Practice Recommendation for Measuring Washout Rates in 123I-BMIPP Fatty Acid Images

The purpose of this practice recommendation is to specifically identify the critical steps involved in performing and interpreting 123I-β-methyl-iodophenyl-pentadecanoic acid (BMIPP) single-photon emission computed tomography (SPECT) and measurement of washout rate (WR) from the heart. This document will cover backgrounds, patient preparation, testing procedure, visual image interpretation, quantitation methods using planar and SPECT studies, and reporting of WR. The pitfall and some tips for the calculation of 123I-BMIPP WR are also included. The targets of global and regional WR calculation include ischemic heart disease, cardiomyopathy, heart failure, and triglyceride deposit cardiomyovasculopathy, an emerging rare heart disease.


Patient preparation
Patients are required to fast for at least 6 hours (6-12 hours) before undergoing image acquisition.Water intake is allowed.The energy for data acquisition is centered at 159 keV with a 20% (±10%) or 15% (±7.5%) window.Despite low-energy collimators, caution is required regarding increased scatter and septal penetration especially from 529 keV gamma rays.

Notes for examinations
-Fasting is required before examinations.No food is allowed for at least 6 hours (6-12 hours) before examination except for water intake to avoid influence of foods on myocardial 123 I-BMIPP uptake and washout rate (31).
-Patient motion artifacts should be avoided.
-Radioisotope leakage during intravenous injection should be avoided.
-Patients must be carefully and precisely positioned for early and late image acquisition.
-Data acquisition protocols for early and late image acquisition should be identical.

Electrocardiography (ECG)-gated data acquisition
Non-gated images should be used to calculate WRs because ECG-gated images are influenced by rejected arrhythmias.

SPECT image analysis
Short-, vertical long-, and horizontal long-axis images are generated, and the regional distribution of 123 I-BMIPP and WRs can be evaluated from early and late polar maps.

Visual interpretation
Cardiac accumulation of 123 I-BMIPP is confirmed using anterior planar images.Time-dependent count decay should be corrected because 123 I-BMIPP is usually washed out from the heart within 4 hours, which is within the timeframe when early and late images are acquired.Myocardial washout can then be interpreted visually using the same scale that was displayed after decay correction (Figures 1 and 2).
Metabolic defects and the homogeneity of 123 I-BMIPP distribution can then be evaluated using standard short-, vertical long-, and horizontal long-axis images after standard reconstruction.

Notes for interpretation
-Time-dependent decay can be calculated as 0.5^(duration between early and late images/13.2[h]).For example, 3-hour decay can be corrected by multiplying counts derived from late images by 1.17 (Table 2).
-Washout is invisible when early and late SPECT images are each displayed separately on a scale of 0%-100%.
However, myocardial washout can be easily interpreted visually when the display range is adjusted for decay correction in the late image.Figures 2 and 3 show late images for planar and SPECT studies, respectively.

Quantitation of washout rates
Washout rates (%) can be calculated for planar and SPECT images using polar maps as:   The WR can be calculated from the average heart counts.Background correction is not applied.Hearly, early heart count; Hlate, late heart count; Hlate-dc, late heart count after decay correction.When interval between early and late images is 3 hours (physical half-life of 123 I = 13.2 hours), calculated decay correction factor was 1.17 (Table 2).
counts on early and late images (Figure 2).The cardiac ROI should be placed on the heart and should not extend outwards.A background ROI is not required.
2) Calculating WRs using a SPECT polar maps: Washout rates are calculated as average counts according to Eq. 1.
after early and late counts are averaged (Figure 3).
Appropriate selections of slice ranges at the base and apex are important to create a polar map.Early and late images that can be misaligned when basal slices near the valve plane are selected could result in inaccurate WR results.
This method can be used for TGCV, ischemic heart diseases, cardiomyopathy, and heart failure.
3) Regional and segmental WR calculation using SPECT polar maps: Commercial software can calculate WRs using three, five, or 17 segments, or three regions.These algorithms can be applied for example, to compare WRs in normal regions with those that have reduced metabolic activity between normal and ischemic myocardia, and among three coronary artery territories (Figure 4).

Notes for WR calculation
-The processing range of basal and apical slices must be carefully determined when the WR is calculated using  A: Clinical diagnosis of TGCV with WR 3%.B: Coronary artery disease with WR 18%.Late image is shown with color scale in which maximum count is multiplied by a time-decay correction factor of 3 h (1.17).Global WR can be calculated by defining WR (Eq. 1) after decay correction.Compared with average of pixel-based WR (left lower corner of WR polar map, 2%), global WR (3%) was in agreement when regional WR values were homogeneous.However, averaged pixel-based WR might be influenced by misaligned early and late slices or defective regions.
large metabolic defects and misaligned settings of the slice range could affect pixel-based average WRs.
-Background subtraction using a mediastinal ROI could cause fluctuations in WR and is not recommended for calculating WRs from planar images (29).
-The scale of the polar map can be count-based after decay is corrected on late polar maps.This is preferable to using a percentage scale (0%-100%) to confirm differences in counts between early and late images.Washout rates can be calculated using counts averaged from early and late polar maps and Eq. 1 (Figure 3).
-When patients have large metabolic defects due to previous myocardial infarction or severe fibrosis, defective segments can decrease regional WRs, and misaligned defect segments could cause fluctuations in regional WRs.Outlier WRs that are calculated regionally from defect segments should be excluded (Figure 4) (29).Global WRs should not be misinterpreted by regionally deranged metabolic activity since a decreased WR is critical for a diagnosis of TGCV.In addition to global WRs, regional distribution should be carefully interpreted when calculating WRs.
-Washout rates can be calculated from averaged early and late counts derived from summed short-axis images of base to apical slices (29).
-Time decay correction factors can be calculated as (Table 2): 1 / 0.5^(elapsed time between early and late images/ 13.2 [h])

Radiation exposure
The respective absorbed radiation doses (mGy/MBq)

Figure 1
Figure 1 Three short-axis SPECT slices and anterior planar images of patients.A: Patient with WR 2% and clinical diagnosis of TGCV.B: Patient with WR 11%.Color scale of late images was corrected for time decay between early and late imaging.This can be achieved either by multiplying counts derived from late images by decay correction factor to the late image or modifying the maximum count in the late image adjusted for time decay.Correction for time decay led to similar heart counts between early and late images in patient A, whereas heart count was decreased in late image in patient B.TGCV, triglyceride deposit cardiomyovasculopathy; WR, washout rate.

Figure 2
Figure 2 Examples of washout rates calculated using early (A), late (B) and decay-corrected late (C) planar images.The WR can be calculated from the average heart counts.Background correction is not applied.Hearly, early heart count; Hlate, late heart count; Hlate-dc, late heart count after decay correction.When interval between early and late images is 3 hours (physical half-life of 123 I = 13.2 hours), calculated decay correction factor was 1.17 (Table2).WR, washout rate.

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A long-axis image can be used as a reference for the selection of slice ranges, and a polar map display of WR is convenient for checking outlier WR values.One algorithm calculates WRs using average counts in early and late images, and another averages pixel-based WRs on polar maps (29, 30).Although the results of the two methods generally agree when patients are defect-free, Ann Nucl Cardiol 2023；9(1) ：3-10 -Nakajima et al.JSNC-JSNM Report on BMIPP Washout Rate

Figure 3
Figure 3 Examples of WRs in SPECT polar maps.A: Clinical diagnosis of TGCV with WR 3%.B: Coronary artery disease with WR 18%.Late image is shown with color scale in which maximum count is multiplied by a time-decay correction factor of 3 h (1.17).Global WR can be calculated by defining WR (Eq. 1) after decay correction.Compared with average of pixel-based WR (left lower corner of WR polar map, 2%), global WR (3%) was in agreement when regional WR values were homogeneous.However, averaged pixel-based WR might be influenced by misaligned early and late slices or defective regions.WRs, washout rates.

Table 1 shows
SPECT or SPECT-CT 123I-BMIPP imaging procedures that can be selected according to the nuclear medicine specialty.Patients are intravenously injected with

Table 1
Methods for 123 I-BMIPP imaging 123I decay with a halflife of 13.2 hours.1) Calculating WRs using a planar image: A circular (elliptical or heart-shaped) region of interest (ROI) is set on the heart, and the WR is calculated as average heart Ann Nucl Cardiol 2023；9(1) ：3-10 -6 -Nakajima et al.JSNC-JSNM Report on BMIPP Washout Rate

Table 2
Physical decay of 123 I and decay correction factors.