Influence of Thin Slice Reconstruction on CT Brain Perfusion Analysis

Abstract

Objectives: Although CT scanners generally allow dynamic acquisition of thin slices (1 mm), thick slice (≥5 mm) reconstruction is commonly used for stroke imaging to reduce data, processing time, and noise level. Thin slice CT perfusion (CTP) reconstruction may suffer less from partial volume effects, and thus yield more accurate quantitative results with increased resolution. Before thin slice protocols are to be introduced clinically, it needs to be ensured that this does not affect overall CTP constancy. We studied the influence of thin slice reconstruction on average perfusion values by comparing it with standard thick slice reconstruction.

Materials and methods: From 50 patient studies, absolute and relative hemisphere averaged estimates of cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT), and permeability-surface area product (PS) were analyzed using 0.8, 2.4, 4.8, and 9.6 mm slice reconstructions. Specifically, the influence of Gaussian and bilateral filtering, the arterial input function (AIF), and motion correction on the perfusion values was investigated.

Results: Bilateral filtering gave noise levels comparable to isotropic Gaussian filtering, with less partial volume effects. Absolute CBF, CBV and PS were 22%, 14% and 46% lower with 0.8 mm than with 4.8 mm slices. If the AIF and motion correction were based on thin slices prior to reconstruction of thicker slices, these differences reduced to 3%, 4% and 3%. The effect of slice thickness on relative values was very small.

Conclusions: This study shows that thin slice reconstruction for CTP with unaltered acquisition protocol gives relative perfusion values without clinically relevant bias. It does however affect absolute perfusion values, of which CBF and CBV are most sensitive. Partial volume effects in large arteries and veins lead to overestimation of these values. The effects of reconstruction slice thickness should be taken into account when absolute perfusion values are used for clinical decision making.

Trial registration: ClinicalTrials.gov NCT00880113.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. A typical CT perfusion summary map and blood volume map.

An example of a thick slice (5 mm) CTP summary map (left) for acute stroke assessment of a patient with an occluded left middle cerebral artery. The red area indicates the infarct core and the green area indicates the penumbra (tissue at risk). The right parameter map shows the cerebral blood volume (CBV). Both maps were generated by the Philips EBW 4.0 Brain Perfusion software (Philips Healthcare, Best, The Netherlands). The thresholds for the infarct core and penumbra were respectively CBV 145% of the contralateral side, as suggested by Wintermark et al. [].

Fig 2. A raw CT perfusion scan at different slice thicknesses.

An example of a raw, sagittal reformatted CT perfusion slice at axial slice thicknesses of 0.8, 2.4, 4.8, and 9.6 mm.

Fig 3. Arterial input functions and venous output functions.

The mean arterial input function (AIF, left) and venous output function (VOF, right) at different slice thicknesses. Scheme 2 (see Table 1) was used for processing this data. Before averaging all AIFs and all VOFs were aligned to their time-to-peak. The areas under the curves are listed in Table 3.

Fig 4. Thin and thick slice perfusion maps.

An example of axial and sagittal reformatted non-contrast CT (NCCT) slices and perfusion maps with an axial slice thickness of 0.8 mm (top row) and 4.8 mm (bottom row). Scheme 2 (bilateral filtering, see Table 1) was used for processing this data. The slice positions are indicated by a dashed line in the NCCT images. The cerebral blood volume (CBV) map shows a right posterior infarct core (low blood volume), whereas the cerebral blood flow (CBF) and mean transit time (MTT) maps clearly show the surrounding penumbra (low blood flow and elevated transit time). An elevated permeability-surface area product (PS) suggests increased vascular permeability. The 4.8 mm axial slices show slightly elevated CBF and CBV values, and decreased MTT values. The 4.8 mm sagittal slices have a pixelated appearance due to the anisotropic voxels. The raw sagittal slices are shown in Fig 2.

Fig 5. Absolute perfusion values in the ipsilateral and contralateral hemispheres.

Box plots showing the absolute perfusion values that were measured in the tissue within the ipsilateral hemisphere (red) and in the contralateral hemisphere (green). Outliers, marked by circles, are defined as points that are more than 1.5× the interquartile range above the 75% quartile or below the 25% quartile.