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4-D Imaging in Cerebrovascular Disorders by Using 320-Slice CT

Rationale and Objectives

The authors report study protocols and initial clinical experience in assessing patients with acute and chronic cerebrovascular disorders using the recently introduced technique of volume computed tomography (VCT).

Materials and Methods

Thirteen patients with presumptive cerebrovascular insufficiency underwent VCT using a 320-slice scanner (detector width, 160 mm), including time-resolved whole-brain perfusion and cerebral angiography (four-dimensional computed tomographic angiography [CTA] and computed tomographic perfusion [CTP]). Unenhanced cranial CT (cCT) and helical cervicocranial CT (three-dimensional CTA) were added according to clinical requirements. Study protocols are presented, and image quality, data management, and radiation exposure issues are discussed.

Results

In 12 of 13 patients, the procedure was performed successfully on admission; in the other patient, the study was aborted for clinical reasons and repeated. Total scan time amounted to about 5 minutes, and data reconstruction times were up to 10 minutes. About 9000 primary images were generated, partially in the enhanced Digital Imaging and Communications in Medicine format, thus requiring new data postprocessing and management strategies. Image artifacts restricted the use of single-rotation cCT and incremental VCT (three-dimensional CTA). Overall exposure figures (computed tomographic dose index and dose-length product) were increased by 65% on average when three-dimensional CTA was added to volume cCT and four-dimensional CTA and CTP (5.0 mSv and 2178 mGy · cm, respectively).

Conclusion

Preliminary clinical experience indicates that whole-brain four-dimensional CTA and CTP is a robust technique that provides relevant clinical information with respect to whole-brain perfusion as well as cerebral hemodynamics. The exposure benefit of deriving time-resolved perfusion and vessel images from one source data set is compromised when adding three-dimensional CTA to the protocol. Other acquisition techniques specific to VCT, such as single-rotation cCT and incremental three-dimensional CTA, suffer from restrictions in terms of image quality at present.

With the introduction of multislice technology into computed tomography (CT), the comprehensive, noninvasive assessment of the cervicocerebral vasculature using computed tomographic angiography (CTA) became possible ( ). Moreover, cerebral volume coverage could be extended up to 40 mm, thus enhancing cerebral perfusion imaging in patients with stroke ( ). Yet the disadvantage of incomplete brain volume coverage when using CT-based perfusion imaging as opposed to magnetic resonance imaging persisted and could not be fully compensated for by additional measures, such as the “toggling-table technique” ( ).

Until now, the recently introduced technique of volume CT (VCT) has been predominantly evaluated with respect to its suitability for cardiac imaging using 256-row and 320-row scanners ( ). Because of their detector widths of up to a 160 mm, scanners of this new generation can cover the whole brain in a single rotation, thus for the first time enabling CT-based whole-brain perfusion imaging.

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Materials and methods

Patients

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VCT

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Acquisition Protocols

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Table 1

Acquisition Protocols for 320-Slice Computed Tomography, Volume Mode and Standard Mode (Right Columns)

Parameter Volume cCT Four-Dimensional CTA and CTP Incremental cCT Helical Three-Dimensional CTA Tube voltage (kV) 120 80 120 120 Tube current (mA) 320 100 250 150 Rotation time (s) 1.0 1.0 1.0 0.5 Pitch — — — 0.61 Axial section thickness (mm) 0.5 0.5 4 and 8 0.5 Reconstruction interval (mm) 0.25 0.5 4 and 8 0.4

cCT, cranial computed tomography; CTA, computed tomographic angiography; CTP, computed tomographic perfusion.

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Data Management

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Radiation Exposure

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Results

Patients

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Figure 1, Four-dimensional computed tomographic perfusion (CTP) (a–b) and computed tomographic angiography (CTA) (d) in acute stroke as well as follow-up magnetic resonance imaging (c) . On admission, the three-dimensional rendered top-down CTP views showed delayed bolus arrival, indicating perfusion disturbances ( a , time-to-peak map) of the whole left-sided anterior territory. A compensatory increase in cortical cerebral blood volume is predominantly depicted, with the exception of a circumscribed frontal area (arrow) showing reduced cerebral blood volume. This finding was indicative of decompensated hypoperfusion causing infarction and confirmed on follow-up magnetic resonance imaging ( c , T2-weighted axial slice, top-down view for comparison) 1 day later. Conventional CTP imaging does not comprehensively assess such an infarction because of volume coverage limitations. (d) In accordance with the time-to-peak map, four-dimensional CTA depicts delayed bolus arrival on the left side because of internal carotid artery occlusion and insufficient collateral blood supply.

Figure 2, Four-dimensional computed tomographic angiography and computed tomographic perfusion in chronic, bilateral internal carotid artery occlusion. Dynamic CTA (a–c) demonstrates timely collateral blood supply to the anterior circulation via the basilar artery. Regional cerebral volume (e) is not altered, whereas the time-to-peak map (f) indicates minor hypoperfusion within the anterior border zones as well as temporo-occipitally.

Figure 3, Arteriovenous malformation. (a) Four-dimensional computed tomographic angiography, frontal projection. Size, location, vascular supply, and different contrast phases of the right-sided frontoparietal arteriovenous malformation are shown in comparison to corresponding digital subtraction angiographic images (b) .

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Data Acquisition

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Postprocessing, Data Management, and Radiation Exposure

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Table 2

Radiation Exposure Figures for Cranial 320-Slice Computed Tomography, Volume Mode and Standard Mode (Right Columns)

Parameter Volume cCT Four-Dimensional CTA and CTP Incremental cCT Helical Three-Dimensional CTA CTDI (mGy) 42.3 ± 4.6 98.7 ± 12.7 76.8 ± 16.8 36.7 ± 0.2 DLP (mGy · cm) 593.7 ± 16.4 1584.1 ± 201.2 1105.5 ± 217.9 1419.2 ± 139.6 Effective dose (mSv) 1.37 3.64 2.54 3.26

cCT, cranial computed tomography; CTA, computed tomographic angiography; CTDI, computed tomographic dose index; CTP, computed tomographic perfusion; DLP, dose-length product.

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Discussion

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Conclusion

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References

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