Onsite Regeneration of Crystal Data in a 3/8” Foresight Detector Using a Digital Workflow

Abstract

Generally after the accreditation of the gamma camera, its unfailing optimal operation is guaranteed with established departmental preventive maintenance schedules. However, it is not easy to always religiously follow the same maintenance routines. Therefore, instead of preventive maintenance schedule, a reactive maintenance schedule was adopted in our department for a short span of time. In our center, the humidity level in the gamma camera room was recorded to be 90%, which was much higher than the recommended value of 50%–60%. This was due to the inundated water in the gamma camera room due to an accidental breakage of fire-extinguishing water pipeline, adjacent to the room. Postaccident, higher values of nonuniformity (both intrinsic and extrinsic uniformity) were observed. The reactive maintenance was undertaken by medical physicist and field engineer to record and confirm the crystal hydration in the detector and to restore the system for the clinical studies. The digital workflow program successfully rectified the problem of crystal hydration. It is highly recommended to perform timely reactive action which may prevent the permanent damage to the systems.

Introduction

In times of unprecedented situations, there is a temptation to eliminate the preventive maintenance, considering “Why maintain the equipment working well below the full operating potential?” or lets delay the service or perform the maintenance half yearly rather than weekly or quarterly. Other factors such as nonavailability of the maintenance staff, delay in arrival of the service engineer, nonavailability of accessory parts locally, or their delayed import further disrupt the routine preventive maintenance schedules in nuclear medicine departments. However, without the maintenance, unplanned downtime increases. Therefore, instead of preventive maintenance, a reactive maintenance schedule was adopted in our department for a short period. In the current case scenario, we present an example of reactive maintenance, where the observed humidity level in the gamma camera room was 90%, much higher than the recommended value of 50%–60%.[1,2] This was due to the inundated water in the gamma camera room due to an accidental breakage of fire-extinguishing water pipeline, adjacent to the room. On careful visual monitoring of the uniformity image, patches of nonuniformity were observed.[3] It was suspected that crystal hydration has occurred in the detector. Thus, the field service engineers were contacted. The maintenance was undertaken to record and confirm the crystal hydration in the detector and to restore the system for the clinical studies.

Technique

The single-head gamma camera (Symbia E, Siemens, Germany) room got flooded with water after accidental breakage of the main fire-extinguishing water pipeline adjacent to the gamma camera room [Figure 1]. The detector housing contained 3/8” NaI (Tl) crystal (a Foresight detector with 59 photomultiplier tubes) and was mounted with the low energy general purpose collimator. The detector was in the home position, 135 cm above the water on the floor. All efforts were made to prevent the seepage of the water into the gantry and electric tower. Later, extrinsic uniformity assessment was performed using a flat sheet rectangular flood source of Cobalt-57 (370 MBq, Featherlite, Eckert and Ziegler Isotope, CA, USA), as a part of reactive maintenance schedule. The higher values of integral (6.94%) and differential (4.61%) nonuniformity in the useful field of view were observed in the detector [Figure 2]. The off-peak imaging (confirmatory test for crystal hydration) was performed to check and verify the cause of nonuniformity.[3,4] The off-peak image with corrections was acquired with a point source of 925 KBq of Technetium-99 m (^99mTc) placed 1.6 meters away from the uncollimated detector surface, acquiring 200 million counts in 1024 × 1024 matrix size. Subsequently, three off-peak images (without the uniformity correction, energy map [Z map], and linearity correction) with 30 million counts were also acquired to check and confirm the crystal hydration in the detector. The pulse height analyzer settings were: (a) On-peak: 140 keV with window widths of 10%, (b) Up off-peak: 154 keV with window widths of 10% (28 keV window from 140 to 168 keV), and (c) Down off-peak: 126 keV with window widths of 10% (28 keV window from 112 to 140 keV), respectively. On completion, the final sets of images were visually analyzed for any hot/cold spots [Figure 3]. After analyzing the images, the field service engineer decided to run a propriety digital workflow named “Partial Reburn (kitless)” program, to rectify the problem of crystal hydration.[5] Briefly, this regeneration process consisted of performing an analog to digital converter (ADC) offset test followed by acquiring a Z map with ^99mTc and calculation of a “coefficient image” using the raw Z map and average Z map images. Finally, the “gradient corrected image” was obtained from the saved “coefficient image,” which compensated for the anomalies in the uncorrected image. After successfully running the “Partial Reburn (kitless)” program, the gradient-corrected image was permanently saved in the crystal data of the detector. Subsequently, the daily intrinsic uniformity (on peak with corrections) was performed with 30 million counts, to check for any crystal hydration spots and generate the new baseline uniformity values. The daily extrinsic uniformity test was also performed postregeneration.

Figure 1.

Postflooded water inundation in the single-head gamma camera room. Plastic sheets and absorbent sheets were used to prevent the seepage of the water inside the gantry tower

Figure 2.

Uncorrected extrinsic uniformity test using cobalt-57 sheet source, of the single-head gamma camera, showing high integral and differential uniformity values. QC: Quality control, FOV: Field of view

Figure 3.

The off-peak image (a) with the uniformity corrections shows uniform flood image. The (b) on-peak image, (c) Up off-peak and (d) Down off-peak images without any corrections shows the hydrated spots (white arrows)

Results and Discussion

Humidity levels were restored to the acceptable level (50%–60%) after proper cleaning of the inundated dirty water. The equipment was switched on, and uniformity test was conducted. The results were conveyed to the field engineer, who quickly diagnosed the problem and started the regeneration process. The off-peak image of the detector with the uniformity corrections [Figure 3a] revealed the uniform flood image, whereas the hydrated spots were clearly visualized in the flood images without corrections [Figure 3b-d]. The “Partial Reburn (kitless)” program successfully rectified the problem of crystal hydration. Daily intrinsic uniformity test showed no hydrated spots and the uniformity values were <3% [Figure 4]. Daily extrinsic uniformity values were within the acceptable range (<3%).

Figure 4.

Daily intrinsic uniformity image (after the regeneration) showing the values are within the acceptable limit. QC: Quality control, FOV: Field of view

The uniformity of NaI (Tl) crystal generally degrades with time owing to many reasons such as photomultiplier tube drift value, nonlinearity, and crystal hydration. Thus, preventive maintenance is required regularly.[6] Following the accidental breakage of the water pipeline, the presence of moisture likely have caused hydration in the detector crystal in the present study. Crystal hydration leads to the loss of the detected scintillations as the photons are either absorbed or scattered within the hydrated portion of the crystal. The loss of photons leads to the artificial reduction in the signal pulse produced from the corresponding photomultiplier tube, and hence, lower energy registered compared to the incident photon deposition.[3,6] This decreases the sensitivity and reduces the resolution of the gamma camera.[7] A hydrated crystal can also lead to the increased scan times and less statistics. Therefore, when the uniformity values exceeded the maximum integral uniformity values, the medical physicist along with the service engineers, critically performs and analyzes the various quality control tests including the off-peak test, to keep the gamma camera operational.

Various tests such as peaking, daily extrinsic uniformity test, and center of rotation test are performed routinely. Off-peak tests are recommended on half yearly basis. In the present study, we performed root mean square, after the humidity level raised above the normal recommended levels. Generally, the ADC offset values are permanently saved as forbidden files in the crystal data of the detector by the manufacturer. In the “Partial Reburn (kitless)” program, an ADC offset test was performed onsite. A new set of values was created and stored in the crystal data. Subsequently, a “coefficient image” was created using the raw Z map and average Z map images for ^99mTc. Finally, the “gradient corrected image” was obtained from the saved “coefficient image,” which compensated for the anomalies in the uncorrected image. Then, this “gradient corrected image” was permanently saved in the crystal data of the detector. This whole process helped in prolonging the life of the crystal. This process also saved a lot of money as the regeneration process was performed onsite requiring no special equipment as described earlier.[5] It was an inexpensive method to use the gamma camera for an extended period without any major repair in our setup. It further helped in avoiding the replacement of an expensive detector (the ultimate solution to the problem of crystal hydration)[7] and greatly reduced the downtime of the gamma camera, although the process was time-consuming (required almost 2 days).

Generally after the accreditation of the gamma camera, its unfailing optimal operation is maintained guaranteed with regular departmental preventive maintenance schedules.[2,4] However, in the current scenario, it was difficult to continue the same maintenance routines. Thus, reactive maintenance was adopted; medical physicists supervising and reviewing the maintenance test scan can critically assess and communicate with the field service engineers when such problem arises. In our setup, the routine maintenance was conducted, and no further patches of nonuniformity were observed in the system till date.

Limitations of the present work

First, this is a time-consuming process, as the regeneration of the crystal requires 2–3 days. Second, the computer program may correct the defects but will not fix a hydrated crystal that has decreased sensitivity and reduced resolution.

Conclusion

Nonuniform areas in the field of view can result in misdiagnosed patients and low-quality clinical images; therefore, it is crucial to perform regular checks of physical parameters (such as humidity and temperature) along with the routine quality control tests to ensure optimal performance of gamma camera. The “Partial Reburn (kitless)” program effectively rectified the problem of crystal hydration in the current situation. A gamma camera requires routine standard checks and proficient knowledge of its governing principles to make sure that its performance stays within satisfactory levels.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.