Abstract
The superior soft tissue contrast of magnetic resonance imaging (MRI) compared to computed tomography (CT) has urged the integration of MRI and elimination of CT in radiotherapy treatment (RT) for prostate. An intraprostatic gold fiducial marker (GFM) appears hyperintense on CT. On T2-weighted (T2w) MRI target delineation images, the GFM appear as a small signal void similar to calcifications and post biopsy fibrosis. It can therefore be difficult to identify the markers without CT. Detectability of GFMs can be improved using additional MR images, which are manually registered to target delineation images. This task requires manual labor, and is associated with interoperator differences and image registration errors. The aim of this work was to develop and evaluate an automatic method for identification of GFMs directly in the target delineation images without the need for image registration. T2w images, intended for target delineation, and multiecho gradient echo (MEGRE) images intended for GFM identification, were acquired for prostate cancer patients. Signal voids in the target delineation images were identified as GFM candidates. The GFM appeared as round, symmetric, signal void with increasing area for increasing echo time in the MEGRE images. These image features were exploited for automatic identification of GFMs in a MATLAB model using a patient training dataset (n = 20). The model was validated on an independent patient dataset (n = 40). The distances between the identified GFM in the target delineation images and the GFM in CT images were measured. A human observatory study was conducted to validate the use of MEGRE images. The sensitivity, specificity, and accuracy of the automatic method and the observatory study was 84%, 74%, 81% and 98%, 94%, 97%, respectively. The mean absolute difference in the GFM distances for the automatic method and observatory study was 1.28 ± 1.25 mm and 1.14 ± 1.06 mm, respectively. Multiecho gradient echo images were shown to be a feasible and reliable way to perform GFM identification. For clinical practice, visual inspection of the results from the automatic method is needed at the current stage.
Highlights
Since the introduction of modern radiotherapy treatment planning (RTP), computed tomography (CT) has been the primary imaging modality used for RTP and delineation of target and organs at risk (OAR)
Gustafsson et al.: Registration free automatic identification contrast compared to CT and a widespread introduction of Magnetic resonance imaging (MRI) into the radiotherapy clinics have been seen in recent years.[1,2]
This study developed an automatic method for identifying gold fiducial markers in an MRI target delineation image, without the need for image registration
Summary
Since the introduction of modern radiotherapy treatment planning (RTP), computed tomography (CT) has been the primary imaging modality used for RTP and delineation of target and organs at risk (OAR). A major drawback using CT images for this purpose is the limited soft tissue contrast. Magnetic resonance imaging (MRI) has superior soft tissue. Gustafsson et al.: Registration free automatic identification contrast compared to CT and a widespread introduction of MRI into the radiotherapy clinics have been seen in recent years.[1,2] It has been proved that MRI can add clinical value to the target delineation process in external beam radiation therapy (EBRT), in particular of prostate.[3,4]
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