For decades, myocardial perfusion imaging has been interpreted in relative terms. One region of the myocardium is compared against another: the territory with the highest uptake becomes the reference, and everything else is measured against it. The logic is elegant, and for most patients with focal coronary disease, it works well. But for a growing and often diagnostically challenging patient population, relative perfusion imaging produces a result that is technically correct and clinically misleading.
Understanding when and why relative imaging fails, and what absolute myocardial blood flow (MBF) quantitation adds to clinical decision making, has become one of the most consequential questions in modern nuclear cardiology.
The Blind Spot in Relative Imaging
Consider a patient with diffuse, three-vessel coronary artery disease. Each major coronary territory is similarly underperfused. On a standard myocardial perfusion image, because every segment is reduced to roughly the same degree, the relative differences between territories are small. The scan appears normal or near-normal. The patient is reassured. Management is deferred. This is balanced ischemia, one of the most important limitations of relative perfusion imaging, where severe global reductions in perfusion may be underestimated or, in some cases, missed entirely.
The same problem, with a different underlying mechanism, arises in microvascular dysfunction, a condition now recognised as a major cause of angina and adverse cardiovascular outcomes in patients without obstructive coronary artery disease. In ischaemia with no obstructive coronary arteries (INOCA), regional perfusion gradients are typically absent. The perfusion map, read in relative terms, is unremarkable. Yet the patient's myocardium is chronically underperfused and, in many cases, at meaningful prognostic risk.
Absolute MBF quantitation addresses both scenarios directly. By measuring actual flow in millilitres per gram per minute, rather than comparing one region against another, it identifies global reduction in perfusion that relative imaging cannot detect, and it provides the physiological underpinning for risk stratification beyond what perfusion patterns alone can convey.
What Absolute MBF Adds to Clinical Decision Making
The clinical implications of moving from relative to absolute flow measurement are not theoretical. A patient with a normal relative perfusion scan who also shows reduced stress MBF (≤1.5 mL/g/min) and a myocardial flow reserve (MFR) below 2.0 meets the criteria for microvascular dysfunction. These finding changes management, directing clinical attention toward intensive medical therapy, lifestyle intervention, and cardiovascular risk factor optimisation rather than reassurance based on an apparently normal scan.
MFR, the ratio of stress MBF to rest MBF, has emerged as an especially powerful prognostic tool. A flow reserve below 2.0 is independently associated with worse outcomes, and this association is particularly strong in patients with diastolic dysfunction, a population that substantially overlaps with heart failure with preserved ejection fraction (HFpEF), itself a syndrome where nuclear cardiology is playing an increasingly important diagnostic and prognostic role.
These are not subtle incremental improvements in sensitivity. They represent a qualitative expansion in what nuclear cardiology can tell clinicians about coronary physiology and patient prognosis.
Rubidium-82 PET: The Reference Standard
Rubidium-82 (Rb-82) PET remains the established reference standard for absolute MBF quantitation. Its advantages are considerable: the typical radiation dose is approximately 3 mSv, well below the ≤9 mSv target set by the American Society of Nuclear Cardiology (ASNC) and a fraction of the dose associated with conventional technetium-based SPECT protocols [1]. It is generator-based, requiring no on-site cyclotron. Its short half-life enables rapid sequential rest and stress imaging, and decades of physiological validation underpin its clinical use.
Rb-82 PET defines the benchmark against which all other methods of MBF quantitation are assessed. When dynamic CZT SPECT, CT perfusion, or other approaches report absolute flow values, their performance is evaluated in comparison to PET-derived measurements. For institutions seeking to offer the highest-grade MBF quantitation, particularly for risk stratification in complex patients, microvascular disease evaluation, or clinical research, Rb-82 PET represents the definitive choice.
Dynamic CZT SPECT: The Accessible Alternative
The expansion of dedicated cardiac CZT cameras has opened a parallel pathway to MBF quantitation. However, several limitations continue to restrict SPECT MBF from achieving full clinical parity with PET:
- Repeatability: A current limitation is its reproducibility, which may be improved by new tracers with higher extraction fraction, more advanced reconstruction algorithms, and greater automation in analysis software [2].
- Lack of standardised normal values: MFR cutoff values vary considerably across platforms, ranging from 1.3 to 2.7 (mean ~1.96). Normal ranges are not yet standardised across devices [3].
- Limited CZT camera availability: CZT cameras remain relatively scarce, and the majority of cardiac nuclear studies are still performed on conventional SPECT cameras, which cannot quantify MBF at all [4].
- Attenuation correction: CT-based correction of tissue attenuation with SPECT is intrinsically less accurate than with PET, affecting MBF absolute values [5].
Dynamic CZT SPECT MBF is clinically promising and increasingly adopted, but currently remains as a second-line alternative to cardiac PET for MBF quantification, both in terms of guideline status and workflow efficiency. Improvements in tracers, software standardisation, and rapid acquisition protocols may close this gap in the coming years.
Challenges and Ethical Considerations
Despite the promising advancements, the application of CRISPR technology in medicine is not without challenges. One significant concern is the potential for off-target effects, where unintended genetic modifications could lead to adverse outcomes. Ensuring the specificity and safety of CRISPR-based interventions remains a critical area of ongoing research.
Ethical considerations also play a pivotal role in the deployment of gene-editing technologies. Issues such as equitable access to treatments, informed consent, and the potential for germline modifications necessitate robust ethical frameworks and regulatory oversight. The scientific community continues to engage in discussions to address these concerns and establish guidelines for responsible use.
The Technical Requirements Are Not Optional
Quantitative MBF measurement demands considerably more from imaging protocols than relative perfusion imaging does. These requirements are not a checklist of nice-to-haves; they are prerequisites for reliable results.
Dynamic, list-mode acquisition is essential. This allows the time-activity curve to be reconstructed post hoc with full temporal resolution. Scatter correction, attenuation correction, and motion correction must all be applied, and vendor-specific kinetic modelling must be used to translate tracer time-activity data into MBF estimates. The dynamic acquisition phase typically lasts 6–11 minutes per rest or stress scan. When the interval between rest and stress acquisitions and any additional static imaging are included, the overall imaging protocol generally ranges from approximately 25 to 50 minutes, depending on the specific one-day or two-day protocol used.
Pharmacological stress is required. Exercise stress does not produce the rapid, well-characterised hyperaemia that kinetic modelling demands, and it cannot serve as the stressor for quantitative protocols. A power injector is essential for delivering tracer with the reproducible bolus characteristics on which input-function estimation depends. This contrasts with hand injection, which introduces variability that propagates directly into MBF error.
Quality control is rigorous and non-negotiable. Accurate input functions, attention to patient positioning throughout the acquisition, and careful QC review of dynamic data are all required at each step. High-stress MBF values are subject to tracer extraction roll-off, a physiological ceiling effect that limits the accuracy of quantitation at very high flow rates, and experienced readers must recognise and account for this.
For programmes implementing quantitative MBF for the first time, protocol standardisation, staff training, and the establishment of local reference ranges should be prioritised before clinical implementation to ensure reliable and reproducible results.
A Framework for Implementation
For programmes considering the transition to quantitative MBF measurement, a staged approach is recommended:
Stage 1: Establish the technical foundation.
Confirm the availability of list-mode dynamic acquisition on existing hardware. Assess scanner type, available kinetic-modelling software, and the feasibility of pharmacological stress with power-injector delivery.
Stage 2: Standardise the protocol.
Work with vendor applications teams and relevant clinical experts to establish a site-specific dynamic acquisition protocol that includes all required corrections. Do not adapt static perfusion protocols; build quantitative protocols from the ground up.
Stage 3: Develop local references.
The published normal MBF values and MFR thresholds in the literature reflect specific scanners, tracers, and patient populations. Collect a reference cohort of patients with low pretest probability of CAD and establish local normal ranges for stress MBF and MFR before applying published thresholds clinically.
Stage 4: Define clinical indications.
Prioritise the patient groups where quantitative MBF value adds the most: patients with suspected balanced ischemia (known multi-vessel disease), patients with unexplained angina and normal relative perfusion (suspected microvascular dysfunction or INOCA), and patients with HFpEF where flow-reserve assessment guides risk stratification.
Stage 5: Audit and refine.
As the programme matures, track outcomes in patients whose management was influenced by quantitative MBF data. The literature supports clear decision thresholds, but programme-level audit of how these translate into patient outcomes is the foundation of clinical credibility.
Why This Matters Now
Nuclear cardiology is at an inflection point. The clinical and physiological rationale for absolute MBF quantitation is well established. The evidence linking reduced MFR to adverse outcomes is substantial. The technology, whether through Rb-82 PET or dynamic CZT SPECT, is available, validated, and increasingly accessible.
What remains uneven is implementation. Across Southeast Asia, as in much of the world, the majority of perfusion imaging is still performed and interpreted in relative terms. Patients with balanced ischemia receive false reassurance. Patients with microvascular dysfunction go unidentified. The full physiological information available from a well-equipped nuclear cardiology programme remains partially or entirely unused.
The gap between what nuclear cardiology can measure and what it routinely does measure is narrowing, but it has not yet closed. Closing it requires not only the right equipment and tracers, but the protocol rigour, training infrastructure, and clinical commitment to use them correctly.
Absolute MBF is not a future aspiration. It is a current capability, with clear clinical application and growing evidence of prognostic impact. The question for any nuclear cardiology programme is not whether to offer it, but how soon and how well.
QT Instruments (under Biomedia Holdings) supports nuclear cardiology programmes across Southeast Asia in establishing and optimising quantitative perfusion capabilities, including Rb-82 PET infrastructure and clinical training.
Editorial Note: The content of this article has been prepared using educational material, scientific presentations, and key takeaways from the ASCI 2026 Congress, Singapore. It is intended for educational purposes and reflects the discussions presented during the congress.
References
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