<title>Evaluation Of Multiplanar Imaging Capabilities Of Four Current Computed Tomography (CT) Scanners</title>

Abstract

Abstract Four state of the art CT scanners were evaluated with respect to optimal techniques for multiplanar imaging. The four scanners were a G. E. 8800, Pfizer 0^50, Picker Synerview 600 and Siemens Somatom 2. Patient movement artefacts can be minimized by choice of techniques that provide rapid data acquisition. By deferring reconstruction, using batch mode acquisition,, suppressing screen display and operator interaction and minimizing tube loading as many as 33 thin slices can be acquired in < 8 minutes. This rapid scan technique makes use of the narrow collimation over a large (5-0 cm) distance quite reason­ able. A high contrast star resolution phantom was scanned using these rapid scan techni­ ques. The multiplanar Images produced from narrow slices are much higher resolution than those created with more widely collimated slices. A low contrast (2.5$) resolution object scanned with the same rapid acquisition method shows improved resolution for the narrow collimation even in the presence of increased noise accompanying the narrow collimation.IntroductionMultiplanar images generated from a series of transaxial CT scans have proven useful In several studies.1 ~4 Reducing the slice thickness generally improves resolution in the multiplanar image but this can be impractical. For example, reducing the slice width from 4 to 2 mm will require acquisition of twice as many scans to cover the same distance. Patient movement during the increased scan time can negate any resolution gain from the narrower collimation. A second objection arises from the increased tube loading that generally accompanys the increased mAs used with narrow collimation. If the mAs is not Increased as the collimation is narrowed the transaxial images will contain much more noise. Increased noise can adversely affect the low contrast resolution. However In­ creased mAs can cause tube loading which results In tube cooling delays that increase the scan time.The object of this study was to define those conditions that would minimize the patient movement problems and determine the effect of these parameters on both high and low contrast multiplanar images of the several scanners involved.MethodsA cross section of the phantom constructed for this study is shown In figure la. Four different objects are suspended In an 8 Inch acrylic bottle. The most important objects for this study are a 10° (36 section) star test pattern and two vertebral bodies (Cl and C2). The other two objects, a set of acrylic spheres an acrylic sheet with a logarithmic hole pattern are discussed elsewhere.5 The star pattern lies in the sagittal plane and the vertebral bodies are suspended with their axis perpendicular to the scan plane (the most common clinical orientation). The contrast of the star relative to the background can be changed by changing the filling fluid. Thus alternate wedges of acrylic and water yield a high contrast task (^ 14$) while filling with 50% dextrose yields a low contrast task (^ 2.5$). Figure Ib and Ic show typical coronal Images of the vertebral bodies.The scanners Investigated are listed in Table 1 along with parameters relevant for multiplanar images. The scanners are representative of the current state of the art including two rotating detector (third generation) and two fixed detector (fourth generation) devices.All the scanners include at least three collimation options: wide, ranging from 8 to 10 mm; medium ranging from 4 to 5 mm and narrow ranging from 1.0 to 2.0 mm. Wide colli­ mation is useful only for the multiplanar imaging of the most gross nature.5 Our interest is in higher resolution multiplanar Images. We have therefore concentrated on images generated with medium and narrow collimation.Bed movement is a second critical parameter in the multiplanar image. The scanners with one exception (See Table I) have bed increments as small as 1.0 mm. Other multiplanar studies used overlapping scans and convolution procedures to some advantage.6 Improvements in the narrow collimation of these scanners to the point that the collimation can be made