Improved Spatial Resolution

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MHCT can be used to improve spatial resolution. This is especially apparent in the z axis (orthogonal plane to slice acquisition plane), where slice thickness determines the z axis resolution, but improved spatial resolution is also visible in the axial plane. By allowing a greater number of images to be acquired in the same amount of time, one can obtain thinner image slices. The spatial resolution advantage in the axial plane is due to reduced amount of partial volume averaging artifacts.

The ability to image the chest in a single breath hold using very narrow collimation will likely have an important impact on CT pulmonary angiogra-

phy. The use of narrow collimation with this technique has been shown to be an important factor in improving the visibility of subsegmental arteries [11]. A preliminary investigation by Patel et al. [12] compared pulmonary artery visibility in three groups of patients (n = 20 in each group) with suspected pulmonary embolus who underwent different CT pulmonary angiography protocols as follows: Group 1: single-detector scanner, 3-mm collimation; Group 2: MHCT scanner, 2.5 mm collimation; and Group 3: MHCT scanner, 1.25mm collimation. Although visualization of lobar arteries was similar among all groups, visualization of segmental arteries was slightly improved with MHCT scanning. The most notable difference, however, was in the assessment of subsegmental arteries, which were visible in 36% of cases in Group 1, 46% in Group 2, and 68% in Group 3. These data suggest that MHCT is superior to single-detector CT for peripheral pulmonary artery visualization, particularly when narrow (1.25-mm) collimation is employed.

The ability to image the entire chest with thin slices will likely also have an important impact on studies performed for lung nodule detection and assessment of the airways. With regard to lung nodule detection, it has been shown that thin-section images are more sensitive than thick-section images for detecting small, subcentimeter lesions [13]. Thin slices also allow for improved nodule characterization. With regard to airway imaging, the ability to obtain thin-section images over a large anatomical region such as the entire bronchial tree will enhance the ability to perform 2D and 3D renderings of the airway (Fig. 8).

A particular advantage of the MHCT scanner relates to its multiplanar reconstruction capabilities. In the coronal, sagittal, and oblique planes, image resolution is determined mainly by image acquisition slice thickness. Although stairstep artifact can be reduced with thinner reconstruction intervals, one is still limited by partial-volume artifact that causes blurring of the reconstructed image. By allowing thinner images to be acquired during a single-breath-hold scan, image resolution of multiplanar images can be dramatically improved. If volumetric image acquisition is obtained with 1-mm thick slices (also referred to as an isotropic volumetric scan), then image reconstructions can be performed with ~1-mm voxels (three-dimensional pixel with x, y, and z dimensions of 1 mm). This allows any reconstructed image in the oblique plane to have comparable spatial resolution to the original axial image. In the following example (Figs. 9, 10, and 11), a box of fruit (apple, orange, strawberries, grapes, and peach) was scanned with varying image acquisition slice thicknesses of 1, 3, and 5 mm; pitches of 1, 2, and 3; and varying reconstruction intervals of 1, 3, and 5 mm. The coronal reconstructed images demonstrate the best spatial resolution for images that were scanned at 1-mm thickness

Figure 8 MHCT 2D and 3D reconstruction images of the airway in a patient with non-small-cell lung cancer. All reconstructions were performed from the same CT data set. (A) Two-dimensional curved oblique reconstruction image along axis of left main-stem bronchus shows narrowing (arrows) due to extrinsic compression by adjacent lymph nodes. Note loculated right pleural effusion (E). (B) Volume rendering of airway and lungs (posterior view) shows narrowing of left mainstem bronchus (arrows) as well as obstruction of bronchus intermedius. (C) Bronchus intermedius obstruction (arrow) is seen to better degree on anterior view of volume rendering of airway. (D) Virtual bronchoscopy (internal rendering) image of peripheral airways distal to left mainstem narrowing shows normal appearance of airway lumen.

Figure 8 MHCT 2D and 3D reconstruction images of the airway in a patient with non-small-cell lung cancer. All reconstructions were performed from the same CT data set. (A) Two-dimensional curved oblique reconstruction image along axis of left main-stem bronchus shows narrowing (arrows) due to extrinsic compression by adjacent lymph nodes. Note loculated right pleural effusion (E). (B) Volume rendering of airway and lungs (posterior view) shows narrowing of left mainstem bronchus (arrows) as well as obstruction of bronchus intermedius. (C) Bronchus intermedius obstruction (arrow) is seen to better degree on anterior view of volume rendering of airway. (D) Virtual bronchoscopy (internal rendering) image of peripheral airways distal to left mainstem narrowing shows normal appearance of airway lumen.

Figure 9 Effect of varying pitch on multiplanar image reconstruction. As pitch increases, image detail is lost, especially along the imaging plane, due to increased interpolation needed for image reconstruction. Note how image detail, especially along the fruit pulp and edges, is degraded with increasing pitch. (A) Coronal reconstruction of a fruit box with pitch of 1 (pitch 1; slice thickness 1 mm; reconstruction interval 1 mm; 5-mm coronal reconstruction). (B) Close-up of coronal reconstruction from Fig. 9A. (C) Close-up of coronal reconstruction of a fruit box with pitch of 2 (pitch 2; slice thickness 1 mm; reconstruction interval 1 mm; 5-mm coronal reconstruction). (D) Close-up of coronal reconstruction of a fruit box with pitch of 3 (pitch 3; slice thickness 1 mm; reconstruction interval 1 mm; 5-mm coronal reconstruction).

regardless of pitch. In contrast, 5-mm-thick slices have the worst spatial resolution, even if they are reconstructed at 1-mm intervals. For most image reconstructions, original image acquisition slice thickness has the greatest effect on image quality, followed by reconstruction intervals and, last, pitch.

Improved quality of reconstructed images with MHCT compared to single-detector CT has been shown in both experimental [14] and clinical studies [15]. In an experimental study, Fleischmann et al. [14] compared stairstep

Figure 10 Effect of varying acquisition slice thickness on multiplanar image reconstruction. Resolution is diminished in the reconstructed plane orthogonal to the imaging plane as slice thickness is increased. Orthogonal image resolution is approximately equal to the acquisition slice thickness for most reconstruction techniques. Objects smaller than the acquired slice thickness, such as grape stems or seeds in this example, become blurred, while larger objects are less affected. (A) Coronal reconstruction of a fruit box with acquisition slice thickness of 1 mm (slice thickness 1 mm; pitch 1; reconstruction interval 1 mm; 5-mm coronal reconstruction). (B) Close-up of coronal reconstruction from Fig. 10A. (C) Close-up of coronal reconstruction with acquisition slice thickness of 3 mm (slice thickness 3mm; pitch 1; reconstruction interval 1 mm; 5-mm coronal reconstruction). (D) Close-up of coronal reconstruction with acquisition slice thickness of 5 mm (slice thickness 5 mm; pitch 1; reconstruction interval 1mm, 5-mm coronal reconstruction).

Figure 10 Effect of varying acquisition slice thickness on multiplanar image reconstruction. Resolution is diminished in the reconstructed plane orthogonal to the imaging plane as slice thickness is increased. Orthogonal image resolution is approximately equal to the acquisition slice thickness for most reconstruction techniques. Objects smaller than the acquired slice thickness, such as grape stems or seeds in this example, become blurred, while larger objects are less affected. (A) Coronal reconstruction of a fruit box with acquisition slice thickness of 1 mm (slice thickness 1 mm; pitch 1; reconstruction interval 1 mm; 5-mm coronal reconstruction). (B) Close-up of coronal reconstruction from Fig. 10A. (C) Close-up of coronal reconstruction with acquisition slice thickness of 3 mm (slice thickness 3mm; pitch 1; reconstruction interval 1 mm; 5-mm coronal reconstruction). (D) Close-up of coronal reconstruction with acquisition slice thickness of 5 mm (slice thickness 5 mm; pitch 1; reconstruction interval 1mm, 5-mm coronal reconstruction).

artifacts with MHCT and single-detector CT by imaging an acrylic rod. These investigators found that stairstep artifacts on volume-rendered reconstruction images were quantitatively and qualitatively smaller with MHCT than with single-detector scanners. In a clinical study, Boiselle et al. [15] qualitatively assessed the degree of stairstep artifact for 2D multiplanar reconstruction CT

Figure 11 Effect of varying reconstruction interval on multiplanar image reconstruction: With increasing reconstruction intervals, artifacts such as stairstepping become more apparent. Note that a 1-mm reconstruction interval does not equal 1-mm resolution in the reconstructed image shown (maximum resolution for these reconstructions from 5-mm acquired slices is approximately 5 mm). (A) Coronal reconstruction of a fruit box with reconstruction interval of 1 mm (reconstruction interval 1 mm; slice thickness 5mm; pitch 1; 5-mm coronal reconstruction). (B) Coronal reconstruction of a fruit box with reconstruction interval of 3 mm (reconstruction interval 3 mm; slice thickness 5 mm; pitch 1; 5-mm coronal reconstruction). (C) Coronal reconstruction of a fruit box with reconstruction interval of 5 mm (reconstruction interval 5 mm; slice thickness 5 mm; pitch 1; 5-mm coronal reconstruction).

Figure 11 Effect of varying reconstruction interval on multiplanar image reconstruction: With increasing reconstruction intervals, artifacts such as stairstepping become more apparent. Note that a 1-mm reconstruction interval does not equal 1-mm resolution in the reconstructed image shown (maximum resolution for these reconstructions from 5-mm acquired slices is approximately 5 mm). (A) Coronal reconstruction of a fruit box with reconstruction interval of 1 mm (reconstruction interval 1 mm; slice thickness 5mm; pitch 1; 5-mm coronal reconstruction). (B) Coronal reconstruction of a fruit box with reconstruction interval of 3 mm (reconstruction interval 3 mm; slice thickness 5 mm; pitch 1; 5-mm coronal reconstruction). (C) Coronal reconstruction of a fruit box with reconstruction interval of 5 mm (reconstruction interval 5 mm; slice thickness 5 mm; pitch 1; 5-mm coronal reconstruction).

pulmonary angiography images in 40 patients imaged with single-detector CT and 40 patients imaged with MHCT. In this study, images obtained with MHCT were graded as having significantly less stairstep artifact (Fig. 12).

As clinical experience with the use of 2D and 3D reconstructed images increases, the spectrum of indications for such reconstructions will likely broaden. For example, preliminary studies have suggested a potential role for multiplanar reconstruction images in the assessment of diffuse interstitial lung disease [16] and lung nodule detection (Fig. 13) [17].

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