Can we REFINE stress-only SPECT MPI protocols using machine learning?

Excerpt

Single photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI) is an indispensable tool for evaluation of patients with suspected or known coronary artery disease (CAD) given its noninvasive nature, widespread availability, and diagnostic and prognostic value.¹ Protocols combining the assessment of both rest and stress SPECT MPI remain commonly utilized to detect and assess the burden of obstructive CAD. However, a strategy of stress-first or stress-only MPI has been advocated by societies to reduce patient and personnel radiation dose and cost, and to improve laboratory efficiency and patient convenience.² The safety of stress-only imaging has been shown in large clinical trials demonstrating comparably low event rates among normal studies using either stress-only or the combined stress/rest approach. Adoption of a stress-first over a rest-stress imaging protocol is further justified by temporal trends indicating that most patients with suspected or known CAD (without prior MI) referred for imaging will have a normal stress MPI. Despite their benefits, stress-first/stress-only SPECT MPI protocols have not been widely adopted in the USA. The reasons for their significant underutilization are multifold but are also important for implementation of a successful stress-only imaging approach. The first consideration is patient selection. Despite the availability of published scoring systems for appropriate patient selection for stress-first/stress-only protocols, there is lack of consensus in this regard.^3,4 Most experts would agree that stress-only imaging should not be performed in those with known or high-likelihood of CAD, prior abnormal scans, or heart failure/reduced left ventricular systolic function. A BMI of greater than 35 kg/m² may be considered another criterion for avoiding stress-only imaging, though one study observed that only 22% of patients undergoing bariatric surgery (mean BMI 49 kg/m²) required additional rest imaging.⁴ Another report indicated that low-dose and ultra, low-dose stress-only protocols should empirically be avoided in patients with BMI between 35 and 39.9 kg/m² and especially in BMI greater than 40 kg/m².⁵ Recently, Rouhani et al published a clinical score model that included 9 variables (age in categories, typical angina, hyperlipidemia, current smoking, hypertension, diabetes, history of MI, history of percutaneous coronary intervention and history of coronary artery bypass grafting surgery).⁶ Depending on the sex of the patient, scores were assigned for the presence or absence of the variable, and a score of 0 to 1 was considered low risk for a non-normal MPI. While a step in the right direction, this model was derived in a single-center SPECT cohort and requires external validation. The model’s area under the curve values were also only in the acceptable range (0.684 and 0.681), raising concerns regarding the overall accuracy. It also requires experienced staff to carefully review the patient’s chart and make determinations on chest pain characteristics and to tabulate all the variables and could be a barrier to implementation in busy practices. In addition, while the model can be used to predict those more likely to have a normal MPI, it does not identify those who may have artifacts or incomplete attenuation correction whereby resting images may still be required. The second barrier to implementation of stress-only/stress-first protocols is the need for rapid review of the stress images by an experienced reader to determine whether resting images should be performed. Delays in this step would mitigate the benefit of improving patient throughput with a stress-first/stress-only approach. This step is more challenging in busy practices or in laboratories without an on-site nuclear cardiologist. The third consideration for stress-only protocols is the need for high-quality images and experienced readers given the lack of resting images for comparison. Advanced SPECT technology including attenuation correction is even more essential and not consistently available in all laboratories and would reduce successful implementation of stress-first/stress-only protocols. Lastly, homogeneous perfusion throughout the myocardium, a criterion for a normal stress-first SPECT MPI, while clearly defined in published studies, is more elusive in real-life clinical practice. Given all of these factors to consider for successful implementation, it is not surprising that stress-first/stress-only SPECT MPI protocols are underutilized. …