Use and uncertainties of mutual information for computed tomography/magnetic resonance (CT/MR) registration post permanent implant of the prostate

Abstract

Post-implant dosimetric analysis for permanent implant of the prostate benefits from the use of a computed tomography (CT) dataset for optimal identification of the radioactive source (seed) positions and a magnetic resonance (MR) dataset for optimal description of the target and normal tissue volumes. The CT/MR registration process should be fast and sufficiently accurate to yield a reliable dosimetric analysis. Since critical normal tissues typically reside in dose gradient regions, small shifts in the dose distribution could impact the prediction of complication or complication severity. Standard procedures include the use of the seed distribution as fiducial markers (seed match), a time consuming process that relies on the proper identification of signals due to the same seed on both datasets. Mutual information (MI) is more efficient because it uses image data requiring minimal preparation effort. A comparison of MI registration and seed-match registration was performed for twelve patients. MI was applied to a volume limited to the prostate and surrounding structures, excluding most of the pelvic bone structures (margins around the prostate gland were approximately 2 cm right-left, approximately 1 cm anterior-posterior, and approximately 2 cm superior-inferior). Seeds were identified on a 2 mm slice CT dataset using an automatic seed identification procedure on reconstructed three-dimensional data. Seed positions on the 3 mm slice thickness T2 MR data set were identified using a point-and-click method on each image. Seed images were identified on more than one MR slice, and the results used to determine average seed coordinates for MR images and matched seed pairs between CT and MR images. On average, 42% (19%-64%) of the seeds (19-54 seeds) were identified and matched to their CT counterparts. A least-squares method applied to the CT and MR seed coordinates was used to produce the optimum seed-match registration. MI registration and seed match registration angle differences averaged 0.5 degrees, which was not significantly different from zero. Translation differences averaged 0.6 (1.2 standard deviation) mm right-left, -0.5(1.5) mm posterior-anterior, and -1.2(2.0) mm inferior-superior. Registration error estimates were approximately 2 mm for both the MI and seed-match methods. The observed standard deviations in the offset values were consistent with propagation of error. Registration methods as applied here using mutual information and seed matching are consistent, except for a small systematic difference in the inferior-superior axis for a minority of cases (approximately 15%). Cases registered with mutual information and with bony anatomy misregistration of greater than approximately 5 mm should be evaluated for rescan or seed-match registration. The improvement in efficiency of use for the MI registration method is substantial, approximately 30 min compared to several hours using seed match registration.

Converting from CT- to MRI-only-based target definition in radiotherapy of localized prostate cancer

“…The estimated mean registration uncertainty was 1.3 mm. In previous studies, the uncertainty in coregistration between MRI and CT images of the prostate has been estimated at 1.7 and 2 mm when based on seeds [15,17], and 1.5 mm when based on fiducial anatomical markers [12]. The maximum rotations of the seeds between the CT and T2 images were less than 5.1º, which might be explained by the use of treatment positioning and a small time interval (average 35 min, maximum 1.5 h) between the two scans.…”

“…Moreover, in studies with the most comprehensive spatial analysis of intermodality differences, the coregistration of MRI and CT images has been based on bony [10,11] and anatomical landmarks [12,14], or the midpoint of intraprostatic markers [15]. In addition, there are only few reports evaluating the magnitude of coregistration errors in RT treatment planning [12,15,17]. A 2-3-mm spatial uncertainty of MRIonly workflow compared to a 3-4-mm uncertainty of a combination of CT and MRI has been estimated [18].…”

MR image‐based synthetic CT for IMRT prostate treatment planning and CBCT image‐guided localization

“…This process unavoidably suffers from uncertainty caused by the MR‐CT image registration. The uncertainty of rigid registration between prostate CT and MR was about 2 mm (STD) evaluated by implanted seeds (4) . These uncertainties may produce a systematic shift in the delineation/propagation and lead to dose discrepancy ultimately (5) …”

Comparison of manual and automatic MR‐CT registration for radiotherapy of prostate cancer

“…Previous studies have performed local rigid MR‐CT registration of the prostate based on registrations using implanted fiducial markers. Landmark‐based registration, 8 , 9 , 10 segmentation‐based registration, (11) or voxel property‐based registration 12 , 13 , 14 have all been applied. Automatic image registration benefits from objectivity in the registrations as the registration quality is observer‐independent.…”