Evaluation of equivalence of upslope method-derived myocardial perfusion index and transfer constant based on two-compartment tracer kinetic model

The purpose of this study was to describe the upslope method-derived myocardial perfusion index using the parameters based on a tracer kinetic model of iodixanol contrast agent and to validate this theoretically derived relationship using an ischemic canine model. The established modified Kety model was used to describe the extravascular diffusion of iodixanol contrast agent, which undergoes no cellular uptake or metabolism. This model consists of two functional compartments, one describing the vascular compartment and the second representing all myocardial capillaries, interstitium, and cells. These compartments are connected by two rate constants, K
₁
and k
₂
, which represent the first-order transfer constants from the left ventricular (LV) blood to myocardium and from myocardium to the vascular system, respectively. In the early phase after the arrival of contrast agent in the myocardium, the relationship between K
₁
and the concentrations of iodixanol contrast agent in the myocardium and arterial blood (LV blood) is described by K
₁
= {dC
_myo
(t
_peak
)/dt}/C
_a
(t
_peak
) (Eq. 1), where C
_myo
(t) is the relative concentration of iodixanol contrast agent in the myocardium at time t, C
_a
(t) is the relative concentration of iodixanol contrast agent in the LV blood, and t
_peak
is the time at the peak of C
_a
(t) and maximum upslope of C
_myo
(t). Six canine models of left anterior descending (LAD) artery stenosis were prepared and underwent first-pass contrast-enhanced multi-detector row computed tomography (MDCT) perfusion imaging during adenosine infusion (0.14-0.21 mg/kg/min) to study a wide range of flow rates. K
₁
was measured using the Patlak plot method and upslope method applied to time-attenuation curve data of the LV blood pool and myocardium. The results were compared against microsphere myocardial blood flow measurements. The Patlak plot-derived K
₁
and upslope method-derived K
₁
showed a good linear association. Regional K
₁
can be measured accurately using the upslope method-derived myocardial perfusion index based on a compartment model.