Periodic Quality Control Methods for SPECT-CT

Summary

This article presents practical monthly SPECT-CT quality control (QC) methods for technicians, including background and uniformity checks, among others. These methods are intended to monitor and maintain the basic operating status of the equipment; however, they cannot replace professional maintenance, which is essential for ensuring equipment and imaging reliability.

Abstract

In the field of medical imaging, gamma cameras, single photon emission computed tomography (SPECT), and SPECT/CT are critical diagnostic tools. Quality control is essential for these devices because equipment malfunctions may lead to degraded image quality, thereby affecting physicians' accurate assessment of medical conditions. The International Atomic Energy Agency (IAEA) and National Electrical Manufacturers Association (NEMA) have recommended standardized, periodic quality control to ensure the stability of equipment operation and the reliability of medical images. By evaluating both planar and tomographic imaging modes, a more comprehensive assessment of equipment performance can be achieved, ensuring stable and accurate operation to provide reliable support for clinical diagnosis.

The importance of quality control is in its capacity to detect equipment issues early, preserve image quality, and guarantee the reliability of diagnostic outcomes. The International Atomic Energy Agency (IAEA) has established pertinent recommendations for quality control and testing of gamma cameras, SPECT, and SPECT/CT systems, emphasizing the importance of testing both planar and tomographic imaging modes of the equipment.

The quality control of medical imaging equipment plays a crucial role in ensuring high-quality images and reliable diagnostic capabilities. By monitoring equipment performance and identifying potential issues promptly, the quality control process helps maintain the image quality of medical imaging and the reliability of clinical diagnostic work. This article outlines the quality control methods employed for SPECT-CT instruments. These methods include background tests, intrinsic uniformity tests, photomultiplier tube gain adjustments, center of rotation tests, CT image quality control, image fusion tests, and other related procedures.

Introduction

Single-photon emission computed tomography-computed tomography (SPECT-CT), as a key device in nuclear medicine diagnosis, directly affects the image quality and the accuracy of diagnostic outcomes^1,2,3,4. Therefore, implementing strict equipment quality control, especially periodic quality control, is crucial for ensuring the stable operation of SPECT/CT equipment and the reliability of diagnostic results⁵. If the problems of the SPECT-CT instrument can be identified and rectified in a timely manner, it cannot only enhance the image quality and prolong the service life of the machine, but also further improve the clinical diagnostic level of the nuclear medicine department^2,6.

The importance of quality control is reflected in several aspects: firstly, it can promptly detect the degradation or failure of equipment performance, preventing misdiagnosis or missed diagnosis due to equipment issues; secondly, through regular calibration and maintenance, quality control ensures the stability and consistency of image quality, providing a reliable basis for clinical diagnosis; lastly, a comprehensive quality control system helps extend the service life of equipment and reduce maintenance costs^1,7,8.

The quality control operations detailed in this article are part of a periodic quality control program recommended by the International Atomic Energy Agency (IAEA) and National Electrical Manufacturers Association (NEMA)^9,10. These operations pertain to the performance of critical equipment and do not necessitate additional technology or equipment. They can be executed by medical technicians following the procedures outlined in this article.

Periodic quality control is the core link of SPECT-CT equipment quality control¹¹. It establishes a scientific and reasonable quality control cycle to conduct regular testing and evaluation of key performance indicators of the equipment, ensuring that the equipment is always in the best working condition^9,12. The frequency of periodic quality control should be reasonably set based on factors such as the usage frequency of the equipment, performance stability, and clinical needs, and must be strictly enforced¹³.

SPECT-CT equipment quality control, especially periodic quality control, is an important part of the nuclear medicine quality assurance system^9,13. By establishing and strictly enforcing a comprehensive quality control system, the performance stability and image quality of SPECT-CT equipment can be effectively ensured, providing reliable technical support for clinical diagnosis and treatment¹¹.

This article introduces several SPECT-CT quality control methods that medical technicians can independently implement. Furthermore, given that existing guidelines merely outline recommended quality control items without specifying the time intervals or cycles for quality control implementation, a periodic quality control method is proposed herein with reference to the latest international standards and guidelines, combined with the experience of clinical workload and the needs of equipment quality control, as well as a method for making point sources that makes the radioactive source more uniform and stable^14,15.

Protocol

1. Preparation

1. Environmental background testing of the machine room
   1. Use a digital multi-function surface contamination meter to test areas around the detector, including the ground, walls, detector shell, racks, inspection beds, and other easily contaminated locations to rule out environmental contamination (Figure 1).
2. Disassembly of the collimator
   NOTE: Prior to operation, remove any obstructions near the inspection bed and detector to ensure sufficient space for bed movement and detector rotation.
   1. Remove the collimator
      1. Use the UP/DOWN key on the top of the Remote Control Unit (RCU) to select the Collimator Exchange item in the Procedure column of the Rack Display Menu. Simultaneously press the Enable key (Side key) and SET key of the RCU (Figure 2A).
      2. When the screen displays Move table to slanted position, step on the inspection bed foot clutch (located under the side of the bed near the detector) to unlock the bed (Figure 2B). Push the bed clockwise along the ground track to the slanted-side limit point until it locks with an audible click.
         NOTE: Perform this on the opposite side of the movement direction to prevent foot injury during bed movement.
      3. Once the inspection bed is secured to the ground stop, the screen will display Place collimators cart (Figure 2C). At this juncture, it is necessary to position the collimator cart in front of the detector collimator cart guiding plate. Finally, press down the brakes on the wheels on both sides to prevent the cart from being displaced due to the inertia of the collimator and causing damage during the collimator disassembly process.
      4. Once the trolley has been fixed to the ground pin jack, the screen will display the instruction Press GO to open mechanism (Figure 2D). At this point, simultaneously press the Enable key and the GO key on the RCU.
      5. After the contouring mechanism fully opens (screen shows Unload collimators) (Figure 2E), remove the collimator using the cart handle. Release the collimator by holding open the safety lock at the detector top, repeating for the other side.
      6. When the screen displays Ensure collimators are secured on cart. Store the cart then dock cart with next collimator set (Figure 2F), step on the square pedal outside the cart's circular floor pin, lift the pin, and move the cart away.
         NOTE: Never move the cart with collimators on only one side to avoid tipping; use only the dedicated wrench for collimator handling.
   2. Installation of virtual collimator
      NOTE: The virtual collimator includes two parts for Detectors 1 and 2: transparent spare collimator plates and their metal supports.
      1. When the screen confirms the Cart in Place. Load the collimators, insert the No.1 and No.2 transparent collimator plates into the detector's collimator track. Insert the corresponding No.1 and No.2 brackets into the gap between the detector and collimator. Align the collimator and the screw holes connecting the bracket to the gantry, and turn each screw to the tightest position in order to fix the virtual collimator.
      2. Once the screen displays Collimators loaded. Press GO to close contour mechanism, press the Enable and GO keys simultaneously in order to close the collimator contour mechanism (Figure 3A,B).
      3. When the screen shows Remove Collimator Cart, slide the cart's virtual limit switch (Figure 3C) upward under the No.1 virtual collimator bracket (indicating the cart has moved out of the limit point). Press the Enable key and GO key to close the contour mechanism.
      4. When the screen displays Move table to Center position, return the inspection bed to the center limit point. Press the Enable key and GO key simultaneously to initiate the detector outward movement and the rack self-test.
      5. When the screen shows Press main PSD on one of the collimators, press the small white button (virtual pressure-sensitive device) (Figure 3D) on the innermost side of Detectors 1 and 2, followed by pressing the long innermost panel of the detector. The screen will confirm a successful virtual collimator exchange.
         NOTE: Hold the bracket handle steady before tightening the screws, and do not use the CT X-ray machine with spare collimators installed to avoid detector damage.
3. Point source production
   NOTE: This step involves the use of radioactive pharmaceuticals, necessitating appropriate radiation protection measures. During the production, movement, and utilization of point sources, radiation protection must be implemented^16,17. For the protection-related content covered in this article, please refer to the Discussion section for details.
   1. In the radiation protection cabinet, use a new 2 mL disposable syringe. Remove the needle cap and place a 5 mm cotton ball (matching the cap's inner diameter) inside the cap¹⁸ (Figure 4). Repeat this process to prepare a second point source with 1 mCi total dose.
   2. Add ^99mTcO₄^- to the cotton ball in small increments until the dose reaches 0.5-0.8 mCi (with a count rate of 20-40 kCt/s using the substitution collimator). Place the completed point source in a lead shielding container.
      NOTE: It is recommended to use a sodium pertechnetate solution with a specific activity of 10 mCi/mL to prepare the quality control source. At this activity level, a cotton ball with a diameter of 5 mm is sufficient to adsorb the required dose volume of the solution, thereby avoiding the impact on quality control effectiveness caused by excessive liquid overflow due to large volumes.

2. Quality control operations

NOTE: Quality control includes nuclear medicine (background, inherent uniformity, rotation center) and CT (tube warmup, air calibration, image fusion) tests. Close room shielding doors and restrict entry during background and inherent uniformity tests. Avoid PET drugs or injected patients outside the room to prevent result errors (See Figure 5 for the Background test interface).

1. Background test
   1. Select L in the Mode column using the RCU's up/down keys. Simultaneously press the Enable key and SET key to convert to L-Mode.
   2. On the NM interface of the acquisition workstation, select Setup, then Daily QC in the QC toolbar. Choose Background Test (Two Detectors) in the left column, check both Detector1 and Detector2, and click Start.
   3. After acquisition, click Next to generate the background test report. Use Open in Browser located in the upper-right corner to open the report in a web browser. Alternatively, click More, located in the upper-right corner, to access additional options. One such option is Save page as, which allows the user to save the report to their computer. Finally, click Save to save the report.
   4. If the report shows Passed (blue), the test is successful. If Failed (red), use the Surface Contamination Meter to identify/remove contamination, then repeat the test before proceeding to inherent uniformity testing.
2. Inherent uniformity test
   1. Keep the detector in L-Mode. Place the prepared point source at a distance greater than 2.5 m from the detector. Rotate Detector 1 (using the RCU's rotation key) to face the point source directly, so that its plane is perpendicular to the point source.
   2. On the acquisition station interface (without exiting the background test), select Image Quality (Detector1) from the left sidebar, click Apply, then Start. After completion, click Next to generate the report.
   3. Repeat for Detector2: rotate it to face the point source, select Image Quality (Detector2) in the acquisition station, click Apply, then Start. Generate the report with Next.
   4. A blue Passed indicates success. For a red Failed, adjust the photomultiplier gain for the affected detector¹⁹ (Figure 6).
      NOTE: The placement position of the point source should be at a distance ≥ 2.5 m from the detector, when the detector is at the default height for quality control operations, the connection line with the center marker point of the detector is perpendicular to any fixed position on the detector plane, such as a wall. This position is determined with the assistance of the manufacturer's engineer during equipment installation. When rotating the detector in default height L mode, the other detector is also applicable to this position.
3. Photomultiplier gain adjustment
   1. On the NM interface, click Setup, select the QC toolbar, and choose Periodic Re-tuning.
   2. Select Iter Cal under the target detector (e.g., Source positioning (Detector1)). Place the point source 2.5 m from the detector, rotate the detector to face the source, perpendicular to its plane, and click Start on the acquisition station.
   3. After program completion, select Energy Gain Calibration from the left column, click Apply for automatic adjustment. Re-run the Image Quality test. If still "Failed," contact the manufacturer's engineer (Figure 7).
4. Collimator exchange
   1. Use the RCU's up/down keys to select Collimator Exchange on the rack screen's Mode interface. Press the Enable key and SET key simultaneously. When the screen shows Move table to slanted position, step on the bed clutch pedal and push the bed clockwise to the slanted-side limit point.
   2. Once the inspection bed is secured by the ground stopper, the screen will display Place collimators cart. At this stage, slide the virtual limit switch on the cart beneath the No.1 collimator support downwards. When the virtual limit switch is engaged, the screen will show Press <GO> to open mechanism. Press the Enable and GO keys simultaneously. After the collimator contour device opens, remove the spare collimator plate and support.
   3. Position the cart with a low-energy high-resolution or general-purpose collimator at the detector's front limit point. Align cart and detector rails, step on the ground pin, press the cart rail's safety lock, and push the collimator onto the detector rails. Lock the collimator with the detector's safety lock switch, repeating for the other side.
   4. When both collimators are in place (screen shows Collimators loaded. Press to close contour mechanism), press the Enable key and GO key to close and lock the contour mechanism.
   5. When the screen displays Remove Collimator Cart move the cart away. After releasing the limit, the screen will show Move table to Center position. Return the bed to the center ground stop. Once locked, press the Enable key and GO key to initiate the detector's outward movement.
   6. When the screen shows Press main PSD on one of the collimators, press the main (front) pressure-sensitive device on the collimator panel. For Press small PSD on one of the collimators, press the small (rear) pressure-sensitive device inside the detector.
   7. The screen will confirm Collimator exchange completed successfully upon finalizing all steps.
5. Center of rotation test
   NOTE: Use a 1 mCi ^99mTcO₄^- point source. The collimator may be Low Energy High Resolution (LEHR) or General Purpose (LEGP). In the setup interface, select C.O.R QC from the QC toolbar. Choose between H-Mode (required for tomography/fusion) and L-Mode (required for cardiac L-mode acquisition) based on daily workflow.
   1. Quality control methods:COR Test H-Mode
      1. Select Source Positioning Acq. under COR Test H-Mode in the left column. Click Apply, enter the machine room, and press the RCU's Enable key and SET key to initiate H-Mode. Place the matching white tripod on the bedside near the detector.
      2. Extend the tripod's long arm to the detector center. Position the point source syringe at the arm's end, near the detector center.
      3. Adjust the point source and bed height until the source image fully enters the red circles (on both Detectors 1 and 2). Press the Enable key and GO key (or click Start on the acquisition station) to begin the test (Figure 8).
      4. After acquisition, click Next, then select COR Acquisition from the left sidebar, click Apply, and press the Enable key and GO key to start collection. Post-acquisition, select COR Processing, click Apply, then Next to process data.
      5. A blue "Passed" confirms success; a red "Failed" requires contacting the manufacturer's engineer.
   2. Quality control methods: COR Test L-Mode
      1. After H-Mode, select Source Positioning Acq. under COR Test L-Mode without repositioning the point source. Press the RCU's Enable key and SET key to enter L-Mode. Adjust the source if needed to align with the red circle, then follow the same steps as H-Mode.
6. Quality control of computed tomography (CT) images (Figure 9)
   NOTE: Ensure the room is unoccupied and shielding doors are closed during these tests.
   1. Tube warmup
      1. Perform tube warmup before the first daily CT scan or if the CT has been idle for > 2 h.
      2. Select Port 2 via the 2-PORT KVM SWITCH or click the CT icon (lower left of the NM interface). In the CT interface, select Daily Prep from the bottom toolbar, click Tube Warmup and press the blinking Scan button on the CT console to start exposure.
      3. After completion, the Daily Prep interface will show a green 02:00 in the lower left, confirming successful preheating.
   2. Fast calibration
      1. In the Daily Prep interface, select Fast Calibration. Press the blinking Scan button to initiate exposure.
      2. The program will complete the following tasks in sequence: i) Mylar Window Check, ii) Cold Warmup, iii) Warmup 1, iv) Warmup 2, v) Collimator Calibration, vi) FAP Check Scans.
      3. Post-completion, return to the initial CT interface by clicking Quit. Switch back to Port 1 (via the 2-PORT KVM SWITCH) or select NM in the upper left toolbar.
7. Image Fusion Test
   NOTE: Complete the COR test first. Use the manufacturer-provided QC phantom. The materials used in this experiment were radioactive sources of ^99mTcO₄^- with an activity level of 6 aliquots loaded in 5 mL syringes and a specific activity of 1 mCi/5mL. The preparation of the radioactive sources was completed in a radiation protection cabinet, using a sodium pertechnetate solution with a specific activity of 10 mCi/mL for dilution. After preparation, the sources were placed in a lead shielding device for transportation and storage until use. The use of radioactive sources must strictly comply with protective measures and radioactive waste disposal methods (refer to the Discussion section for details).
   1. Pre-test preparation
      1. Prepare six 10 mL syringes with 1 mCi ^99mTcO₄^- each, and let them sit for 30 min. Place the syringes into the phantom's six numbered recesses. Retract the bed to 0 position, the phantom at 70-90 on the scale (with syringes 4, 5, 6 facing the detector), and add a 15 Kg weight near the detector side (close to 0).
   2. Fusion Test
      1. In the NM interface, click Settings, select X-Ray to NM Registration under QC, then Registration Test (Tomo). Choose Registration Test Acquisition from the left sidebar, click Start, enter the room, and press the RCU's Enable key and SET key. After rack movement (screen shows GO), press the Enable key and GO key (or click Start on the acquisition station) to begin NM tomography acquisition.
      2. After NM acquisition (screen shows Press GO to CT), press the RCU's Enable key and GO key to enter the CT interface. Click the radiology icon's Confirm button, press the blinking Scan button to acquire CT images, then click CT Acq Completed (lower left) to return to the NM interface. Click Registration Test Processing to transfer data to the processing station; keep the interface open.
      3. On the processing workstation, locate the Registration file, select Tomo and CTACTomo data, choose SPECT-CT Registration QC under Miscellaneous, and click Start. After processing, return to the acquisition station, click Fetch Result, and check for a blue "Passed" (success) or red "Failed" (contact the manufacturer's engineer).

Results

Here, we propose a convenient periodic SPECT-CT equipment quality control method for medical technicians to implement. Figure 1 investigates radioactive contamination in machine room environments. Figure 2 illustrates the step-by-step process for detector replacement, Figure 3 shows the installation method of the virtual collimator, Figure 4 depicts the configuration of the point source,

Discussion

SPECT-CT represents a crucial component of the nuclear medicine department's imaging apparatus, particularly in light of the growing integration of diagnosis and treatment. The accuracy of the image, the credibility of the higher requirements, and the equipment quality control serve as a vital guarantee of accurate and reliable equipment imaging. For units that have not purchased the equipment maintenance provided by the manufacturer for various reasons, it is of particular importance for the personnel of these units...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Collimators Cart	GE Health Care	Discovery NM/CT 670	Cart for loading low-energy high-resolution collimators
Collimators LEHR	GE Health Care	Discovery NM/CT 670	Low-energy high-resolution collimator, used for filtering low-energy photons during gamma camera acquisition.
Dry cotton swab	Cixi Huabao Medical Supplies Industry and Trade Development Co., Ltd.	Sterile type 75mm	Used for making point sources, cotton swabs made of absorbent cotton are formed into balls with a diameter of about 2.5 mm for adsorbing 99mTcO4- solution.
Single-use sterile syringe	Zhejiang Longde Pharmaceutical Co., Ltd.	Single-use sterile syringe with needle 2mL	Used for making point sources, the needle cap part of the syringe serves as the carrier for the point source.
SPECT/CT	GE Health Care	Discovery NM/CT 670	The equipment being charged.
Surface Contamination Monitor	IMI International Inc.	Inspector Alert V2	Surface contamination monitoring.
Virtual Collimator	GE Health Care	Discovery NM/CT 670	Accessories used for quality control