Keywords
PET - PET-CT - daily and weekly QC - Ge-68 -
18F-FDG
Introduction
Digital PET-CT
The performance of positron emission tomography-computed tomography (PET-CT) scanners
has improved significantly over time, one of the reasons being the integration of
digital silicon photomultiplier (SiPM) detector technology.[1]
[2] Other advances include improving of the axial field of view coverage, advances in
time-of-flight (ToF) technology, spatial information through optimization of crystal
size, and other performance parameters. This hardware upgrade results in an improvement
in image quality, timing resolution, sensitivity, effective sensitivity, and reduction
in scan acquisition time as well as optimization of patient radiation dose.[1]
[2]
[3]
[4]
[5] Fluorodeoxyglucose F-18 (18F-FDG) has been the radiopharmaceutical used in most PET scans. But now along with
the advances in PET scanner technology, the variety of radiopharmaceuticals used for
diagnosis has also increased, leading to different types of imaging procedures by
PET-CT. These include 68Ga-PSMA, 68Ga-DOTATATE, 18F-DOPA, 18F-PSMA, 18F-Choline, 18F-NaF, and many more.[6]
[7]
[8]
[9]
[10]
Quality Control Procedures in Digital PET-CT
The quality control (QC) procedures for PET scanners are covered by the NEMA (National
Electrical Manufacturers Association), the International Electrotechnical Commission
(IEC),[11]
[12] and manufacturer standards as minimum requirements. The global agencies and NEMA
establish standards for imaging testing and procedural guidelines recommendations
for all manufacturers for the evaluation of PET scanner performance parameters and
the outcomes. These guidelines have also evolved over time with PET technology and
performance evolution.[13] This initial PET scanner performance evaluation is used to establish a baseline
of measurements, and then the periodic assessment of the scanners' system performance
is achieved on an annual, semi-annual, quarterly, weekly, and daily basis.[14] These QC must be carried out at regular intervals according to the specifications
of the scanner manufacturer.[15]
[16] This QC plays a critical role in reproducing accurate diagnostic results. Daily,
weekly, quarterly, half-yearly, and later periodic Q play a valuable role in monitoring
PET system performance stability and changes. Any issues and malfunctions encountered
during QC procedures must also be recorded, reported, and resolved in order to perform
a prior clinical scan.[17]
[18]
QC Radioactive Sources in PET-CT
The most commonly used radioactive long-life sealed sources are germanium-68 (Ge-68)/sodium-22
(Na-22). It is compatible and used by most PET-CT scanners. The Ge-68 and Na-22 QC
sources are available in different types such as point source, annulus phantom, and
cylindrical source. In most PET-CT scanners, the quality assurance (QA) process is
performed with a sealed cylindrical Ge-68 radioactive source to ensure optimal PET
reproducibility.[19]
[20]
[21] Ge-68 is used as a long half-life PET source for attenuation correction and calibration
of PET scanners.[22] The half-life of Ge-68 is 270.95 days, decay by electron capture gamma and X-rays:
10.3 keV (46%), 9.25 keV (25.6%), 9.22 keV (13.1%), to the daughter radionuclide gallium-68.
Ge-68 is commonly used as the cylindrical and annulus phantom of Siemens and GE PET-CT
scanners, respectively. Na-22 with a half-life of 2.6 years, decay by positron (90.2%,
1.27MeV), and electron capture (9.7%) are used as a point and disc source by Philips
PET-CT scanners.[22]
[23]
[24] The annulus phantom developed for GE Healthcare's PET-CT scanner is packed with
Ge-68 radionuclide with a radioactivity range of 37-55 MBq (1-1.5 mCi) and the source
lifetime is 2 years for replacement and optimal performance to reproduce precise results.
Similarly, the Siemens Healthineers cylindrical phantom is also filled with Ge-68
radionuclide with a radioactivity range of 74 to 92.5 MBq (2–2.5 mCi) and a source
lifetime of 1 to 2 years. Philips healthcare PET-CT scanners are designed for point
sources of Na-22 with an activity of 3.7MBq and one of the lowest radioactivity with
a lifetime of 2 years.[24] These radioactive sources used for QC procedures contribute to radiation exposure
of personnel.[25]
uMI550 Digital PET-CT
The uMI550 digital PET-CT system (Shanghai United Imaging Healthcare, China) is one
of the first digital PET-CT scanners designed considering the current challenges of
radioactive Ge-68 phantoms on a global scale with the latest technology integrated
into SiPM and hardware to meet the minimum QC requirements.[26] The design and technology of this uMI550 PET-CT system have transformed the daily
QC methodology, where the daily QC can be performed without using the Ge-68 radioactive
source phantom, and in the weekly QC, this system has been further developed to use
the radioactivity-based QC using 18F-FDG or 18F-sodium fluoride (18F-NaF) uses as an alternative option. In the standard and traditional practice in
PET-CT imaging, be it analog or digital PET-CT systems, QC of the PET system must
be ensured prior to the clinical scan to ensure the accuracy of the quantitative and
qualitative analysis of standard uptake values (SUV) measurements and to ensure reproducibility
in oncological and nononcological treatments. Most PET-CT models are manufactured
by Siemens Healthineers, GE Healthcare, Philips Healthcare, Neusoft, and Canon Medical.[24]
[25] These systems rely on daily QC using long-lived Ge-68 or Na-22 radionuclides. These
systems are limited by the daily and periodic QC reliability of the Ge-68 cylindrical
phantom source. The main challenges related to a radioactive source are (a) the cost
of procuring the shielded radioactive source, (b) the lifetime of this source is maximum
2 years and needs to be replaced every 2 years, (c) safety and shielding requirements
due their radioactive nature in accordance with local and national regulations, (d)
exposure to radiation from daily bare-handed handling, (e) the cost of returning the
expired source to the United States of America/Europe, and the country's state and
(f) regulatory permitting documentation with each procurement or return and maintenance
of these records. Due to unavoidable circumstances at the international or national
level, there will be delays in providing sources in a timely manner, and without this
PET-CT system, it cannot be used for clinical scans and will affect the patient's
diagnosis and treatment overall. With continued advances in technology and attempts
to reduce radiation dose to personnel, it is time to examine the efficacy of employing
a nonradioactive source or alternate technical method to perform daily QC procedures.
Using an alternate technical method that does not require any radioactive source for
QC procedures is essential not only in terms of completely eliminating radiation exposure
to personnel during daily QC but also in reducing departmental and logistics costs.
This study is nonetheless a valuable contribution to the field as it shares operational
and performance experience with uMI550 digital PET-CT during routine QC procedures
that do not require a radioactive Ge-68 source to perform daily QC and experience
with 18F-FDG as a substitute for Ge-68/ Na-22 source for weekly QC.
Materials and Methods
This study was conducted using the uMI550 digital PET-CT scanner installed at Medicover
Cancer Institute Nellore Andra Pradesh, India in 2020. The QC data were recorded and
analyzed from December 2020 to March 2022. This is one of the initial installations
of United Imaging Healthcare's uMI550 digital PET-CT system in the country.
uMI550 Digital PET-CT Scanner
This system is equipped with ToF and SiPM detector technology. The scanner features
a 24 cm wide axial field of view embedded in a 2.76 × 2.76 mm lutetium yttrium oxyorthosilicate
(LYSO) crystal, paired with a SiPM detector and integrated into a 40 physical rows
CT scanner and a 2.2 cm wide CT detector cover.[26] It offers some unique operational advantages such as not requiring a radioactive
source to perform the daily QC test and therefore reducing the total cost of operation
and downtime as it does not reply on long life Ge-68 source phantoms. This reduces
daily handling of Ge-68 radioactive sources compared to other digital and traditional
analog PET-CT scanners. For weekly QC testing, this system also requires a radioactive
source for QC testing, as many QC performance parameters always require the radioactive
source to verify performance. The uMI550 scanner is designed and integrated with 18F-based radionuclide data in QC operation instead of Ge-68/Na-22 to support and be
compatible with 18F-based radionuclides. These unique features result in similar operational stability
benefits without additional operational costs, and reduce radiation exposure from
7 days to once a week. This is a new concept and method of QC in PET-CT imaging. The
durability and stability of this system are observed prospectively and this method
is also compared with other PET-CT scanners. The system tolerance limit and QC test
parameters are listed in [Table 1].
Table 1
Daily and weekly QC tests parameters and tolerance limits
|
Imaging system
|
Frequency
|
Radioactive source
|
Test
|
Procedures
|
Tolerance criteria
|
|
PET
|
Daily
|
No
|
Count rate (kcps)
|
Detect and check the count of random and scatters without radioactive source
|
25.0–160.0
|
|
Daily
|
No
|
Temperature
|
System initialization and temperature measurement at hardware of detector and SiPM
for operational condition
|
14.0–37.0
|
|
Daily
|
No
|
System humidity
|
Checking humidity level within room and at hardware assembly for safe operation of
detector
|
30.0–70.0
|
|
Daily
|
No
|
Voltage
|
Checking for stable and constant voltage for SiPM safe operation within limit
|
33.00–37.00
|
|
Weekly
|
Yes
|
LUT drift
|
PET rod phantom filled with 18F-FDG, positioned to perform QC for LUT drift
|
0.00–0.46
|
|
Weekly
|
Yes
|
Energy drift
|
PET rod phantom filled with 18F-FDG, positioned to perform QC for energy drift
|
0–8
|
|
Weekly
|
Yes
|
TOF status
|
PET rod phantom filled with 18F-FDG, positioned to perform QC for TOF status
|
0.00–30.00
|
|
Weekly
|
Yes
|
CMap status
|
PET rod phantom filled with 18F-FDG, positioned to perform QC for CMap status
|
0–5
|
|
Weekly
|
Yes
|
Temperature
|
System initialization and temperature measurement at hardware of detector and SiPM
for operational condition
|
14.0–37.0
|
|
Weekly
|
Yes
|
System humidity
|
Checking humidity level within room and at hardware assembly for safe operation of
detector
|
30.0–70.0
|
|
Weekly
|
Yes
|
Voltage
|
checking for stable and constant voltage for SiPM safe operation within limit
|
33.00–37.00
|
Abbreviations: 18F-FDG, fluorodeoxyglucose F18; LUT, look-up table; PET, positron emission tomography;
QC, quality control; SiPM, silicon photomultiplier; TOF, time of flight.
Comparison with Other PET-CT Systems
The daily and weekly QC of the uMI550 system was also compared with another manufacturer
of PET-CT systems. The comparison includes the radioactive source requirement for
QC and, frequency of QC with radioactive and without radioactive sources (QC parameters
checks and workflow as shown in [Table 2]).
Table 2
Comparison with other PET-CT scanners
|
Test
|
Frequency
|
QC requirements
|
Siemens PET scanners
|
GE PET scanners
|
Philips PET scanners
|
GE PET scanners
|
United Imaging PET scanners
|
|
|
|
Valladares et al 2019[24]
|
Valladares et al 2019[24]
|
Valladares et al 2019[24]
|
Hallab 2022[25]
|
Present study
|
|
Sealed QC Source
|
Daily/Weekly
|
PET image quality and quantitative analysis reproducibility
|
68Ge cylindrical phantom sealed source
|
68Ge PET annulus phantom sealed source
|
22Na-point and disc sealed source
|
68Ge PET annulus phantom sealed source
|
No Radioactive source for daily QC/weekly QC with 18F-FDG
|
|
Daily QC
|
Daily
|
To initialize the hardware test and confirm the operational status of the PET detectors
and connected electronics, temperature, humidity, voltage, count rate, and, and system
sensors for optimal performance within the limits specified by the manufacturer
|
68Ge/68Ga cylindrical phantom: Partial PET detector setup
Normalization
Scatter ratio
Block noise
Block efficiency
E correction factor
Sinograms evaluation Measured randoms
Image plane efficiency
Scanner efficiency (E)
|
68Ge/68Ga PET annulus phantom: Coincidence (sensitivity)
Energy peak
Singles
Deadtime
Timing
Gain changes
|
22Na-point and disc source: System initialization
Hardware sensor test
PMT gain calibration
Energy
Timing
Emission sinogram
Baseline collection
|
68Ge/68Ga PET point source: Coincidence (sensitivity)
Singles
Deadtime
Timing
Energy peak
Gain changes
|
No radioactive source: System initialization
Hardware test
Count rate
Energy
Temperature
Humidity
Voltage 0 & 1 data path status
|
|
Weekly QC
|
Weekly
|
Ensuring proper calibration and validation of acquired values by using a radioactive
source phantom against a scanner reference value for optimal performance of quantification
results
|
Complete PET detector setup
Normalization
Sinograms evaluation
Block noise
Block efficiency
Measured randoms
Scanner efficiency (E)
Scatter ratio
E correction factor
Image plane efficiency
|
Coincidence (sensitivity)
Singles
Deadtime
Timing
Energy peak
Gain changes Sinograms evaluation
|
System initialization
Hardware sensor test
PMT gain calibration
Energy
Timing
Emission sinogram
Baseline collection
|
Coincidence (sensitivity)
Singles
Deadtime
Timing
Energy peak
Gain changes Sinograms evaluation
|
Inhouse rod Phantom filled with 18F-FDG (1mCi): System initialization
Hardware test
Count rate
Energy drift, LUT drift, sinogram, TOF status and CMap status
|
|
Image quality
|
Annually or major service
|
Evaluation of the reconstructed PET image for qualitative and quantitative analysis
|
NEMA IQ phantom: Contrast recovery Background variability
|
NEMA IQ phantom: Contrast recovery Background variability
|
NEMA IQ phantom: Contrast recovery Background variability
|
NEMA IQ phantom: Contrast recovery Background variability
|
NEMA IQ phantom: Contrast recovery Background variability
|
Abbreviations: 18F-FDG, fluorodeoxyglucose F18; LUT, look-up table; NEMA IQ, National Electrical Manufacturers
Association Image Quality; PET-CT, positron emission tomography-computed tomography;
QC, quality control; TOF, time of flight.
Daily QC Methodology
The daily PET QC test on this uMI550 digital PET-CT system was performed every 24 hours
according to the manufacturer's recommendations. This test was performed early each
morning before proceeding with patient clinical scans according to department protocols.
As mentioned above, according to the manufacturer's recommendation, the daily QC of
this scanner does not require a radioactive source. This system is based on a semiconductor-based
SiPM detector. The daily QC of the uMI550 system has been designed to focus on important
parameters that must be checked daily without using the radioactive source and must
be within acceptable limits. Performing the daily QC test was quick and easy and contained
only a few steps in protocol. First, the PET-CT table must be in the isocenter with
the gantry. There were no requirements for positioning any phantom. The technologist
conducts the daily QC procedure from the console room. This daily QC test was quick
and took about 10 minutes and generated a result report. For this scanner, the daily
QC check parameters are system temperature and humidity as SiPM detectors in the digital
PET-CT system take these two into account, the system count rate using the background
activity, the data link status, and the voltage stability of SiPM. Since no radioactive
source is required, therefore there is no radiation exposure to the operator.
Weekly Rod Phantom and QC Methodology
The uMI550 digital PET-CT requires the weekly QC with a radioactive source similar
to other manufacturers' PET-CT systems as shown in [Table 2]. This scanner was QC compatible with 18F-FDG (18F-based radionuclide), eliminating operational costs and downtime. During the weekly
QC test, the system checks the key parameters that need to be calibrated with a radioactive
source, and these parameter values need to undergo QC once a week. This QC test requires
(a) the preparation of rod phantom, (b) the positioning of QC phantom on the scanner
table, (c) the QC procedure command on the console system, (d) the QC report, (e)
and removing the rod phantom source.
Rod Phantom Preparation
This system was equipped with a compact rod phantom to perform weekly QC with 18F-FDG as an alternative to the standard sealed Ge-68 source phantom. The volume capacity
of this rod phantom is 80 to 85 mL and this phantom can be manufactured in a PET radiopharmacy
laboratory using 18F-FDG/NaF radioactivity. The amount of radioactivity was 37 MBq ± 20% (1 mCi). The
rod phantom was prepared and first filled with normal water to about 95% of the volume
capacity and about 37 MBq was taken from the 18F-FDG vial behind the lead-shielded L-bench and put into the phantom, later the phantom
was closed and the proportions shuffled four to five times, then the remaining space
was filled. The preparation time of this phantom was less than 3 minutes and this
phantom resulted in approximately nine times less radiation exposure to the technologist
when positioning and performing weekly QC compared to a standard whole-body PET-CT
scan in which the patient was injected with 370 MBq (10 mCi). Rod phantom is shown
in [Fig. 1].
Fig. 1 Rod Phantom for weekly quality control.
Weekly QC Procedure
The rod phantom was prepared in radiopharmacy laboratory. After preparation, the technologist
brought the phantom into the scanning room and positioned it on a dedicated bracket
on the table according to the instructions in the manual provided by the manufacturer.
Positioning of the phantom and scanning of the isocenter with the laser is shown in
[Fig. 1]. After positioning the phantom in the PET isocenter on the couch, the technologist
performed the weekly QC test according to the protocol in the control panel system.
The procedures for conducting QC were similar to other manufacturer's systems. In
the weekly full QC test, the checking parameters are the look-up table (LUT) drift
of the LYSO crystal, detector performance, the energy drift, the ToF status of the
time resolution performance, the C-map status, humidity, temperature, voltage, and
sinogram map as shown in [Table 3]. This weekly QC test took about 20 minutes and a QC report was generated. It checks
the acquired values with reference values of the system.
Table 3
Weekly quality control results
|
LUT drift
|
Energy drift
|
TOF status
|
CMap status
|
Temperature
|
System humidity
|
Voltage 0
|
Voltage 1
|
Result
|
|
Sl no./Normal range
|
(0.00, 0.46)
|
(0,8)
|
(0.00, 30.00)
|
(0, 5)
|
(14.0, 37.0)
|
(30.0, 70.0)
|
(33.00, 37.00)
|
(33.00, 37.00)
|
P: Pass
F: Fail
|
|
1
|
0.1
|
0
|
15.8
|
0
|
High: 31.3 Low: 27.9
|
44
|
34.98
|
34.98
|
P
|
|
2
|
0.12
|
1
|
15.4
|
1
|
High: 31.8 Low: 26.7
|
38
|
34.44
|
34.44
|
P
|
|
3
|
0.15
|
0.5
|
13.8
|
0
|
High: 33.3 Low: 21.5
|
39
|
34.58
|
34.58
|
P
|
|
4
|
0
|
0.2
|
14.4
|
2
|
High: 30.2 Low: 22.4
|
42
|
36.22
|
36.22
|
P
|
|
5
|
0.11
|
0.3
|
15.4
|
1
|
High: 29.7 Low: 26.3
|
41
|
38.84
|
38.84
|
P
|
|
6
|
0.12
|
0
|
15.2
|
0
|
High: 33.7 Low: 24.1
|
47
|
33.22
|
33.22
|
P
|
|
7
|
0.1
|
0.1
|
14.8
|
0
|
High: 28.5 Low: 20.1
|
43
|
34.68
|
34.68
|
P
|
|
8
|
0.14
|
1
|
14.2
|
1
|
High: 34.4 Low: 26.8
|
44
|
34.98
|
34.98
|
P
|
|
9
|
0.1
|
0.2
|
12.8
|
0
|
High: 32.2 Low: 20.6
|
43
|
34.66
|
34.66
|
P
|
|
10
|
0.13
|
0.2
|
16.6
|
1
|
High: 30.8 Low: 23.5
|
42
|
36.42
|
36.42
|
P
|
Abbreviations: LUT, look-up table; TOF, time of flight.
QC Report
After completing the weekly QC procedures on the console system, the results of the
QC report showed pass or fail or warning, and a pdf report was also generated with
detailed information on QC parameters. Weekly QC parameter consistency was observed
with 18F-FDG over the period. Fluctuations in QC parameters were also noted. A delay in QC
operation was also observed due to the radioactivity of the rod phantom. All weekly
QC parameters are listed in [Table 3].
After the successful completion of the weekly QC, the radioactive rod phantom was
removed from the couch and placed in the lead-protected decay container. The scanner
was prepared for clinical use. Since the 18F-FDG radioactivity used for weekly QC was low and had a short half-life, it decayed
within a few days and the phantom can be reused after a week to ensure complete decay
of 18F-FDG radioactivity.
Result
With this scanner, over 200 daily QC tests without a radioactive source Ge-68 phantom
and 50 full weekly QC tests using an 18F-FDG rod phantom were performed and a test report was prepared according to the manufacturer's
instructions. [Table 1] shows the daily and weekly QC test parameters and their tolerance limit according
to the manufacturer's system manuals. [Table 4] shows the daily QC results and [Table 3] shows the weekly QC results. [Fig. 1] shows the rod phantom used for weekly QC. [Fig. 2] shows a sinogram image and [Fig. 3] shows detector block and single map images with a captured rod phantom filled with
radioactivity. [Figs. 4] and [5] show the consistency of the daily and weekly QC test parameters.
Fig. 2 Sinogram.
Fig. 3 Block image. CT, computed tomography; PET, positron emission tomography.
Fig. 4 Daily quality control test parameters consistency.
Fig. 5 Weekly quality control test parameters consistency. LUT, look-up table; ToF, time-of-flight.
Table 4
Daily quality control results
|
Count rate (kcps)
|
Temperature
|
System humidity
|
Voltage 0
|
Voltage 1
|
Data path
|
Result
|
|
Sl no./normal range
|
(25.0, 160.0)
|
(14.0, 37.0)
|
(30.0, 70.0)
|
(33.00, 37.00)
|
(33.00, 37.00)
|
Status I
|
P: Pass
F: Fail
|
|
1
|
High: 37.4 Low: 32.2
|
High: 31.1 Low: 28.2
|
39
|
34.98
|
34.98
|
Status I
|
P
|
|
2
|
High: 39.2 Low: 33.5
|
High: 32.4 Low: 24.5
|
41
|
35
|
35
|
Status I
|
P
|
|
3
|
High: 37.8 Low: 29.3
|
High: 33.1 Low: 23.2
|
42
|
35.86
|
35.86
|
Status I
|
P
|
|
4
|
High: 42.4 Low: 31.8
|
High: 30.2 Low: 23.1
|
38
|
34.86
|
34.86
|
Status I
|
P
|
|
5
|
High: 39.5 Low: 30.6
|
High: 27.5 Low: 20.4
|
36
|
34.42
|
34.42
|
Status I
|
P
|
|
6
|
High: 36.9 Low: 33.5
|
High: 26.2 Low: 19.4
|
43
|
35.32
|
35.32
|
Status I
|
P
|
|
7
|
High: 37.2 Low: 28.9
|
High: 25.7 Low: 19.8
|
44
|
35.54
|
35.54
|
Status I
|
P
|
|
8
|
High: 35.3 Low: 28.7
|
High: 25.1 Low: 19.4
|
37
|
34.46
|
34.46
|
Status I
|
P
|
|
9
|
High: 41.1 Low: 34.2
|
High: 24.5 Low: 18.9
|
48
|
36.22
|
36.22
|
Status I
|
P
|
|
10
|
High: 32.7 Low: 28.5
|
High: 28.5 Low: 20.8
|
45
|
34.44
|
34.44
|
Status I
|
P
|
Discussion
QC of medical imaging equipment is vital to ensure their proper functioning and to
attain accurate quantitative results and stability throughout the life of the device.
Daily and weekly QC tests play a critical role in monitoring the performance and consistency
of the baseline values and their deviation and whether the deviation values are relevant
to examine and rectify them at the service level to maintain the baseline values.
Routine monitoring of QC results also means the deficiency is identified at an early
stage and remediation can be performed without interrupting routine clinical scans.
Over the decades, the daily QC of the PET scanners have been performed utilizing the
radioactive source. The radioactive source for daily QC is broadly a sealed source
of either Ge-68 or Na-22 and has a half-life of 1 or 5 years. The usage life relies
on the minimum radioactivity in the source according to the manufacturer's recommendations
for optimal performance and an accurate result. Due to their radioactive nature, these
radioactive sources require regulatory and environmental agency approval before procurement
and use. There is a substantial amount of documentation, testing, and security to
manage these sources during transport or use within the clinical facility. These procedures
also place an economic burden on the facility and delays in utilizing the PET-CT clinical
scan equipment.[1]
[16]
[22]
[23]
[24]
[27]
[28] Dealing and handling of radioactivity, either for QC or for clinical scans in nuclear
medicine, lead to radiation exposure of the operating radiographer or technologist.
Performing daily and weekly QC is one of the unavoidable situations where the operator
needs to hold the phantom source filled with radioactivity by hand and position it
on the PET scanner. This manual handling of radioactivity during the positioning of
the phantom in the PET scanner leads to radiation exposure of the operator. In the
present study, the manufacturer's established protocols for periodic QC testing were
followed and performed. The novel approach of daily QC testing without a radioactive
source was performed and the results were recorded for 15 months. Several international
bodies and manufacturers have published QC guidelines and recommendations for a PET
imaging modality to ensure adequate performance of the systems to achieve accurate
qualitative and quantitative outcomes in clinical practice, as well as the frequency
of these tests.[1]
[16]
[22]
[23]
[24]
[27]
[28] QC as a segment of a QA program aims to monitor that a system is performing as expected.
In order for the PET part to function properly, the detectors and measuring electronics
must function properly. By checking detector stability, sudden fluctuations in the
detector assembly can be diagnosed early. Review of energy window, LUT, and timing
and ToF status results in optimal performance of quantification parameters such as
SUV and metabolic tumor volume. Equally important are the other parameters such as
temperature, voltage, and humidity work in the optimal range. Optimal SiPM performance
depends heavily on temperature, voltage, and humidity. Normalization is one of the
most important corrections to overcome the inconsistency between the efficiencies
of each detector and maintain the uniformity of the final reconstructed PET images.
In the present study, daily QC was carried out using a new technique from the manufacturer.
Over 200 daily QC tests were conducted and recorded without using a radioactivity
filled Ge-68 phantom or any other radioactive source. No daily QC errors or warnings
were observed during this period. All daily QC parameters collected were recorded
to be within the range according to the manufacturer's baseline values listed in [Table 1]. All recorded values were found to be consistent with baseline. Over 50 weekly QC
tests have been performed and recorded using this scanner. The weekly QC of this scanner
also requires the radioactivity to perform the QC test. The uMI550 PET-CT scanner
is capable of performing weekly QC with 18F-FDG energy, while other manufacturer's scanners are activated using Ge-68 energy.
Since this scanner is also compatible with 18F-FDG radionuclides, the manufacturer has provided one dedicated inhouse rod phantom
for weekly QC. All weekly QC testing was performed by using this in-house rod phantom.
The radioactivity required to perform QC with 18F-FDG is 37 MBq (1 mCi). No weekly QC test failures were observed and reported. All
recorded values were consistent and within the range of baseline values according
to the manufacturer's equipment manual. When compared with the QC test results of
PET-CT scanners from Philips, Siemens, and GE Healthcare,[1]
[16]
[24] the QC tests of these scanners were found to be dependent on a Ge-68 and Na-22 source,
which must be procured to carry out the QC ([Table 2]). Any source or phantom use filled with radioactivity must be approved by the national
regulatory agency (in India, Atomic Energy Regulatory Board [AERB]) that entails various
application procedures and pre-import approvals, as well as financial costs to safely
transport the radioactivity.[29] The uMI550 PET-CT scanner comes with an in-house rod phantom and eliminates the
need for approval from regulatory and environmental authorities to procure the sealed
radioactive source for QC. This results in commercial as well as operational advantages
over other PET-CT scanners. In our experience, performance was extremely positive,
implementation was easy, and time and money were saved on both daily and weekly QC
procedures. These daily QC procedures were compared with Hallab et al[16] they have reported the daily QC procedures of their GE PET-CT scanner using a radioactive
Ge-68 point source. This Ge-68 point source was shielded and attached to the back
of the gantry with a shield. In QC mode, the point source automatically extends and
performs QC. This QC method also does not involve any radiation exposure to operating
staff. The limitation was that it used to take up a lot of space in the gantry. This
method is no longer available with the latest PET scanners. The latest GE PET scanner's
QC was published by Valladares et al.[24] It was reported that PET-QC was performed using a Ge-68 annulus phantom. This QC
procedure requires manual handling of the annulus phantom and positioning in the PET
scanner to perform daily QC. Similarly, Matheoud et al[1] reported PET-QC in a Siemens PET scanner using a cylindrical Ge-68 phantom. Compared
to other studies, the present method was found to be unique in eliminating operator
radiation exposure during daily QCs.[1]
[16]
[24]
Limitation of Study
This is a single-center study using the uMI550 PET-CT system. Further multicenter
studies could be conducted on multiple scanners with comparable performance range.
Conclusion
This study shares operational and performance experiences with the new approach of
the uMI550 digital PET-CT for daily and weekly QC procedures. Daily QC procedures
do not require any radioactive source and are simple and quick to perform. Weekly
QC with this scanner is performed using an in-house with 18F-FDG source compared to the traditional sealed Ge-68 source. This translates into
commercial and operational benefits for the operators and center.
Implications for Practice
The uMI550 digital PET-CT helps reduce operating cost, reduce downtime as it does
not rely on the Ge-68 source; it also reduces radiation exposure to operational personnel
during daily QC procedure.
