Cold spot in the uniform Co-57 image may not necessarily be due to photomultiplier tube failure or variations in photomultiplier tube tuning: A technical note

Sir,

The reason for cold spot in the uniform Co-57 image has been reported as the variation in photomultiplier tube (PMT) tuning or PMT failure.[1] In this report, we found that this problem can also be related with the electronic board that stores the correction maps (such as uniformity, energy, and sensitivity) and applies necessary correction on the acquisition data acquired during the static or dynamic acquisition with the gamma camera.

On performing the routine daily extrinsic uniformity test on Symbia E Dual Head Gamma Camera (Symbia E, Siemens Medical Solutions, Illinious, USA), we found a cold spot on gamma camera with detector head 2. The uniformity test was performed with Co-57 flood source (Co-57, Serial Number: 1717-119, 10 mCi, Reference date: 1-February-2014, from Eckert and Ziegler Isotope Products, Medical Imaging Laboratory, 24937 Avenue Tibbitts, Valencia, California 91355, USA). The uniformity image was acquired in 256 × 256 matrix size for 500 k counts on individual detector. The image we obtained is depicted in Figure 1. Initially we thought that the cold spot might be either because of failure of one of the PMTs corresponding to that position or the corresponding PMT may be out of tune in comparison to their surrounding PMTs. We later shut down the system and restarted it. Even then the cold spot persisted. Hence, the system engineer's support was taken to resolve this issue. We later performed several interventions to sort the issue. The interventions we performed and the corresponding results we obtained are elaborated in Table 1. As detailed in Table 1, The PMT tuning was performed and still the cold spot persisted. Before coming to conclusion that PMT number 57 has failed and it needs replacement, the acquisition electronic board (AEB) was replaced. After replacing another spare electronic board, the intrinsic and extrinsic Co-57 flood source image were acquired and we obtained the uniform image without the cold spot.

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Figure 1 The Co-57 flood image, 20% symmetric window, 256 × 256 matrix size and 500 k counts. A discrete, semicircular cold indentation can be appreciated in the lower region of the field of view. Semi-circular pattern delineated by borders of increased count. This was due to a defect in the electronic board which stores and applies correction factors on static and dynamic images

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Table 1 Serial interventions performed and the results obtained

The basic electronics of a gamma camera takes signals from all the PMTs and produces three signals, two of which define the X and Y coordinate of the detected gamma rays, and the remaining one defines the energy of the at event (Z). The unstable PMT leads to variation in the PMT response and non-linearity in the X, Y positioning pulses along the field of view (FOV). This can lead to non-uniform image.[2] The variation in uniformity across the FOV can also be due to the hygroscopic nature of the NaI (Tl) detector. NaI (Tl) captures the water vapor and becomes yellow leading to decrease in light transmission in that area.

The AEB is the place where the linearity correction, energy correction, sensitivity corrections are stored and are applied on the fly. The block diagram of AEB and how it communicates with the PMT and the preamplifier (Preamp) is shown in Figure 2. On the crystal, the PMT is attached, and from each PMT the signal drives to the AEB. Each detector has AEB and from both the AEB communicates with each other and from one AEB the final signal of both the detector is sent for further processing and display of the image. Detector 1 has AEB, PMT/Preamp, OEM power supply PS (receives the AC input from MEDU [board] and sends output to AEB), high voltage module (HVM) that supplies voltage to the PMT/Preamp. There is a command control which communicates with AEB and HVM to and fro, to acknowledge proper supply of voltage). PMT/Preamp output goes to AEB, information about the PMT and Preamp temperature also goes to AEB. AEB communicates with PMT/Preamp through command/control signal. On the crystal, PMT/Preamp is attached. AEB receives data through flex interconnect. AEB of detector 1, detector 2 and SNAC computer each has one Ethernet card, through which they communicate. AEB receives voltage supply from low voltage supply. Whatever detector 1 has, detector 2 also has the same equipment or whatever applies to detector 1 also applies to detector 2. Either of detector 1 or detector 2 can be made master or slave. Suppose detector 2 is a master, and then the signal from detector 1 and detector 2 will be collected at detector 2 and collectively it will be transferred to SNAC. The data that displayed on the PPM through SNAC it is also transferred to the console computer for further processing. There may be a problem with the connector available on the AEB that cannot be verified on site on the AEB level. That is why the AEB was replaced with the board.

The appearance of hot and cold spots in the flood image may be due to the variation in PMT tuning or PMT failure.[3] From Figure 2, it is clear that problem can also be in the AEB, or the cable that is driving the signal from the PMT to the AEB. This report highlights the importance of considering the problem with AEB can also be a reason for the cold spot in the uniform Co-57 image apart from the usual reasons like the variation in PMT tuning or PMT failure.

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Figure 2 Acquisition electronic board (AEB) and its communication with the PMT and preamplifier (Preamp). Light photons are converted into electronic voltage, amplified by Preamp for impedance matching, and then come to AEB, and from AEB the signal goes to further electronics for processing

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