is difficult to obtain. In order to determine the signals level, Co60/1uCu was employed. By looking the differentiate of the frequency signal level before and after place the point source, the broken PMT and preamplifier could be determined. There are two ways to determine both type devices. The physical placements of PMTs were shown in Figure 3 above.To determine the PMT functionality, the signals were tap from the PMT output directly. The other way is each output of PMT is substituted into same preamplifier. Both methods were completed for verification purpose. The other method is by using counting system where the point source is placed for each PMT and count the signal using NIMBIN counting system.

C. Counting system

Same as others radiation detection system, besides PMTs the TransCam gamma camera consists of ADC, preamplifier, amplifier, discriminator and data acquisition system as well as PC for displaying. In testing stage, the collimator was removed in order to get better signal of 1mCu Co60. The rest parts such as a PC and most of data acquisition card are not working. Moreover, the fuse are busted once the of camera-head and data acquisition cards are connected.

+12v

+3v5

+5v

4 5 3

6 2

1

7

8

AGIN

U+U-

100R

500K TP2

TP1

The AGC board consists of a microprocessor with a RS232 interface connected with a master processor to control the gain of each preamplifier individually. In the ideal operation, the gain values can be set to 1000 from the PC which set the DAC voltage to approximately 3.8Volt. Since the AGC board is broken down, the DAC voltage is granted from direct external power supply.

a. Preamplifier

There are 37 preamplifiers on camera-head, dedicated for each PMTs. The preamplifier diagram was shown in Figure 4. To determine the preamplifier functionality, the signals were tap from each preamplifier output. The input of each preamplifier is coming from same PMTs output. Means, the point source was placed at one PMT only and the output of PMT was feed to preamplifier one by one. The frequencies of signal at certain trigger level were measured.

Figure 4 Preamplifier diagram (left) and preamplifier placement on camera-head (right)

b. Crystal scintillation

The camera-head is dissembled from the AGC board to the crystal layer. The AGC board, motherboard and PMTs were removed from the assembling location. The crystal surface was cleaned and found out there are few yellowish spot on crystal surface.

RESULT AND DISCUSSION

From the beginning, the TransCam gamma camera is not be able functioning at all. In order to make it operate at least, the power supplies for each module were tested. After replacing the ±15Volt module, the equipment can be run. The module developed is shown as Figure 5. Even the develop module is uneven but the TransCam could be operate. Then the task is continuing by tracing the signal flow from input which is produced by gamma photon to the counting system.

Figure 5 ±15Volt original module (left) and new build module (right)

The PMTs functionality testing result that only two PMTs are broken, which are number 3 and 15. The frequency level for each PMTs was shown in Table 2. All of the preamplifiers and summing circuit is in good condition and still can functioning well but the gain value for each preamplifier may need correction. It can be proved by the different value of frequency for each PMTs and preamplifiers. To build the minimum complete system, the TransCam gamma camera needs new data acquisition board, PC, motion control system and few others accessories such as foot brake and replacement of two broken PMTs.

ACKNOWLEDGEMENT

The authors are very grateful to PDC and PIA technical people by the support of this work through the project.

REFERENCES

1 ADAC Laboratories, TransCam Operation Manual, ADAC Laboratories, 1992

2 Thomas Jonasson, Revival of Gamma Camera, Thesis Royal Institute of Technology, 2003

3 I. Valastya, A. Kerek, J. Molna, D. Nova, J.Ve, M. Emri, L. Tro, A SPECT demonstrator —revival of a gamma camera, Nuclear Instruments and Methods in Physics Research A 563 (2006) 274 – 277

4 N. Giokaris, G. Loudosa, D. Maintas, A. Karabarbounisa, M. Lembesia, V. Spanoudakia, E. Stiliarisa, S. Boukisa, A. Gektinc, V. Pedash, V. Gayshan, Partially slotted crystals for a high-resolution g-camera based on a position sensitive photomultiplier, Nuclear Instruments and Methods in Physics Research A 550 (2005) 305–312