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Nuclear Medicine Instrumentation and Quality Control: A Review
Vesper Grantham, MEd, RT(N), CNMT
*Assistant Professor, College of Allied Health, The University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma.
Address correspondence to: Vesper Grantham, MEd, RT(N), CNMT, Assistant Professor, College of Allied Health, The University of Oklahoma Health Sciences Center, PO Box 26901/CHB-451, Oklahoma City, OK 73190. E-mail: firstname.lastname@example.org.
Radiation technologists are presented with a variety of detection needs, requiring a thorough understanding of radiation detectors. Proper operation of the detectors is maintained by scheduled quality control checks and troubleshooting, which are unique to each radiation detector. Knowledge and awareness of the various radiation detection instruments and their quality control procedures are imperative to follow radiation safety practices, provide adequate patient care, and produce accurate diagnostic exams.
Radiation detectors consist of a variety of devices used to detect radiation from a specific region of the electromagnetic spectrum. In nuclear medicine, the patient is the source of the radiation after receiving a radiopharmaceutical particular to the nuclear study. In addition to imaging, a technologist may have a variety of detection needs; therefore, a complete understanding of radiation detectors is required.
These detection needs include:
- Measuring the activity of a patient's dose
- Measuring the radiation exposure of a material or room
- Measuring/imaging the amount of activity within a patient
- Measuring the amount of activity within a sample
Proper operation of the detectors is maintained by scheduled quality control checks and troubleshooting. Quality control procedures are unique to each radiation detector and will be outlined in this article. Published guidelines specify that quality control of equipment must be performed routinely and documented. These guidelines are issued by several organizations, including the Society of Nuclear Medicine,1 the American Society of Nuclear Cardiology,2 the American College of Radiology,3 the Joint Commission on the Accreditation of Healthcare Organizations, and the Nuclear Regulatory Commission, in addition to agreement states and other radiology societies. Specific procedures required for each instrument may be further defined by the manufacturer and the National Electrical Manufacturers Association. In addition, current accreditation bodies indicate the minimum quality control to be performed on equipment; these bodies include the American College of Radiology3 and the Intersocietal Commission for the Accreditation of Nuclear Medicine Laboratories.4
Survey Meter (Geiger-Mueller Detector)
The survey meter, also known as the Geiger-Mueller detector or Geiger counter, is a type of gas-filled radiation detector. The sensor is an inert gas-filled tube (usually helium, neon, or argon with halogens added) that briefly conducts an electrical current when a particle or photon of radiation temporarily ionizes the gas.
The tube amplifies this conduction by a cascade effect, known as gas amplification, and outputs a current pulse, which is then displayed by a needle or lamp and/or audible clicks. Because of the gas amplification phenomenon in the detector, the survey meter is a very sensitive instrument to detect small amounts of radiation exposure. Nuclear medicine technologists typically use the meter to determine whether there is radiation contamination. The exposure reading on the survey meter is given in roentgens per hour (R/h) or milliroentgen (mR/h) in low-exposure settings typical to nuclear medicine.
The battery check, sealed source check, and calibration are 3 different quality control procedures performed to ensure proper operation of the survey meter (Table 1).5
The battery check is performed daily by the nuclear medicine technologist and assesses the sufficiency of the battery powering the instrument. The meter located in the readout scale will be located within the battery region if the power is adequate.
The nuclear medicine technologist also performs the sealed source check to assess the sensitivity and consistency of the meter. The probe is placed directly over a sealed source to measure the exposure. This exposure reading is compared with the annual calibrated source reading. These readings should be within ± 10% of each other.
The last quality control procedure is the annual survey meter calibration. This procedure is usually performed by the physicist by placing the meter in front of a high-activity cesium sealed source and exposing the meter. The source should produce an exposure reading of at least 30 mR/h at 100 cm. The exposure reading that should be read by the meter is calculated using the inverse square law. The exposed reading should be adjusted to read the same as the calculated value, or within ± 10%.6
The dose calibrator is a unique type of gas-filled radiation detector that detects the exposure of a radiation source. Once detected (in a syringe or vial), the dose calibrator converts it to units of activity in curie or becquerel (Ci or Bq), based on the radionuclide's gamma constant. To ensure the proper operation of the dose calibrator, 4 quality control procedures must be performed: accuracy, constancy, linearity, and geometry (Table 2).
Accuracy is a quality control measure performed annually, and is an assessment of the validity of the calibrator's activity reading compared with the activity of a calibrated sealed source. Two sealed radiation sources greater than 50 µCi and one with an energy between 100 to 500 keV are assayed 3 times each and averaged.6
The average activity readings for the sources are compared with the decay corrected calibrated activity. The decay equation is used for the correction, AT=A0e(-0.693T/T1/2); AT = activity after time T, A0 = initial activity, T = elapsed time, and T1/2 = half-life. The calculated activity and the average activity reading should be within ± 5% of each other.6
Constancy is performed daily by the nuclear medicine technologist and assesses instrument reliability from day to day. A cesium-137 sealed radiation vial (or other γ-emitting sealed radiation source) greater than 50 µCi is placed in the calibrator well. The radionuclide activity is recorded by the technologist and compared with prior day activities and the decayed accuracy readings to ensure acceptability. The daily constancy readings should be between ± 5% of the decay corrected accuracy readings.6
Linearity procedures assess the instrument's ability to measure a range of low-activity doses to high-activity doses accurately and are performed quarterly. A dose of a high-activity, short-lived radionuclide is used and assayed over a given period. Actual measures are compared with calculated decayed activities and should be within ± 5%.6
The Calicheck System may also be used to perform the linearity procedure in a shorter period of time. With the Calicheck System, lead attenuation sleeves are used to simulate decay of the radionuclide.
Geometry is a quality control procedure performed during installation and only performed during acceptance testing or if the calibrator is relocated or repaired. Geometry ensures the ability of the instrument to accurately measure activities in different configured containers such as a syringe, vial, or pill. A given amount of radionuclide is assayed in a syringe and the activity is recorded. Next, small increments of saline are added to the syringe to increase the volume and the activity is measured. The activity should remain fairly consistent regardless of the changing volume, again within ± 5%.6 Often, this procedure is performed with all the dose configurations used in the department.
Well Counter and Uptake Probe
A scintillator is a material that converts energy lost by ionizing radiation into pulses of light when they absorb radiation. In most scintillation counting applications, the ionizing radiation is in the form of X rays, γ rays, and α or β particles ranging in energy from a few thousand electronvolts to several million electron volts (keV to MeV).
The scintillation well counter and uptake probe are actually 2 detectors with the ability to be configured to a single computer. They are solid sodium iodide crystal detectors demonstrating high efficiency with γ rays. The well counter counts patient samples such as blood, urine, and other radiation samples. The uptake probe counts activity within the patient (typically the neck region for thyroid uptakes).
Disintegrations or disintegrations per minute (dpm) being emitted by the radioactive source are measured in units of counts or counts per minute (cpm). The counts from the detector are often used to calculate a physiological event such as thyroid uptake or dose excretion.
To ensure the proper operation of the well counter and uptake probe, there are 5 different quality control procedures (ie, calibration, sensitivity/constancy, efficiency, chi-square, and energy resolution) that must be performed (Table 3).
Calibration. The nuclear medicine technologist calibrates the operating voltage daily with the press of a button.
Sensitivity/constancy. This daily measure is performed by the nuclear medicine technologist and assesses the instrument's consistency from day to day. A cesium-137 sealed source is placed in the well or in the front of the probe, depending on which detector is being used. The counts are recorded by the technologist and compared with the counts of the previous day's recordings. These values should remain fairly consistent.
Efficiency. Efficiency is performed quarterly and evaluates the ability of the instrument to detect radioactive disintegrations. It is impossible to measure 100% of emitted radioactive disintegrations, but there is an expected efficiency rate for each instrument. This is calculated by taking the counts per minute detected by the instrument and dividing by the actual disintegrations per minute from the source (efficiency = cpm/dpm x 100).7 The disintegrations per minute are calculated based on the activity of the source. The calculated efficiency is compared with the manufacturer's specifications.
Chi-square. Chi-square is a statistical quarterly assessment of the instrument. Ten separate counts are taken for each sample. Although variation in the counts is anticipated, after 10 counts, there should not be too little or too much. The variation is detected by the chi-square equation and determined if they are within acceptable limits. This function is performed with the press of a button in modern scintillation detectors.
Energy resolution. This measure is performed annually and determines the ability of the instrument to discriminate between different energies. Typically, the instrument will automatically calculate the energy resolution when the calibration is performed, and is based on the formula (full width at half maximum [FWHM]/peak energy (keV)] x 100). Ideally, the lower the energy resolution, the better.7
The scintillation camera is used in nuclear medicine imaging and is also known as the Anger or gamma camera. The majority of imaging performed in general nuclear medicine is performed with the gamma camera. Scintillation occurs when γ photons emitted from the source or patient interact with the sodium iodide crystal to produce light.
The primary components of the scintillation camera include the collimator, scintillation crystal, photomultiplier tube, positioning logic network, pulse height analyzer, and display.
In nuclear medicine, patients are the source of radiation after being injected with a radiopharmaceutical. The γ photons exit the patient and pass through the first component of the camera, the collimator. The collimator ensures that the image will record only photons moving directly from the organ to the crystal.
After passing through the collimator, the photons interact with the sodium iodide crystal detector and produce pulses of light that are detected by the photomultiplier tubes. The tubes convert the light into electrical signals and amplify these signals. These signals are transferred to the positioning logic network, where their original location in the patient is determined. The pulse height analyzer determines which detected photons will contribute to the final image based on the respective energy of the signals. These energies should fall into the energy window set by the technologist based on the specific radionuclide used.
Scintillation camera quality control
Seven different quality control procedures are necessary for a general nuclear medicine camera: energy peaking, uniformity, efficiency, resolution, linearity, high calibration flood, and collimator integrity (Table 4).3,4
Energy peaking. Daily, the technologist checks the peak of a known radioactive source by the camera's energy spectrum. The energy peak of the camera should correlate with the peak counts of the radionuclide source.
Uniformity. Uniformity is a daily assessment to measure the camera's ability to produce uniform images of a uniform source or accurate images. A uniformity flood source of Tc-99m or Co-57 is placed on the detector of the camera and an image is taken. To ensure acceptable uniformity, quantitative analysis is performed. Quantitatively, the uniformity should be below 5% and preferably in the range of 3% with today's camera abilities.
Efficiency. This analysis can be performed in combination with uniformity. Efficiency assesses the ability of the instrument to detect any radioactive disintegration emitted. When analyzing the efficiency in combination with the uniformity, the time can be noted and compared with previous uniformity floods. When analyzing the efficiency separately, a known amount of activity is counted and the activity converted to disintegrations per minute. The efficiency is calculated by dividing the counts per minute imaged by the calculated disintegrations per minute and multiplying by 100. Efficiency = (cpm/dpm) x 100.
Resolution. Resolution is performed weekly and assesses the camera's ability to produce image detail and sharpness. This test is performed similar to the uniformity test; however, a resolution bar phantom is placed between the camera detector head and uniformity source to produce the resolution image. The images are assessed qualitatively to evaluate the resolution acceptability.
Linearity. Linearity is performed with resolution and assesses the images for horizontal and vertical line straightness or linear lines.
High calibration flood. A high-count flood (100 million counts or greater) is used as a calibration source for the system's images. This high-count flood is applied to static, dynamic, and single photon emission computed tomography (SPECT) images to improve image quality and decrease nonuniformities inherent in the system.
Collimator integrity. Collimator integrity is performed annually (at minimum) by comparing the extrinsic and intrinsic uniformity floods. The collimator is inspected visually for damage such as dents.
Single Photon Emission Computed Tomography
Most nuclear medicine cameras have the ability to take 2-dimensional (2-D) images as the detector rotates a full 360º around the patient. This ability describes SPECT capabilities. These multiple 2-D images are summed together by reconstruction software to produce a 3-dimensional (3-D) image and tomographic slices. This imaging technique allows radiologists and physicians a better view of the patient because of the sliced and 3-D imaging capabilities.
In addition to general quality control measures, there are 2 key quality control procedures performed to assess the imaging device for SPECT capabilities. The center of rotation (COR) is a measure performed weekly to ensure that the center field of view of the camera detector matches with the computer software. A small point source is imaged with SPECT acquisition and then analyzed quantitatively by the computer. This analysis will assist the technologist to determine whether or not the COR is acceptable. The SPECT resolution is performed quarterly with a SPECT phantom filled with a small amount of radionuclide and diluted with water. Images of the phantom are taken and processed, and qualitatively assessed for uniformity, linearity, and resolution.
Positron Emission Tomography Camera
The majority of nuclear medicine detection involves single γ-emission imaging. Alternatively, positron emission tomography (PET) uses paired 511 keV γ photons that are generated from an annihilation reaction. The paired 511 keV photons travel in opposite directions, 180º apart. Instead of using a collimator, these 2 photons are used to produce an image. Once the photons are detected by the ring of crystals circling the patient, the technologist performs the reconstruction of the images, producing tomographic slices of the patient and a 3-D image.
Positron emission tomography camera components
Although similar to a scintillation camera, the PET system has several key differences (Figure).8 An important distinction is that PET cameras do not use collimators, but rather use the annihilation reaction mentioned prior to produce images. Additionally, instead of a single sodium iodide crystal for photon detection, these systems use hundreds of detector crystals. Crystals are organized into buckets with several photomultiplier tubes per bucket. The arrangement of the buckets in 360º rings in the gantry give the camera improved resolution and sensitivity over that of traditional scintillation camera systems.
Positron emission tomography camera quality control
A summary of general quality control procedures is presented here. These procedures can be quite extensive and vary between institutions and manufacturers (Table 5).4
Blank scans. Blank scans are performed daily and provide accurate transmission scan information for the attenuation correction derived from external radioactive sources. This scan represents the sensitivity response of the transmission source without any attenuating material present in the gantry. The blank scan is used as a uniformity reference to account for the radioactive decay of the transmission source, deadtime corrections, and detector sensitivity fluctuations. The daily acquisition and analysis of the blank scan or sinogram is important because attenuation correction based on poor or outdated blank scans will introduce artifacts to the patient emission data. Typical analysis of the blank scan includes reconstruction of the sinogram, which demonstrates projection data between each pair of detectors operating in coincidence. The sinogram is evaluated for "streaks," which indicate problems with the detector(s), signal cable(s), high-voltage drift, and/or gain changes, to name a few. Some systems evaluate the sinogram quantitatively by producing a measurement of detector uniformity. The blank scan has been compared with the daily uniformity flood used for the gamma camera.
Normalization. Normalization calibrations are performed quarterly to measure the efficiency for all the detector projections in the system. This is performed by rotating low-activity rod sources and acquiring data that will be used to balance the efficiency between the detectors. This type of calibration is very similar to the high-count uniformity correction used on the gamma camera and is used during image reconstruction.
Absolute activity calibration. Absolute activity calibration factors are used to convert pixel values into a measure of absolute activity per voxel. A known amount of activity is diluted with a known volume of water into an imaging phantom. The phantom is imaged, reconstructed, and processed into a set of correction factors. These correction factors allow the conversion of a patient scan into a representation of the percentage of injected dose per volume or gram of tissue. This quality control is important for the system's determination of standard uptake values of tissue and tumors; it should be performed quarterly.
In addition to the quality control procedures mentioned, a system will require other operation calibrations. These may include, but are not limited to, energy window calibrations, gain settings, and coincidence timing calibration.9
Gamma surgical probes are used in a variety of radioguided surgical procedures, primarily focusing on the treatment of malignant disease. After administration of a radionuclide such as technetium-99m or indium-111, surgical intervention using the gamma probe can assist in dose localization. The probe can be used to localize malignant tumor sites that have accumulated the radiopharmaceutical or, more often, localize the sentinel lymph nodes, the first lymph nodes draining the lesion site.10 Manufacturers have successfully developed and introduced several different types of γ probes, each designed to address the variety of technical difficulties experienced in clinical environments.
The tip of the probe contains a Cs crystal capable of detecting γ-ray emissions. The probe is designed to detect small quantities of radionuclide uptake in the high scatter, highly variable background environments, which assists the surgeon in differentiating areas of true radionuclide uptake during exploration.
The analyzer shows counts using a digital readout and an auditory signal. The frequency of the auditory signal is directly proportional to the level of radioactivity detected.
There are a variety of collimators that can be used to vary the probe's field of view from wide (for surveying) to narrow (for probe-guided resection), and can be matched to suit tissue/background levels. The radiation shielding and collimation around the detector provide a high degree of spatial resolution and protection from scatter radiation while preserving sensitivity, allowing for more precise localization.
For quality assurance, the system includes a radioactive check source containing the radionuclide Co-57.
Surgical Probe Quality Control
Procedures performed with a Co-57 radioactive source with activity between 5 to 25 µCi are recommended by the manufacturer (Table 6).
The power should be confirmed prior to each use to ensure the battery and/or electrical powering device is working sufficiently.
Also prior to each use, the constancy of the surgical probe should be checked. This is performed by measuring a known radioactivity sealed source and determining whether the counts are consistent from previous measurements or accurate with the calculated source activity.
A background measurement should be performed prior to each use to ensure the probe is registering minimal counts (close to 0) when not in the proximity of any radioactivity (patient or sources). All tests should also be performed after repair.
In addition to the imaging and radiation detectors, technologists use a variety of other equipment that also requires quality control. This equipment includes the glucometer, defibrillator/crash cart, xenon trap machine, and nebulizer. A brief explanation of each and its quality control procedures are provided.
A glucose reading is necessary prior to the administration of a PET radiopharmaceutical. The glucometer is used to determine the patient's glucose reading. Most glucometers require daily calibration and accuracy checks to ensure accurate patient glucose readings.4
Every department will have access to a crash cart with a defibrillator. Often the responsibility falls on the nuclear medicine technologist to ensure the defibrillator is powered and operational. This is confirmed by a battery test and a voltage check. In addition, other checks on the crash cart may be necessary, such as the medication lock.
Xenon Trap and Nebulizer
The xenon trap machine and aerosol nebulizer are used to perform lung ventilation studies. The nebulizer does not require any quality control, but should be visually inspected for damage and cleaned when necessary. To keep the xenon trap machine operating efficiently, several quality control procedures should be performed. The Drierite and soda lime crystals within the machine help to trap moisture and carbon dioxide. The manufacturer recommends the crystals be changed between every 3 to 5 patients to maintain maximum efficiency. A xenon leak test is recommended monthly to ensure that the machine is not leaking xenon into the room air. Finally, the charcoal filter, responsible for trapping the radioactivity, should be changed annually.
The following scenarios provide examples of instrument malfunctions that occur in the nuclear medicine department. The review of such scenarios will assist in troubleshooting one's own departmental instrumentation.
GM Survey Meter
While performing the battery and source check on the survey meter, the technologist discovered that the exposure reading for the cesium source was over the department's 10% limit of the calibrated source reading. The technologist repeated the quality control procedure with the same results. After discussing the problem with other technologists, one technologist admitted to accidentally dropping the detector. This physical damage accounts for the inaccurate calibration and inconsistent constancy readings. The survey meter was recalibrated and the meter was usable.
After performing the daily constancy quality control on the dose calibrator, the technologist noticed that one of the readings was off from the previous readings and out of the acceptable ± accuracy range. The technologist repeated the measurement with the manual radionuclide setting, which showed consistent acceptable results. The technologist determined that the automatic button had lost its electrical setting and had to be recalibrated. The manual dial setting was used for that radionuclide for the rest of the day until the technologist or physicist could recalibrate the button.
Well Counter/Uptake Probe
A technologist discovered that the constancy check for the uptake probe was varying slightly in counts registered per minute from day to day. The technologists each repeated the background, calibration, and constancy with similar results. It was discovered that the distance between the source and the probe that the technologists were using was inconsistent. This difference between the counting procedures explains the variability in the counts measured. More precise distance positioning allowed for more consistent readings from day to day. This discovery can also improve patient counts if similar variances in distances are used between the patient and the probe for counting.
After performing the uniformity for each camera in the department, the technologist started his patients' procedures for the day. While processing the third myocardial perfusion study, the technologist noticed that the images were of much poorer quality than usual. The technologist analyzed the uniformity that was performed in the morning and discovered the uniformity percentage to be 9%. The technologist decided to repeat the uniformity, and while setting up for the procedure, discovered that the medium energy collimator was left on from a gallium study the day before. The technologist changed the collimator and repeated the uniformity, which was acceptable. All of the patients who were imaged that day had to have their tests repeated. This scenario would have been prevented if the technologist would have analyzed the uniformity when the quality control was performed, instead of just doing the procedure. With all quality control, analysis is imperative and should never be overlooked. There is no benefit in performing the quality control procedure if the results are not analyzed.
The surgical probe is a very stable instrument. The major problem encountered with this device is when quality control is not performed prior to use or has never been performed. Issues are then encountered during or right before surgery.
Nuclear medicine technologists administer radiopharmaceuticals to patients to monitor the characteristics and functions of tissues or organs localized by the radioactivity. Abnormal areas show higher-than-expected or lower-than-expected concentrations of radioactivity. Nuclear medicine differs from other diagnostic imaging technologies because it determines the presence of disease on the basis of biological changes rather than changes in organ structure.
Nuclear medicine technologists operate cameras that detect and map the radiopharmaceutical in a patient's body to create diagnostic images. After explaining test procedures to patients, technologists administer the radioactivity dose by injection, inhalation, or other means. Technologists position patients and start a gamma scintillation camera (or scanner) that creates images of the distribution of the radiopharmaceutical as it localizes in, and emits photons from the patient's body. The images are produced on a computer screen or on film for a physician to interpret. Radiographers, diagnostic medical sonographers, and cardiovascular technologists also operate diagnostic imaging equipment, but their equipment creates images by means of a different technology.
When working with radiopharmaceuticals, technologists adhere to safety standards that keep the radiation dose to workers and patients as low as possible. Technologists keep patient records and record the amount and type of radionuclides that they receive, use, and discard.
Knowledge and awareness of the various radiation-detection instruments and their quality control procedures is imperative to follow radiation safety practices, provide adequate patient care, and produce accurate diagnostic exams.
1. Society of Nuclear Medicine performance and responsibility guidelines for NMT: revision 2003. Society of Nuclear Medicine Procedure Guidelines Manual. Available at: http://interactive.snm.org/docs/pg_ch16_0803.pdf. Accessed March 15, 2007.
2. Nichols K, Bacharach S, Bergmann S, et al. Instrumentation quality assurance and performance [American Society of Nuclear Cardiology Web site]. J Nucl Cardiol. November/December 2006. Available at: http://www.asnc.org/imageuploads/Imaging%20Guidelines%20Instrumentation.pdf. Accessed April 9, 2007.
3. American College of Radiology. Nuclear medicine guidelines. Available at: http://www.acr.org/s_acr/sec.asp?CID=1074&DID=14838. Accessed March 15, 2007.
4. The Intersocietal Commission for the Accreditation of Nuclear Medicine Laboratories (ICANL). Essentials and standards for nuclear medicine accreditation. Available at: http://www.icanl.org/icanl/pdfs/ICANL_NucMedStandards5-03.pdf. Accessed March 15, 2007.
5. Steves AM, Wells PC. Review of Nuclear Medicine Technology: Preparation for Certification Examinations. 3rd ed. Reston, VA: Society of Nuclear Medicine; 2004.
6. United States Nuclear Regulatory Commission. Regulatory guide 10.8 - guide for the preparation of applications for medical use programs. Available at: http://www.nrc.gov/reading-rm/doc-collections/reg-guides/general/active/10-008/index.html. Accessed April 9, 2007.
7. Cherry SR, Sorenson JA, Phelps ME. Physics in Nuclear Medicine. 3rd ed. Philadelphia, Pa: WB Saunders; 2003.
8. Votaw JR. The AAPM/RSNA physics tutorial for residents: physics of PET. Radiographics. 1995;15:1179-1190.
9. Christian PE, Waterstram-Rich K. Nuclear Medicine and PET/CT Technology and Techniques. 6th ed. St. Louis, Mo: Mosby; 2007.
10. Classe JM, Fiche M, Rousseau C, et al. Prospective comparison of 3 gamma-probes for sentinel lymph node detection in 200 breast cancer patients. J Nucl Med. 2005;46:395-399.
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Nuclear Medicine Instrumentation and Quality Control: A Review
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