Investigaciones Relacionadas

MRI is known as a noninvasive medical imaging method. MRI uses a strong magnetic field (usually 0.5 to 3.0 Tesla) and radio frequency (RF) pulses (with frequency of ~42.5 MHz/Tesla) to generate the cross-sectional images of the body. MRI yields high-contrast, detailed images of soft tissues. However, for air and bone imaging the quality of MR images is poor, and additional techniques such as using contrast agents are required.

MRI is capable of producing 3D images in the form of a set of cross-sectional 2D images. There are three orthogonal standard imaging planes defined in radiology to present cross-sectional views: the axial, sagittal, and coronal imaging planes. The axial plane (also known as the transverse plane) divides the body into superior and inferior

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parts (Figure 1.5 (a)). The sagittal imaging plane (also known as the lateral plane) is perpendicular to the axial plane and divides body into left and right parts (Figure 1.5 (b)). The coronal imaging plane (also known as the frontal plane) is perpendicular to the axial and sagittal planes and divides the body into anterior (ventral) and posterior (dorsal) parts (Figure 1.5 (c)).

Figure 1.5: Three standard imaging planes in radiology. (a) Axial plane, (b) sagittal

plane, and (c) coronal plane.

1.5.3.1 Prostate MRI

Although MRI is not used as a clinical standard test for PCa [63-65], MRI has demonstrated its potential and important role as an imaging modality for PCa

management [63-68]. Over the past two decades, in many centres MR imaging has been

Axial

Sagittal

Coronal

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used for PCa diagnosis, staging, treatment planning and therapy guidance [65-67, 69]. Most commonly, prostate MRI is performed at 1.5 or 3.0 Tesla magnetic field strength. In some centres, an ER coil and/or pelvic phased array coil (also known as a body coil) are used for prostate MRI to increase the signal-to-noise ratio (SNR) and improve the spatial resolution of the images [7, 70]. Although the optimal use of the ER coil for prostate MRI is still under study, there is some evidence of improvement in diagnosis and staging of PCa using ER MRI [7, 70, 71]. Figure 1.6 shows the same axial cross-section of the prostate on T2w MRI acquired with and without ER coil from the same patient.

Figure 1.6: Axial view of T2w prostate MRI acquired (a) without, and (b) with ER coil. Both images are midgland slices of the same patient

There are several different MR imaging pulse sequences available for prostate that form multiparametric MRI; e.g. T1-weighted MRI, T2w MRI, dynamic contrast enhanced (DCE) MRI, diffusion weighted imaging (DWI), and MR spectroscopy (MRS) [65]. (a) (b) Prostate Rectum R L A P R L A P Prostate Rectum

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1.5.3.2 Multiparametric MRI

T1- and T2-weighted MRI: T1- and T2-weighted MRI are both used for PCa

detection [72]. T1-weighted MRI is also used for detection of hemorrhage after prostate biopsy [72]. The zonal anatomy of the prostate is appreciated better on T2w MRI compared to T1-weighted MRI, and therefore usually T2w MRI is used as a main

imaging approach for anatomy description of the prostate and adjacent tissues [73-75]. In T2w MRI of the healthy prostate, the peripheral zone appears brighter than the central and transitional zones, which are mixed dark to semi-bright regions on the image [76]. In T2w MRI, a hypointense area within the peripheral zone is considered to be PCa unless a hyperintense area (i.e. usually associated with the post-biopsy hemorrhage) is observed at the same location on T1-weighted MRI [77]. However, in some cases, PCa is challenging to detect on T2w MRI, because PCa can occur within an isointense region or even a hyperintense area compared to the background [70]. Sensitivity and specificity for T2w MRI in PCa detection have been reported as 52% to 83%, and 46% to 83%, respectively [78].

Contouring of the prostate on MRI is used for localizing the prostate border with surrounding tissues to help clinicians deliver the treatment to the prostate gland, and more specifically, to the PCa tumour sites while preserving healthy surrounding tissues from harm. Due to its better anatomical definition, the contouring task is often performed on T2w MR images. Furthermore, T2w MRI is useful in assessing the PCa extent and its spread beyond the prostate border. Hence, prostate border localisation on this MRI sequence could be helpful to staging.

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Dynamic contrast enhanced MRI: For acquisition of DCE MRI, an MRI contrast

agent is injected into the body and the changes in contrast agent uptake and washout by the prostate tissue are measured through acquisition of a time series of T1-weighted MR images. DCE MRI is useful for detecting, localising, and staging of PCa [79, 80]. It has shown high sensitivity and specificity for early detection of PCa [79].

Diffusion weighted imaging: DWI is another type of MRI in which the mobility of

water molecules at the microscopic level is measured. DWI measures the apparent diffusion coefficient (ADC) value that reflects the water diffusion pattern in the tissue. The idea behind clinical DWI is that, in general, the water motion in healthy human body tissues with intact cell microstructures is oriented and anisotropic. In a pathological change in tissue these microstructures are destroyed, therefore, the pattern of water diffusion in the tissue is more isotropic [81]. DWI has a short acquisition time and usually provides high-contrast between PCa and normal tissue and is useful in PCa diagnosis. However, because of low SNR, the spatial resolution of DWI is low [82].

MR spectroscopy: MRS is used in combination with MRI to provide more

information about tissue characteristics [83]. Similar to MRI, MRS is also based on the nuclear magnetic resonance phenomenon. It provides information about the metabolic activity of the prostate by measuring the quantities of some metabolites (e.g. choline, citrates, creatine and polyamines) within the prostate gland. The metabolite quantities or the ratio between them indicate different abnormalities of the prostate [82]. One of the most important metabolite change in PCa is related to the level of citrate [84].

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1.5.3.3 Endorectal receiver coil

In MRI and MRS, the smaller the receiver RF coil and the closer the coil is located to the target, the lower the noise level. Body and ER coils are two common RF receiver coils that are used in prostate MRI to enhance the quality of MR images in terms of spatial resolution and SNR [70]. The most common ER coil used in prostate MRI is an inflatable ER coil that consists of a probe with a inflatable latex cover, also called a balloon. The balloon is filled by either air, perfluorocarbon, or barium after insertion into rectum for better positioning and coverage, and less coil motion [85]. Typically, the inflated ER coil has a cylindrical shape with about 8.5 cm length and about 4.5 cm diameter after inflation [7, 70].

Despite improvement in image quality via the ER coil, the ER coil complicates some aspects of imaging. For example, the ER coil substantially displaces and deforms the prostate [7]. On average, it compresses the prostate gland about 15%

anteroposteriorly, and expands it about 8% in the left-right direction [7]. In MRI-targeted image-guided procedures, MRI information is often combined with another imaging modality (such as intra-procedural TRUS). Therefore, the deformation of the prostate shape challenges image alignment between MRI and the other imaging modality. In EBRT, CT imaging (the standard imaging modality for dose calculation) is acquired with no prostate gland deformation and in TRUS-guided procedures, although the endorectal transducer is used, the shape of the transducer and the way it is located inside the rectum is different and therefore the prostate shape is deformed in a different way [7]. Another limitation related to the use of the ER coil is the presence of some image distortion and artifacts such as magnetic susceptibility, coil flare, and rectum movement artifacts [86].

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Figure 1.7 shows some types of imaging artifacts that occur on ER MRI. Some

distortions, e.g. magnetic susceptibility, occur because of the air-inflated ER coil and can be reduced by replacing air with other ER coil balloon filling materials [85].

Furthermore, since the ER coil is placed posterior to the prostate, it generates an inhomogeneity in the received signal and, accordingly, in the image intensities [87]. In MRI, the voxel intensities are higher close to the coil.

Figure 1.7: ER coil distortion on MRI [86]: (a) gland distortion, (b) near-field coil flare artifact, (c) coil-related artifact because of air-inflated balloon, and (d) rectal movement

distortion.

(a) (b)

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Although the ER coil improves image quality overall and some studies have shown a positive impact of using ER coil on MRI-based PCa diagnosis [6, 69, 71, 88-90], the use of the ER coil for prostate MRI is still debated because the coil is not comfortable for the patients and generates image distortions. One study suggested that the ER coil does not significantly improve MRI power in diagnosis of PCa [86]. Another study [91] has shown that in terms of staging accuracy, non-ER 3.0 Tesla MRI is equivalent to ER 1.5 Tesla MRI.