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Three-dimensional MRI of the male urethrae with implanted artificial sphincters: initial results

¹ Department of Medical Physics and Bioengineering, ² Department of Radiology and, ³ Institute of Urology, ⁴ Department of Obstetrics and Gynaecology, University College London, Gower Street, London WC1E 6BT, UK

View larger version (23K): [in a new window]   Figure 1. Diagram of Craggs artificial urinary sphincter. 1: urethral cuff; 2: primary reservoir/balloon; 3: additional reservoir (stress-relief balloon); 4: scrotal pump. The design allows a temporarily increased intra-abdominal pressure (caused by a stress such as coughing) to be transmitted from 3 into 1. This prevents stress incontinence, as well as urethral erosion that could be caused by a constant high pressure on the urethra even during rest when a conventional artificial sphincter was used (Artwork by Martin Knight).

View larger version (35K): [in a new window]   Figure 2. Schematic drawings of data acquisition and post-processing. Pixels (voxels) of high, intermediate and low intensities on original (positive) images are numbered 1, 2, and 3, respectively. (a,b) Two series are acquired, each with a 5 mm slice interval. The second series is scanned with a 2.5 mm offset from the first one. (c) After interweaving the slices of the two series, a (positive) dataset (Slices 0–5) is created, with the slice interval halved to 2.5 mm and the number of slices doubled. A negative dataset (Slices 0'–5') is also created, changing pixels of low-intensity (numbered 3) into high-intensity. (d) Two thresholds appropriate for displaying originally high intensity pixels in Slices 0–5 and negatively high intensity pixels in Slices 0'–5', correspondingly, remove intermediate and low intensity pixels. Background pixels in both positive and negative datasets are not displayed. The remaining pixels (voxels) are areas of interest, which can then be fused into a single 3D image (not shown here), restoring the original spatial relationship between pixels (voxels) 1 s and 3 s.

View larger version (102K): [in a new window]   Figure 3. One of serial slices from a coronal scan.(a) In the original (positive) sequence, the bladder, balloon and cuff of the artificial sphincter are of high-intensity, which can be segmented by simply applying a threshold to remove low-intensity signals, and then rendered in 3D surface display (Figure 4a). However, this will also remove the corpora cavernosa from being displayed because their intensities are below the threshold. (b) After inversing the grey scale, the corpora become high-intensity structures which can be displayed by simply applying another threshold (Figure 4b). In the 2D images, it is difficult to tell whether the cuff is fully sealed around the corpora cavernosa urethrae.

View larger version (44K): [in a new window]   Figure 4. 3D reconstruction with surface displays.(a) Structures above corresponding thresholds in the positive and negative sequences are separately visualized. In order not to obscure the smaller structures (in grey), the positive sequence is placed closer to the readers, so the bladder is placed in front of the lower abdomen. The threshold in the negative sequence is set to intermediate low so that the separations of the three spongy structures are displayed as solid while their inner regions as (artificially) hollow. (b) The structures from both positive and negative sequences are fused together to reveal the relationship between the cuff and the urethra; in this case, the cuff is fully sealed (also see movies on our website for a better 3D perception).

View larger version (78K): [in a new window]   Figure 5. A 3D surface display showing that the cuff is not fully wrapped around the corporus cavernosum urethrae. Again, it is an image after fusing the cuff in the positive and the penis in the negative sequences. The threshold in the negative sequence is set very low so that the entire spongy structures are displayed as solid(compare with Figure 4). Also see a movie on our website.

View larger version (63K): [in a new window]   Figure 6. Multiplanar reformatting of the same 3D dataset inFigure 4a: Three orthogonal images are obtained by cross-sectioning the bladder, showing the motion artefact (arrowheads) in the interlaced data, which appears more severe in the upper side of the bladder. (b) Another three orthogonal images are obtained by cross-sectioning the cuff, showing the well-sealed cuff, although not so apparent as in 3D images. Note the depiction of the tubing (arrows) connecting the cuff and the balloons. Its entire course may not be visualized by 3D surface display due to its heterogeneous signal intensity, but can be traced by interactively reformatting sequential 2D images.