Dynamic CT perfusion measurement in a cardiac phantom

Abstract

Widespread clinical implementation of dynamic CT myocardial perfusion has been hampered by its limited accuracy and high radiation dose. The purpose of this study was to evaluate the accuracy and radiation dose reduction of a dynamic CT myocardial perfusion technique based on first pass analysis (FPA). To test the FPA technique, a pulsatile pump was used to generate known perfusion rates in a range of 0.96–2.49 mL/min/g. All the known perfusion rates were determined using an ultrasonic flow probe and the known mass of the perfusion volume. FPA and maximum slope model (MSM) perfusion rates were measured using volume scans acquired from a 320-slice CT scanner, and then compared to the known perfusion rates. The measured perfusion using FPA (P_FPA), with two volume scans, and the maximum slope model (P_MSM) were related to known perfusion (P_K) by P_FPA = 0.91P_K + 0.06 (r = 0.98) and P_MSM = 0.25P_K − 0.02 (r = 0.96), respectively. The standard error of estimate for the FPA technique, using two volume scans, and the MSM was 0.14 and 0.30 mL/min/g, respectively. The estimated radiation dose required for the FPA technique with two volume scans and the MSM was 2.6 and 11.7–17.5 mSv, respectively. Therefore, the FPA technique can yield accurate perfusion measurements using as few as two volume scans, corresponding to approximately a factor of four reductions in radiation dose as compared with the currently available MSM. In conclusion, the results of the study indicate that the FPA technique can make accurate dynamic CT perfusion measurements over a range of clinically relevant perfusion rates, while substantially reducing radiation dose, as compared to currently available dynamic CT perfusion techniques.

Appendix

In order to measure perfusion through a compartment, it is necessary to determine the volume [V(t)] of contrast material entering the compartment within a certain time interval, and this volume can be described as:

$$V\left( t \right) = \int_{0}^{t} {Q_{i} \left( t \right)C_{i} \left( t \right)dt} - \int_{{t_{{min} } }}^{t} {Q_{o} \left( t \right)C_{o} \left( t \right)dt}$$

(3)

where Q_i(t) and Q_o(t) are the incoming and outgoing flow rates, and C_i(t) and C_o(t) are the incoming and outgoing concentrations of contrast agent, respectively. Equation 3 represents the fluid dynamic form of mass conservation indicating that the total amount of contrast material in the compartment equals the amount that has entered minus the amount that has exited. The term t _min denotes the minimum transit time of contrast material through the compartment, from entrance to exit. Hence, if V(t) is calculated before any contrast material has exited the vascular compartment, at t < t _min , the outgoing contrast concentration is zero [i.e. C_o(t) = 0] and the latter integral can be ignored.

$$V\left( t \right) = \int_{0}^{{t < t_{min} }} {Q\left( t \right)C_{in} \left( t \right)dt}$$

(4)

The derivative of both sides of Eq. 4, divided by the input iodine concentration, C _in (t), yields:

$$Q\left( {t < t_{min} } \right) = C_{in}^{ - 1} (t < t_{min} )\frac{d}{dt}V(t < t_{min} )$$

(5)

Integrating from t to t + Δt and dividing by Δt to give the time-averaged value of Eq. 5 over the sampling period, the final form of flow derived via the proposed first pass analysis (FPA) technique is:

$$Q_{ave} = \left( {C_{in}^{ - 1} \frac{d}{dt}V} \right)_{ave}$$

(6)

where Q _ave is the calculated flow, $\frac{\text{d}}{\text{dt}}{\text{V}}$ is the rate of change of contrast volume in the vascular compartment per unit time, and C _in is the maximum input concentration of incoming contrast material at the time of measurement. The measured flow can be further simplified as:

$$Q = \frac{{\Delta V}}{{\left( {\Delta t} \right)\left( {C_{in} } \right)}}.$$

(7)