Rationale and Objectives
The purpose of this study was to compare lesion-detection performance when interpreting computed tomography (CT) images that are acquired for attenuation correction when performing single photon emission computed tomography/computed tomography (SPECT/CT) myocardial perfusion studies. In the United Kingdom, there is a requirement that these images be interpreted; thus, it is necessary to understand observer performance on these images.
Materials and Methods
An anthropomorphic chest phantom with inserted spherical lesions of different sizes and contrasts was scanned on five different SPECT/CT systems using site-specific CT protocols for SPECT/CT myocardial perfusion imaging. Twenty-one observers (0–4 years of CT experience) searched 26 image slices (17 abnormal, containing 1–3 lesions, and 9 normal, containing no lesions) for each CT acquisition. The observers marked and rated perceived lesions under the free-response paradigm. Four analyses were conducted using jackknife alternative free-response receiver operating characteristic (JAFROC) analysis: (1) 20-pixel acceptance radius (AR) with all 21 readers, abbreviated to 20/ALL analysis, (2) 40-pixel AR with 21 readers (40/ALL), (3) 20-pixel AR with 14 readers experienced in CT (20/EXP), and (4) 20-pixel AR with 7 readers with no CT experience (20/NOT). The significance level of the test was set so as to conservatively control the overall probability of a type I error to <0.05.
Results
The mean JAFROC figure of merit (FOM) for the five CT acquisitions for the 20/ALL study were 0.602, 0.639, 0.372, 0.475, and 0.719 with a significant difference in lesion-detection performance evident between all individual treatment pairs ( P < .0001) with the exception of the 1-2 pairing, which was not significant (these differed only in milliamp seconds). System 5, which had the highest performance, had the smallest slice thickness and the largest matrix size. For the other analyses, the system orderings remained unchanged, and the significance of FOM difference findings remained identical to those for 20/ALL, with one exception: for 20/EXP analysis the 1-2 difference became significant with the higher milliamp seconds superior. Improved detection performance was associated with a smaller slice thickness, increased matrix size, and, to a lesser extent, increased tube charge.
Conclusions
Protocol variations for CT-based attenuation correction (AC) in SPECT/CT imaging have a measurable impact on lesion-detection performance. The results imply that z-axis resolution and matrix size had the greatest impact on lesion detection, with a weaker but detectable dependence on the product of milliamp and seconds.
Attenuation correction (AC) has become necessary in myocardial perfusion imaging (MPI) because of the likelihood of photon attenuation artifacts. In addition to a general reduction of photon counts in larger patients, localized photon attenuation artifacts typically caused by diaphragmatic attenuation in larger men and breast attenuation in larger women can cause difficulties in interpretation. Misinterpretation could lead to unnecessary invasive intervention, such as coronary angiography. This type of error is clinically unacceptable, and a high-quality attenuation map is recommended to correct for these patient-induced artifacts . For these reasons, AC is recommended by the American Society of Nuclear Cardiology and Society of Nuclear Medicine for MPI studies .
AC was initially performed using radionuclide-based transmission images but has been superseded by an x-ray computed tomography (CT)–based technique .
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Materials and methods
Image Acquisition
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Table 1
The CT Acquisition Parameters and CT Doses Delivered by the Five SPECT/CT Systems
SPECT/CT Systems kVp mAs Product Pitch Rec. Slice Thickness (mm) Scan FOV (mm) Matrix Size Pixel Size (mm) CTDI (mGy) Number of Axial CT Images 1 140 30.4 1.9 6.1 435 256 × 256 1.70 3.976 97 2 140 18.2 1.9 6.1 435 256 × 256 1.70 2.380 116 3 140 57.7 1 10 442 128 × 128 3.45 4.600 36 4 140 57.7 1 10 435 256 × 256 1.70 4.112 40 5 120 49.6 0.94 5 600 512 × 512 1.17 3.500 58
CTDI, computed tomography dose index; FOV, field of view (x-y dimension); kVp, kilovoltage peak; mAs, milliamp seconds; Rec, reconstructed; SPECT/CT, single photon emission computed tomography/computed tomography.
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Observer Performance Study
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Statistical Analysis
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Plotting Free-response Data
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Results
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Table 2
Summary Results for the Five SPECT/CT Systems for the Four Analyses (Two for Effect of Acceptance Radius and Two for the Effect of Reader CT Experience)
Analysis F-statistic_P_ Value Avg. number of NL Marks per Normal Case Avg. number of NL Marks per Abnormal Case Avg. number of LL Marks per Abnormal Case 20-pixel AR ( n = 21) F (4,80) = 100.16 <.0001 0.575 0.649 0.674 40-pixel AR ( n = 21) F (4,80) = 95.09 <.0001 0.569 0.626 0.701 CT Experience ( n = 14) F (4,52) = 72.45 <.0001 0.444 0.535 0.679 No CT Experience ( n = 7) F (4,24) = 26.83 <.0001 0.835 0.877 0.664
AR, acceptance radius; Avg, average; n , number of observers; LL, lesion localization; NL, nonlesion localization; SPECT/CT, single photon emission computed tomography/computed tomography.
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Discussion
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Conclusions
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Acknowledgment
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