Issues of Concern
In nuclear medicine, imaging artifacts can be divided into instrumental, radiopharmaceutical, technical, patient-related, and treatment-related as defined below: [2]
Instrumental
With digitized gamma cameras, the acquired image raw data may show artifacts clearly, but post-processing, the artifacts may become subtle or altered and difficult to recognize. These artifacts can be categorized into uniformity, resolution and linearity, multiple window spatial registration, collimators, the field of view, and computer-related.[3]
- Uniformity artifacts can arise from faults in the photomultiplier tube and sodium iodide crystal, as well as other electronic and mechanical problems.
- Resolution and linearity artifacts are principally affected by spatial distortion. Still, modern developments have automatically updated linear correction maps with minimal human involvement for correction, even with PET/MRI using flood histograms.[4]
- Multiple window spatial recognition is the accuracy of dual-isotope positioning with multiple photons of different energy using their normalized reference ranges. Artifacts can occur when using multipeak radionucleotides such as 67Ga, 201Tl, and 111In, causing resolution degradation and subtle errors even with software corrections.[5]
- Attenuation artifacts can be inherent to gamma radiation due to its energy. Collimators address most of the gamma camera attenuation potential problems, but Compton scatter artifacts can still occur.[6] Less energy causes more scatter and more absorbed radiation by the patient. Certain radioisotopes minimize this effect (99mTc instead of 201Tl). Through digital subtraction methods and 3D imaging, the reconstruction of tomographic imaging (PET/CT and SPECT) can avoid these artifacts. It does not change spatial resolution or radiation scatter content but increases contrast and helps with locational relationships of structures. Collimator artifacts have no bright rim around the photogenic area, distinguishing them from crystal defects such as cracks. If the damage is more widespread, flood images can be compared between different collimators for detection. A dent or defect in the collimator can be caused during manufacturing or by mechanical stress. Software processing of the image can also reduce collimator artifacts.[7]
- Field-of-view artifacts can involve many mechanisms. Electronic masking of the unusable edge of view prevents edge-packing effects along the crystal area. This can reduce the field of view by overcompensation or misalignment of the field of view (leaving edge effects in view). These artifacts can be prevented by correct calibration after servicing the machine. Misalignment may be a source of error in any series of static images. If multiple passes are used in acquisition or if a continuous acquisition is used, a “zipper” (linear) artifact or banding (often horizontal linear) artifact may be observed, respectively. This can be prevented by equipment maintenance of whole-body uniformity. Hard copies of images should be avoided as artifacts such as those related to the printing process and damage to the print medium are common. Truncation errors can also occur when the field of view between the PET and CT differ, positioning patients away from the in-plane center, or when patient size is too large for the field of view—even though the software can mostly correct this.[8]
- Computers are now integrated into all aspects of modern imaging, including nuclear medicine. This allows for a wide range of artifacts. Some examples include energy corruption, linearity and uniformity correction maps, failure in any step of digitization and image processing, or failure to display the image accurately.
Multimodality imaging is common in modern nuclear medicine. PET/CT and SPECT artifacts may include high attenuation bodies, truncation, respiration motion, and misregistration. They also include native CT artifacts, such as CT noise and slice thickness.[9] CT noise occurs with larger patients or in lower dose CT scans, exaggerated in the reconstruction. CT slices are too thick. It can create a loss of visualizing smaller details.
Radiopharmaceutical
Altered radiopharmaceutical biodistribution can be classified into 2 main categories: preparation and formulation and administration technique and procedures.[10]
There are various factors involved with the radionucleotide purity, including the radionucleotide itself (eg, isomer competition, radionucleotide contamination from cyclotron), its components (eg, reagent concentration, particulate size, commercial source variation), preparation procedures (eg, mixing order, incubation, competing components), and many others (eg blood pH variations, blood electrolyte concentration, aluminum needle interaction, volatility, decomposition, etc).[10]
Administration technique and procedure artifacts can be classified into 5 groups: extravascular injection, clotting in the syringe, suboptimal delivery in ventilation studies, administration line residue, and patient positioning.[10] Extravasation is common and is further discussed in the section on technical artifacts below.
Multiple uses of radionucleotides and study timing artifacts are rare but must be considered with busy nuclear medicine departments where timing overlap may occur. When more than 1 nuclear medicine scan is planned, the physical and biological half-lives must always be considered. Adequate clearance and decay time minimize the occurrence of such artifacts. Careful notice of radioisotope therapy timing with scintigraphy can also avoid occurrences. Biologic half-lives can be altered by patient actions such as having unscheduled meals (when fasting) or gut motility-altering medications during gastric emptying or colonic transit studies. Food or medications may contain radiocontrast material and should be avoided during certain studies.
Technical
Injection site artifacts are usually avoided by software or lead blocking. This helps identify the site and minimize pixel overflow. Extravasation artifacts are common, including local accumulation and possible lymphatic tracking to regional lymph nodes to give false-positive nodes. The area of blood pooling can also cause Compton scatter effects by adjacent soft tissues, giving a false positive uptake.[11] Unintentional intra-arterial injections can create blood pooling or flow artifacts and delayed image findings.
Artifacts may also be caused by medical catheters and tubes inserted into the body. With venous catheter use, the radiopharmaceutical may adhere to the plastic wall, or residual amounts may remain in the catheter lumen or venous system if numerous valves are present due to improper flushing with saline after administration. Pedal veins can reduce the venous retention artifact if the radiotracer commonly does this (eg, 201Tl, (99m)Tc-sestamibi). Urinary catheter bags or nephrostomy bags can also impair views of the area if improperly positioned.
Patient-related
Anatomical variations such as skull thickness, muscle density, and adipose distribution may affect attenuation. Non-anatomical structures in planar imaging that can cause attenuation artifacts can be external (eg, clothing, coins, keys, belt buckle) or internal (eg, pacemaker, orthopedic devices, previously ingested contrast, breast implants, tampon). If the cause of the artifact cannot be removed (eg, limb plaster cast), then the reporting physician must be informed. Contamination artifacts are the most commonly found artifacts in nuclear medicine. These artifacts usually occur when radiotracer is found in unexpected areas.
Urine-contaminating clothing or footwear frequently occurs. It can be confirmed and corrected by washing the skin or removing the contaminated clothing or different image views—lateral makes it obvious. The high photon concentration can impair hip and pelvis visualization in planar imaging with incomplete bladder voiding. Bladder catheterization and reducing the time between voiding and imaging help minimize such artifacts. Secretions can contain radioisotopes, including residual nasal and lacrimal secretions on handkerchiefs, skin, and clothing, but they may also show interference surrounding the ocular prosthesis. These can be interpreted as false positives.
Motion artifacts can exist in any PET or other radiological scan. Respiratory motion artifacts cause blurring, attenuation errors, false negatives (often near the diaphragm), and possibly misregistration errors.[12] Misregistration usually occurs near the boundaries of organs. An example of a misregistration error is if a liver lesion is near the lung-diaphragm interface may show the lesion in the lung rather than the liver due to the respiratory artifact.[13] Respiratory gated (4D) PET/CT developments include hardware gating, software gating, bayesian penalized likelihood (BPL) PET reconstruction, and texture analysis that aim to overcome inherent respiratory motion artifacts.[14]
PET/MRI combinations are used more frequently but still have limitations due to breathing and motion artifacts. The most common SPECT imaging artifact in the past was of the myocardium, the left breast, or the left hemidiaphragm if the position changes between rest and stress imaging. This position change may be misinterpreted as pericardial effusion or false ischemic areas of the myocardium. Simultaneous or sequential image acquisition of transmission and emission is often used to correct attenuation artifacts, along with first-order and extended acquisition correction techniques. Recent developments with multi-pinhole cadmium zinc telluride (CZT) gamma cameras, new detector materials, iterative reconstruction, and new isotopes (18F-NaF) help minimize artifacts.[15] Despite the advances in myocardial perfusion imaging, patient motion artifacts still exist mainly as coughing, twisting, sliding, and slumping during the acquisition.[16]
Treatment-related
Medical procedures such as radiotherapy affect the tissue by increasing uptake in the inflammatory stage and decreasing uptake in the fibrotic stage with many radiopharmaceuticals. An example of this can be seen with treatment evaluation of bone metastasis if done too early. The post-treatment bone is damaged and remodeled, causing high uptake and false-positive progression for the metastasis when the bone is simply healing. This is sometimes called a flare phenomenon in a fluorodeoxyglucose (FDG) PET.
Recent surgery certainly causes increased uptake as inflammation and healing are occurring. Subcutaneous injection sites may have increased uptake of certain radioisotopes (eg, 99mTc-MDP), and they may obscure regions (lumbar spine, pelvis, abdomen) on anterior planar views and can be confirmed with lateral planar or SPECT. Patients on dialysis and with chronic renal failure can have increased clearance through the liver to compensate for renal impairment, altering the biodistribution of the radiopharmaceutical.[17] These patients can also have various chemical interactions with the radiopharmaceuticals involving factors such as pH changes, electrolyte abnormalities, and dialyzing before sufficient target uptake.
Pathophysiologic and biochemical changes alter the biodistribution of a radiopharmaceutical. Examples include excretory organ failure, altered glucose metabolism (fasting on FDG myocardial imaging), abnormal hormone levels (hypoparathyroidism with the bone scan), inflammatory reactions, and numerous others.[17]
Therapeutic drugs the patient is taking may alter the biodistribution or pharmacokinetics of the radiopharmaceutical. An example could be cardiac medications interfering with stress tests by preventing the heart from reaching the desired rate. Another example would be immunosuppressants reducing chemotaxis and diminishing the uptake of radiolabelled leukocytes. Some medications, such as somatostatin analogs, may alter the uptake of 68Ga-DOTATATE imaging.[18]
Nursing, Allied Health, and Interprofessional Team Interventions
Both medical and non-medical personnel are responsible for the quality of a nuclear medicine service. Although quality can be defined in many ways, using various selected indicators involves interprofessional teams. Sometimes, patients' perceptions of quality and satisfaction may not agree with some of a nuclear medicine department's measurements. We can even find older patients are generally more satisfied than younger ones.[19]
Quality of service involves expertise from physicists, radio-pharmacists, nuclear medicine instrumentation, nuclear medicine technologists/radiographers, allied health professionals, and all other clinicians. Artifacts in nuclear imaging can be identified in many ways, not just by the reporting specialist physician but also by other staff involved. Typically the reporting specialist physicians rely on staff members for updated patient and equipment information, but anybody in the imaging process can improve outcomes. For example, the person who transported the patient to the imaging department may report incontinence or that the patient is likely to cause a motion artifact if agitated so that adjustments might be performed for better imaging.
Compliance with guidelines and protocols and good communication between medical teams make for the best outcomes, and discerning artifacts from images enables better treatment. The more relevant information provided, the better the communication between all teams improves patient outcomes. If someone notices an anomaly and reports it to the correct staff member, corrections can be made to avoid issues and improve patient care. Ideally, artifacts should be prevented and, if not, identified at the time of imaging or reporting.
Nuclear Medicine Artifacts