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.
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
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.
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