Accuracy and reliability assessment of CT and MR perfusion analysis software using a digital phantom

Abstract

Purpose: To design a digital phantom data set for computed tomography (CT) perfusion and perfusion-weighted imaging on the basis of the widely accepted tracer kinetic theory in which the true values of cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), and tracer arrival delay are known and to evaluate the accuracy and reliability of postprocessing programs using this digital phantom.

Materials and methods: A phantom data set was created by generating concentration-time curves reflecting true values for CBF (2.5-87.5 mL/100 g per minute), CBV (1.0-5.0 mL/100 g), MTT (3.4-24 seconds), and tracer delays (0-3.0 seconds). These curves were embedded in human brain images. The data were analyzed by using 13 algorithms each for CT and magnetic resonance (MR), including five commercial vendors and five academic programs. Accuracy was assessed by using the Pearson correlation coefficient (r) for true values. Delay-, MTT-, or CBV-dependent errors and correlations between time to maximum of residue function (Tmax) were also evaluated.

Results: In CT, CBV was generally well reproduced (r > 0.9 in 12 algorithms), but not CBF and MTT (r > 0.9 in seven and four algorithms, respectively). In MR, good correlation (r > 0.9) was observed in one-half of commercial programs, while all academic algorithms showed good correlations for all parameters. Most algorithms had delay-dependent errors, especially for commercial software, as well as CBV dependency for CBF or MTT calculation and MTT dependency for CBV calculation. Correlation was good in Tmax except for one algorithm.

Conclusion: The digital phantom readily evaluated the accuracy and characteristics of the CT and MR perfusion analysis software. All commercial programs had delay-induced errors and/or insufficient correlations with true values, while academic programs for MR showed good correlations with true values.

Supplemental material: http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12112618/-/DC1.

Figure 1a:

Data structure of digital phantom for CT. Digital phantom data consist of 16 sections (a). Image shows section (Slice) location. Section 1 (b, left) contains curves for arterial input function (arrow on b, left) and venous output function (arrowhead on b, left). Tissue curves are embedded in sections 2–16 (b, middle). The tissue section contains 7 × 7 quadratic tiles (b, right), which has a different delay and MTT. The first five sections (sections 2–6) have curves of exponential R(t), with each section having a particular value of CBV (1, 2, 3, 4, and 5 mL/100 g, respectively) (a). The next five sections (sections 7–11) and the final five sections (sections 12–16) have curves of linear R(t) and box-shaped R(t), respectively.

Figure 1b:

Data structure of digital phantom for CT. Digital phantom data consist of 16 sections (a). Image shows section (Slice) location. Section 1 (b, left) contains curves for arterial input function (arrow on b, left) and venous output function (arrowhead on b, left). Tissue curves are embedded in sections 2–16 (b, middle). The tissue section contains 7 × 7 quadratic tiles (b, right), which has a different delay and MTT. The first five sections (sections 2–6) have curves of exponential R(t), with each section having a particular value of CBV (1, 2, 3, 4, and 5 mL/100 g, respectively) (a). The next five sections (sections 7–11) and the final five sections (sections 12–16) have curves of linear R(t) and box-shaped R(t), respectively.

Figure 2a:

Perfusion maps. True-value and perfusion maps of digital phantom (section 16, box-shaped R[t], and CBV [5 mL/100 g]) are shown with identical color lookup table (not shown). The color scale is automatically adjusted for visual assessment in each map of CBF, CBV, and MTT. A constant window level and window width (8 and 16 seconds, respectively) are used for T_max. In the true-value maps of CBF and MTT, seven columns with different CBF or MTT show different colors. However, there is no vertical color gradient, because all seven rows have identical CBF or MTT and only the delay is different. The true-CBV image is uniform in color, because all 49 tiles have the same CBV value in a section. (a) CT perfusion. Delay-dependent vertical gradation is apparent in most of CBF (G1, P1, P2, T1, T2, PM1, ST1, and ST2) and MTT (G1, H, P1, P2, T1, T2, PM1, PM2, ST1, and ST2). In CBV, a delay-dependent vertical gradient is observed in H and T1. MTT dependency is also noted in G2, H, S1, T1, T2, and ST2 as a horizontal gradient. (b) Perfusion-weighted imaging. Delay dependency is apparent in most algorithms for CBF and MTT maps. Delay-dependent vertical gradient on CBV map is observed for P. MTT dependency is also noted for G, S1, and ST2 as a horizontal gradient. N.A. = not available.

Figure 2b:

Perfusion maps. True-value and perfusion maps of digital phantom (section 16, box-shaped R[t], and CBV [5 mL/100 g]) are shown with identical color lookup table (not shown). The color scale is automatically adjusted for visual assessment in each map of CBF, CBV, and MTT. A constant window level and window width (8 and 16 seconds, respectively) are used for T_max. In the true-value maps of CBF and MTT, seven columns with different CBF or MTT show different colors. However, there is no vertical color gradient, because all seven rows have identical CBF or MTT and only the delay is different. The true-CBV image is uniform in color, because all 49 tiles have the same CBV value in a section. (a) CT perfusion. Delay-dependent vertical gradation is apparent in most of CBF (G1, P1, P2, T1, T2, PM1, ST1, and ST2) and MTT (G1, H, P1, P2, T1, T2, PM1, PM2, ST1, and ST2). In CBV, a delay-dependent vertical gradient is observed in H and T1. MTT dependency is also noted in G2, H, S1, T1, T2, and ST2 as a horizontal gradient. (b) Perfusion-weighted imaging. Delay dependency is apparent in most algorithms for CBF and MTT maps. Delay-dependent vertical gradient on CBV map is observed for P. MTT dependency is also noted for G, S1, and ST2 as a horizontal gradient. N.A. = not available.