Patient-specific tracer activity in MPI SPECT: A hands-on approach

Previously, several studies have reported that a decreasing image quality in heavier patients in myocardial perfusion imaging (MPI) using single-photon emission computed tomography (SPECT) can be compensated by using a body-weight-dependent tracer activity or scan time,1-3 as illustrated in Figure 1. Although we derived and validated a activity-scan-time formula for a conventional SPECT scanner, this formula cannot simply be used for all SPECT scanners.1 Differences in detector sensitivity, technical specifications such a collimator design and geometrical detector configuration, and acquisition and reconstruction settings limit the generalizability of the derived formula. Ideally, a tracer activity-scan-time formula should therefore be derived for each SPECT scanner using the method as described previously.1 However, this could be technically challenging and is time consuming. In this technical note, we therefore introduce, as a first-order approach, an alternative simplified method to obtain a body-weight-dependent protocol, which can easily be adopted in every day patient care. Figuer 1 Example of constant image quality in MPI SPECT scans of three male patients without any perfusion defects with varying body weights. From left to right: 66 kg (22.6 kg·m−2), 85 kg (25.1 kg·m−2 ... Deriving a Body-Weight-Dependent Protocol In cardiac SPECT, the application of a fixed tracer activity and scan-time protocol results in a decreasing number of photon counts in heavier patients due to increased photon attenuation, as demonstrated earlier1,3 and illustrated in Figure 2A, D. As image quality primarily depends on the number of measured photon counts, a constant number of detected photon counts provides an image quality less dependent on patients’ size.1,3 Figure 2 Schematic overview of the transition from a fixed tracer activity and scan-time product (A×T) to a minimized patient-specific A×T. From left to right: a fixed A×T (A) resulting in a decreasing number of photon counts and image ... A patient-specific protocol will allow obtaining a constant number of detected photons independent of patients’ size.1,3 A method to derive such a protocol is described recently.1 Ideally, the derivation and validation of a patient-specific protocol are performed for each SPECT scanner to account for differences in hardware, software, and acquisition and reconstruction settings. However, to limit the burden of using this extended method, we hereby introduce an alternative, simplified approach, which can easily be adopted in every day patient care. In this approach, we assume that local physicians consider their SPECT image quality of patients with average body weight, AVGweight, to be adequate, using the local tracer activity and scan-time combination. To convert this to other patients, a multiplication factor (MF) can be determined using MF=0.13AVGweight0.64×bodyweight(kg)+1-0.13×AVGweight0.36. 1a This formula is derived from the validated tracer activity and scan-time formula as presented in our recent study by normalizing it to an average patient.1 In a patient population with an average body weight of 80 kg, the MF formula can be described by MF = body weight (kg) × 0.0079 + 0.37 1b In the next step, the body-weight-specific tracer activity or scan time can be calculated using Patient-specific tracer activity (using a fixed scan time) = standard activity × MF 2a Patient-specific scan time (using a fixed tracer activity) = standard scan time × MF 2b As can be seen, MF is 1.0 for a patient of 80 kg when applying Eq. 1b. In that case, the patient-specific tracer activity (or scan time) is the same as the standard administered activity (or scan time). For heavier patients MF is higher than 1, and for less heavy patients it is lower than 1. Table 1 shows an example with the outcome of these equations in practice. The suggested MF is only eligible for conventional SPECT cameras1 and patients weighing between 60 and 130 kg, as weights outside this range were not used in deriving the formula.1 One could worry that the application of a patient-specific tracer activity or scan-time protocol deviates from the current guidelines.4,5 However, these guidelines are relatively old and partly outdated due to technological advances and revised insights. Motivated deviation can therefore be justified. Table 1 Multiplication factors to adjust the tracer dose or scan time per projection angle as a function of patient’s weight, using Eq. 1b. Furthermore, two examples for introducing either a patient-specific tracer activity or scan-time protocol ... Beneficial Effect of Patient-Specific Tracer Activities Introducing a body-weight-dependent protocol will not only result in image quality that depends less on patients’ size, it also allows for a reduction in the administered activity and, hence, radiation dose to the patient, as shown in a previous study6 and illustrated in Figure 2C, F. Nowadays, leaner patients are generally administered a higher activity than clinically necessary. In heavier patients, the currently applied fixed tracer activity is generally low or at best just sufficient. Implementing a patient-specific protocol will therefore result in a better image quality independent of patients’ size. It might even allow an overall tracer activity or scan-time reduction, without compromising diagnostic accuracy.