• Introduction
Nuclear medicine images today are not as standardized as images, provided by computed tomography (CT) and magnetic resonance imaging Rather, an infinite variety of images may be (MRI). provided by different institutions, depending on the facilities in each institution. The Working Group for Investigation and Research on Nuclear Medicine Image Quantification and Standardization started work in 2002 with the aims of improving the reliability and objectivity of nuclear medicine images to create I Masamichi YANAGISAWA Nihon Medi-Physics Co., Ltd. evidence-based nuclear medicine tcchnology (EBNMT) on a national basis. The Working Group has distributed questionnaires to members of the Japanese Society of Nuclear Medicine Technology manufacturers, (JSNMT), device and printer manufacturers, the results of which have been published in the JSNMT journal Nuclear Medicine Technology, and on the JSNMT website (in Japanese). In the survey of JSNMT members, 87% responded that there is a need for guidelines on standardized acquisition, processing, and display, indicating that great hope is being placed on the Working Group's
activities and results.
The survey of device manufacturers showed that although there is a shared recognition of the need for standardization, some manufacturers also expressed the desire for users to understand their devices. This report summarizes the standard images and some of the pitfalls of myocardial perfusion single- photon emission computerized tomography (SPECT) from the final report of the Working Group.  A checklist has also been included at the end.  We hope that readers will refer to these standard images and checklist during image acquisition, processing, display, and evaluation in their own institutions.  Speaking on behalf of the JSNMT it is our hope that this report will provide a valuable reference for the appropriate performance of nuclear medicine testing in order to establish EBNMT.
• Myocardial SPECT
Evidence-based medicine (EBM) relating to the application of stress myocardial perfusion SPECT in coronary artery disease has been established in areas such as the diagnosis of ischemic heart disease, the determination of treatment to evaluate the severity of stenosis of coronary artery lesions to determine appropriate treatment, response to treatment, and risk assesment. 1 〉 Appropriate testing methods and high- quality technologies are required to implement such EBM, and acquisition conditions such as acquisition time and pixel size, preprocessing filters, processing conditions such as scatter/attenuation correction, and display conditions such as gradation, display scales, and color scales are all important in myocardial SPECT. In this section, we describe the techniques and knowledge required for myocardial SPECT image acquisition, processing, and display and we provide standard images for myocardial SPECT.
1. Devices used for myocardial SPECT
Myocardial SPECT image acquisition using a single-detector gamma camera was performed in most institutions over a 180° scan range from RAO45° to LPO45° to shorten acquisition time in supine position. The development of dual-detector and triple-detector gamma cameras has enabled 180° or 360° scan using L-shaped detectors with 90° or 76° angles between them. Lesser institutions are now using single- SPECT images of a normal volunteer acquired by 180° and 360° scan Figure  1 detector gamma cameras, as they have been replaced by dual-detector gamma cameras. These cameras are capable of 180° scan by means of detectors joined at 90° angle or 360° scan by opposing detectors at 180° For angles, depending on the choice of institutions. triple-detector devices, 360° scan is generally used. The main difference between images acquired by 180° and 360° scan is that 180° scan provides high contrast and high spatial resolution, whereas with 360° scan the images are stable, with little distortion in the inferior
In  this and posterior walls (shown in figure 1). 2) report, we provide the standardized images for triple- detector 360° scan (shown in figure 2 ), standardized images for 180° scan with a dual-detector gamma camera using L-shaped detectors with 90° angle (shown in figure 3), and acquisition conditions (Table 1). In general, a low-energy high-resolution (LEHR) or low-energy general-purpose (LEGP) collimator is used for both 201Tl and 99m Tc, although an LEGP collimator is often used for 201 Tl in particular as the count rate is insufficient for short acquisition times or electrocardiogram (ECG) -synchronized acquisition (gated SPECT).   123I has a 529 keV peak in addition to the main peak at 159 kcV, and the effect of down scatter must therefore be taken into account, meaning that the use of low–medium energy general-purpose (LMEGP) collimators is preferable. Collimators should be chosen with reference not only to their 5% penetration energy, but also to the balance between sensitivity and resolution.
2. Acquisition conditions for myocardial
SPECT
Myocardial SPECT involves administering a radiopharmaceutical agent and performing imaging, either under stress or at rest.  The stress test may comprise exercise stress on a treadmill or ergometer, or pharmacological stress with an agent such as adenosine, dipyridamole, or dobutamine. In terms of the radiopharmaceuticals and doses used for myocardial SPECT, when 201 TlCl is used as a myocardial perfusion agent the dose is normally 111 MBq, and for 99m Tc-MIBI and 99m Tc-tetrofosmin it is generally 296–740 MBq.  123 I-BMIPP is used to assess myocardial fatty acid metabolism and 123 I- MIBG to assess myocardial sympathetic nerve function, with a dose of 111 MBq generally used in each case.   The imaging start time and pretreatment agents    vary    according    to for these their pharmacodynamics, and the following points should be noted. At rest, 201 TlCl is slightly affected by washout and early images may be obtained in 15–30 min after administration, with late images also obtained 3–4 h later in some cases.  Under stress, early images are obtained in 5–10 min after administration to take into account of the effect of upward creep, 3 ) and late images are acquired 3–4 h later to evaluate redistribution.   To avoid the uptake of 201 TICI in the gastrointestinal tract, patients should fast for 3–4 h before imaging as pretreatment and during imaging, with only water intake permitted. High levels of 99m Tc-MIBI and 99m Tc-tetrofosmin accumulation in the hepatobiliary system precedes the myocardial uptake during the early stage after the administration, causing artifacts, and imaging is therefore generally performed 30–60 m after administration.  The disposition of these two agents is not exactly the same. 4 )  The acquisition for 99m Lc- tetrofosmin can be performed slightly earlier, since it has a comparatively high cardiac-hepatic uptake rate compared to 99nxTc-MIBI. The consumption of fatty foods such as milk or chocolate after the administration of 99m Tc myocardial perfusion agent is effective in promoting excretion from the hepatobiliary system. Food consumption prior to administration may conversely promote excretion from the liver and gall bladder.  When imaging is performed twice with a 99m Tc agent, either on the base of stress-rest or rest- stress protocols, an interval of 3–4 h should be left between the first and second administrations, and the second dose should be 2–3 times higher than the first dosc. 5) When using 123 I-BMIPP, early images are obtained in 15–20 min after administration, and late images may be obtained 3–4 h later. Fasting before administration is required as pretreatment.    For 123 I- MIBG, early images are acquired in 15–20 min after administration and late images 3–4 h later.   The use of reserpine, tricyclic antidepressants, and labetalol hydrochloride may inhibit the myocardial uptake of 123 I-MIBG. The standard settings for data acquisition are matrix size 64 × 64, pixel size 5–7 mm, step angle 5°–6°, and acquisition time within 20−40 s in each direction, but if possible an acquisition time of around 40 s is preferred. 6 )    Recently, many institutions have started to perform gated SPECT at the same time, 7 ) and the acquisition count setting is important in order to obtain a highly accurate evaluation of left ventricular function.   When using a polar map (bullseye) display to calculate the washout rate, it is necessary to confirm whether or not correction is being performed for acquisition time and nuclide decay.  The number of
R-R divisions in gated SPECT is normally set at eight for 201 Tl and 16 for 99m Tc. Energy window settings are generally 71 kcV ± 10%-15% for 201 Tl.   Acquisition with an added 167 kcV ± 10% increases the acquisition count by around 10%. The setting for 99n Tc is generally 140 kcV ± 10% and that for 123 I is generally 159 keV ± 10%.  In 2008, when this report was published, the majority of institutions did not perform scatter or attenuation correction ( Table 2 ). 8 )   Currently, more institutions are using SPECT-CT scanners in performing myocardial perfusion imaging with the same position as needed for SPECT acquisition. CT scanning results in additional radiation exposure, and care is required when setting acquisition conditions.
3. Points to note for myocardial SPECT
acquisition
Although the myocardial SPECT acquisition count or acquisition time per projection is dependent on the dose of the nuclide used and the patient's constitution, the conditions should be set so that a sufficient myocardial count can be obtained even in late imaging with 201 TlCl, which is affected by washout from the myocardium.  To investigate the acquisition count, the region of interest (ROI) is set in the myocardial region on the LAO45° planner image from SPECT projection data, and the myocardial projection count is measured ( Figure 4 ).   The acquisition count should ideally be over 100 counts per pixel to reduce for statistical error influence ( Figure 5 ), but acquisition time should be set at 15–20 min as scanning in a posture with both arms raised for a prolonged period of time may result in the generation of artifacts due to body movements.  With large patients, however, the settings must be adjusted to ensure a constant acquisition count by prolongation of the acquisition time if 201 TlCl is used, or increasing the dose if 99nn Tc is used as the myocardial agent.   This definition of ≥ 100 counts is also derived from the concept that the statistical noise included in the acquisition count can be kept within 10%, and as the scatter component also varies depending on collimator performance, it does not represent a uniform determination.   When gated SPECT is used, the acquisition count must be investigated for non-gated SPECT with all the R-R divisions added together. During actual acquisition, the patient is securely immobilized with a belt to restrict body movements, and measures such as aids to maintain the body position with both arms raised are also required. Scanning is also performed at close range with a diameter of rotation of 200–250  mm to improve its resolution.
4. Myocardial SPECT processing conditions
Myocardial SPECT image reconstruction is often carried out by filtered back projection (FBP), but when 99m Tc is used as the myocardial agent it may caused streak artifacts due to hotspots in areas such as the liver, gall bladder, and small intestine, and an increasing number of institutions are therefore using ordered subset expectation maximization (OSEM). It has recently become possible to use algorithms that incorporate spatial resolution correction for OSEM, enabling shorter acquisition times and reduced doses. The filters used in myocardial SPECT image reconstruction generally comprise Butterworth filtering for preprocessing and ramp filtering for image reconstruction.  With Butterworth filtering, a high cutoff frequency yields better resolution but more noise, whereas a low cutoff frequency yields poorer resolution but smoother images ( Figure 6 ), and the settings must therefore take into account the device resolution, acquisition count, and noise elimination. Scatter and attenuation correction are also performed in myocardial SPECT, but the anatomical complexity of the thoracic region means that their use requires caution. The triple energy window (TEW) 125 method used in scatter correction improves the contrast between the cardiac cavity and the myocardium, but the inferior wall is corrected to lower It is also necessary to be aware that the count values. is reduced by 30%−40%. Next, attenuation correction using a method such as Chang's method that assumes a homogenous absorber is problematic, as the thoracic region contains many organs that are heterogeneous. absorbers, including the lungs, spine, heart, and mammary glands.   Recent developments include the production of an attenuation map by X-ray CT or another method for the use of the CT-based attenuation correction (CTAC) incorporated in OSEM, and segmentation with scatter and photopeak window data for attenuation correction (SSPAC). 13) It is crucial to perform appropriate scatter correction at the same time as attenuation correction.
5. Points to note for myocardial SPECT
processing
The projection data obtained from myocardial SPECT are checked for body movement in cine model prior to image reconstruction.   Body movements are mainly seen as defects in SPECT images ( Figure 7 ). Therefore, correction to remove body movement artifacts must be performed if the device has with a body movement correction program.   However, it is impossible to correct for changes in posture or repeated severe coughs or deep breaths of the patient during acquisition.   At best, correction can only deal with misalignments in the X and Y directions for each set of projection data. The Butterworth filter is a smoothing filter that uses the combination of a cutoff frequency and order to eliminate or reduce the high-frequency component, creating smoother images.  When determining the cutoff frequency, a simple method is to start by calculating the SPECT full width at half minimum (FWHM) and regard components above a frequency of 0.5/FWHM (cycles/cm) as noise, but visual evaluation of the clinical data before making a final judgment should not be forgotten. It is still possible to correct for count reduction by attenuating the lower wall of the myocardium on images after scatter/attenuation correction, but this may result in false positives by reducing the counts of the anterior wall or the apex.   Many institutions now use a combination of conventional uncorrected images and CTAC images.  CTAC is a procedure whereby attenuation maps produced by X-ray CT are compared with SPECT images to confirm the absence of any misalignment.  It is particularly important to pay attention to inconsistencies at the diaphragmatic level or apex as a result of the heartbeat or respiration.
6. Myocardial SPECT image display
Myocardial SPECT images are often displayed both in gray and in color.  It is also important to set the appropriate gradient scale to express the relationship between count and concentration, whether linear or square. Filmless operation has recently become standard, and for the gray display of myocardial SPECT images, it is now normal to make the concentration curve for the exponential or quadratic function (square) between count and concentration with the upper level of the display scale at around 100%−110% and the lower level at around 10%−20% (Figure 8). 91  The cutoff for the lower level is adjusted by the power of the number. For the color display of myocardial SPECT images, a linear gradient is used with the upper level of the display scale at 100% and the lower level at 0% as standard.  Areas of healthy myocardial perfusion, mild, moderate, or severe ischemia, and occlusion can be distinguished by variations in the color of color codes, and the determination may change as the result of major variations in the upper and lower levels. Particular care is required as the mistaken use of a non- linear gradient may result in misdiagnosis. In myocardial SPECT image display, it is important that the evaluation of ischemia and viability be performed accurately, and that the right ventricle and the background are also visualized to provide information on cardiac function. When a linear gradient is used, the background concentration is high and the depiction of ischemic areas is difficult (Figure Increasing the lower level to 40% removes data 9). about the right ventricle together with the background ( Figure 10 ).   The use of inappropriate gradient and display scales may affect the visual evaluation of the contrast and size of lesions.
7. Points to note for myocardial SPECT
display
Myocardial SPECT images are displayed in a total of 9−16 divisions of 5−7 mm per slice, with the vertical long axis (VLA) from the septum to the lateral wall, the short axis (SA) from the base of the heart to the apex, and the horizontal long axis (HLA) from the inferior wall to the anterior wall ( Figure 11 ).   As myocardial SPECT compares early and late images, diagnosis is easier if the SPECT slice cross sections for these early and late images are in the same position.    A standard layout for myocardial SPECT displays has been proposed in the Nuclear Medicine Image (NMI) with Cardiac Option of the NMI Profile as part of the Integrating the Healthcare Enterprise initiative, and this should be used as reference.    VLA, SA, and HLA myocardial SPECT images are displayed so that their maximum myocardial counts are consistent. If a 99m Tc myocardial agent is used, the concentration display may not be the same in patients with high uptake outside the myocardium, and care is required to ensure that the maximum counts in each of the displayed images are the same.  Effective measures for dealing with this involve excluding extramyocardial hotspots from the obtained SPECT images during reconstruction, masking hotspots on images after reconstruction, 10 ) and truncation processing for counts exceeding the maximum myocardial count. 11) If the secondary capture SPECT images are transmitted to a picture archiving and communication system (PACS) and viewed on a diagnostic display after appropriate processing on the display of a nuclear medicine image processing device, it is vital that the brightness and gradient properties of both the processing device display and the diagnostic display are consistent to ensure that the images are displayed uniformly. The task of establishing uniformity of display is one that should be solved by technicians themselves in order to provide high-quality images with good reproducibility.  Personal preferences on the part of the interpreting physician or requesting physician may have a major effect on the final decision. However, we hope that technicians will offer technical explanations and be willing to strive to achieve the aim of promoting the consensus,    with standardization of displays based on the advice contained in these guidelines.  The Japan Industries Association of Radiological Systems Standards has published a method for using the JIRA TG8-QC pattern for the visual evaluation of displays.  Please refer to the Quality Assurance (QA) Guidelines for Medical Imaging Display Systems. 15)
8. Color display optimization
Color display optimization should start with the careful choice of which color scale to use. In myocardial perfusion SPECT, priority should usually be given to whether or not the visualization of defects corresponds to the diagnostic criteria used by the interpreting physician or requesting physician. For example, simply changing the color scale on a myocardial polar map (bullseye) display may have a major effect on the diagnosis (see Figure 14 in color). The color codes displayed are input as counts, with their output expressed as 8-bit (256-gradient) values corresponding to red (R), green (G), or blue (B).    It is possible to create numerous different color scales by allocating different count–gradient properties to R, G, and B, (scc Figure 1 in color).  They can also be converted to black-and-white grayscale by allocating the same count-gradient properties (see Figure 2 in Variation of the RGB color codes changes the color). shades of low-count regions into borderline ones and normal regions into high-count ones, which may make a major difference to the impression given by the color scale for diagnosis (see Figure 3 in color). The first step involved in appropriate color display is to talk to the doctor who will perform the diagnosis before selecting the display color codes.  Most nuclear medicine images are displayed in rainbow colors from black to red at 0%−100% for the relative values of the counts, but if the 100% level is not present in the myocardial region (because of extramyocardial hotspots), it may appear that myocardial uptake has decreased. The use of a code other than red, such as white or pink (rainbow white, rainbow pink) for 100% regions makes it possible to confirm the presence of the 100% level in the myocardial region.  The use of a color gradient other than a linear gradient (such as a square gradient) for the color display results in major changes in color tone distribution, and care is required as this may affect diagnosis.   As shown in Figure 4 (see in color), in general, when rainbow colors (the figure shows PRISM colors) are used, regions where the count has dropped to 50% are depicted in green (red triangular arrow).  The doctor who performs the diagnosis should be aware of the level of the decrease in count expressed by the rainbow shades.  In a linear display, there is no major breakdown of the rainbow shades even if the lower level is cut at around 10%, but if a square gradient is used, the shades break down and regions with a decreased count may be overestimated. With the exception of color codes close to monotone, such as GE color and Hotmetal, a linear gradient is recommended for rainbow colors.
9. Conclusions
Myocardial count is the most important factor in myocardial SPECT acquisition, and the acquisition time and dose should be adjusted to obtain around 100 counts per pixel.  As the preprocessing filter settings for myocardial SPECT processing take into account the SPECT device acquisition count and resolution, good images cannot be produced unless a sufficient myocardial count can be obtained.  Immobilizing the patient and reducing the SPECT diameter of rotation are effective for reducing image artifacts and improving resolution. The quality of myocardial SPECT is determined by the projection data, and it therefore goes without saying that if the acquired data are poor, this will result in degraded reconstructed images. The heart is surrounded by heterogeneous absorbers, and given the addition of the heart's own beat and respiratory movements, highly accurate scatter and attenuation correction are currently difficult in myocardial SPECT.  Even if scatter correction is carried out, an additional CT scan for the purpose of attenuation correction should be performed, and this should be compared with the conventional uncorrected images. The clinical evaluation of myocardial SPECT images following the more widespread introduction of SPECT-CT devices and new image reconstruction algorithms will be important for improving the diagnostic performance of myocardial SPECT, and at the very least gradation and display scale settings and display evaluation are essential for the adequate assessment of ischemia and viability.
10. Final thoughts
Imaging protocols for nuclear medicine devices have yet to be fully standardized not only in Japan, but also in European countries and North America.  The emergence of combination CT and MRI and nuclear medicine devices is resulting in the clinical use of more. and more image reconstruction methods and correction techniques, raising the importance of pressing ahead with the standardization of imaging protocols designed for these new devices. In addition, amid a global trend to try to reduce the doses of radiopharmaceuticals, it is now time to reconsider imaging protocols for low-dose scanning, particularly of children. The JSNMT will both continue to consider the standardization of imaging protocols and publish reports.  It is our hope that this report will contribute to the standardization of nuclear medicine scanning techniques, not only in Japan but in other Asian countries and worldwide.
10. Reference
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