Introduction

Magnetic resonance imaging (MRI) of the prostate has become an indispensable tool for diagnosing, staging, or assessing the aggressiveness of a histologically confirmed prostate cancer (PCa) [1]. For best performance, in this approach, the MRI should be done under high quality standards [2].

The use of an endorectal coil for prostate imaging is primarily historical. The basic assumed rationale for its use is the theoretical improvement of image quality particularly for older and low magnetic field systems. Moreover, the literature has strong opinions both ways, as ERC use has well-known advantages and limitations [3,4,5,6,7]. The major factor put forward to support its use is the improved quality of images, by enhancing image resolution and SNR [8]. However, the disadvantages are also relevant and include patient discomfort, reduced workflow efficiency, longer acquisition time due to coil positioning, and coil-related artifacts (including some prostate gland deformation, distortion, or both) [9, 10].

The decision to use an ERC is multifactorial and closely related to another capital issue in prostate imaging: whether to use 1.5 or 3.0 T systems [11]. Regarding this question, two statements are commonly emphasized: (1) If a 3T system were available, then it should be favored for prostate imaging (although it is recognized that newer and more advanced 1.5T scanners may sometimes produce better quality images than some older 3T systems); and (2) Prostate MRI should not be performed on MRI systems under 1.5T [12].

Although minimal standards for prostate MR imaging have been published by multiple entities, there is no explicit recommendation for receiver coil, whether endorectal or surface (pelvic phased array) [13]. The PI-RADS Committee is cautious when assessing this issue, and on version 2.1, in order to perform state-of-art prostate MRI, both receiver coils options are admitted, but members of Committee favor the surface only approach. Regarding the scanners’ magnet strength, 3T is preferable with the surface coil and the use of an endorectal coil for low-field magnets is not formally indicated, assuming good images can be achieved with the modern 1.5T systems, along with a multichannel receiver surface coil. However, most authors actually favor the use of 3T equipment, if available, or the use of an endorectal coil when using older generation 3T or 1.5T systems [13, 14]. In fact, the PI-RADS committee reaffirms that, for some 1.5T systems, the use of an ERC is preferable [13].

Once the coil approach has been defined, it is important to define the exam protocol, as adjustments in sequence parameters might be required, especially in FOV and Matrix size.

Image quality—ERC vs non-ERC approach

The first report of the use of an endorectal coil for prostate imaging dated from 1989, by Martin et al. [15]. First, ERC was used alone, yielding images with limited FOV and that were difficult to adjust as the signal was not uniform (being very high close to the coil and low far from the rectal wall). The combination of an ERC with a surface coil was a major advance in prostate MR image quality [16] and, currently, there is consensus that this approach does enhance image resolution (Fig. 1), regardless of the MRI system used, either 1.5 or 3.0T, by improving the SNR [17]. This improved SNR yields greater spatial resolution, which can in turn aid the assessment of crucial structures, such as the prostatic capsule and the neurovascular bundles [18]. However, the use of ERC is also associated with technical issues, such as near-field artifact and more frequent generation of susceptibility artifacts, especially for diffusion-weighted (DW) images [9], and even distortion of the gland (Fig. 2).

Fig. 1
figure 1

Image quality with and without ERC. a Axial T2-w image showing an ill-defined, intermediate signal lesion (ISUP 3 at histopathology) at left, anterior transition zone (asterisk); b and c are DWI images without and with ERC acquired in a 1.5 T scanner at the same time; and d and e are the corresponding ADC maps without and with ERC. The SNR is clearly superior in ERC images and besides the index lesion (*), a secondary lesion (arrow) is better defined in the set of images with ERC

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Fig. 2
figure 2

In this axial T2-w image, the overdistended coil is seen abutting and deforming the posterior surface of the prostate (arrow), one of the disadvantages of the use of the ERC

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Therefore, a crucial point here is whether this improved SNR impacts clinical results or, in other words, if more lesions are seen with the use of an ERC. On the way to addressing this essential question, in the last decade, the gap in quality between images with and without ERC has narrowed, driven by the advent of modern pelvic phased array coils, along with improvements in gradient systems, in such a way that some recent studies suggested this difference no longer has clinical significance [19].

Here, it is important to consider technical issues, as the performance of surface coils shows wide variability between vendors, depending on their design, the number of elements and channels, coil efficiency, and other technical parameters that ultimately define the image quality of the system [20, 21].

Other factors should also be considered when the focus is to optimize image quality, including patients’ characteristics. For instance, it is clear that a patient’s size affects image quality, so for those with high body mass index (BMI) or large body habitus, the use of ERC could be considered to maintain high quality standards [22]. Another important clinical situation is the presence of metallic devices within the pelvic field of view, producing susceptibility artifacts (Fig. 3); in this case, the most common are hip implants. For those, a 1.5T could be the best option as both 3T scanners and ERC are more prone to generate susceptibility artifacts [13]. Of importance, the newer DWI techniques including reduced field of view acquisitions have the potential to reduce susceptibility artifacts [23], yielding higher quality images compared to conventional DWI sequences (Fig. 4).

Fig. 3
figure 3

a and b Images from a patient with a hip implant acquired in T2-w (a) and DWI-ADC map (b) in a 3T system. The severity of susceptibility magnetic artifacts (asterisk in both images) is greater in the high-field equipment and is worse in DW images, where the prostate can barely be seen, compared to T2-w, where the prostate is well depicted (white arrows in both images). The Echo-Planar sequences, common in DWI, are prone to these artifacts

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Fig. 4
figure 4

a T2-w, axial image, acquired in a 3T system with a surface receiver coil only, showing PI-RADS 4 lesions. b Axial conventional ADC map (b = 1400 s/mm2) and c Axial restricted field-of-view ADC map, from the same patient, showing marked improvement in image quality

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Comparison in different clinical settings

One of the major factors in deciding whether to use (or not use) an ERC is the indication for prostate MR imaging [24]. For diagnosis, one of the first studies comparing prostate cancer detection rate with and without the use of an ERC was by Heijmink et al. in 2007 [3], indicating better results with the combined use of ERC and surface coils. However, more recent studies favored the opposite (i.e., no significant difference in detection rates between systems with and without ERC) [5,6,7, 19, 25, 26], except for the study of Costa et al. [8] which indicated a better sensitivity when an ERC was added. In the most recent study focusing on this question, Mirak et al. [19] indicated that both approaches showed similar detection of overall and index prostate cancer. The index tumor detection rates for the endorectal coil and non-endorectal coil groups were 78.5% and 76.3%, respectively. However, of interest, for posterior and peripheral prostate cancers, the endorectal coil group had a significantly higher detection rate, while for the anterior and transition zone PCa, the same group showed a lower detection rate. Also, the meta-analysis of Shaish et al. [27] reported similar findings, i.e., no significant difference for index lesions, when assessing tumor grading based on quantitative ADC: the pooled sensitivity was 83% for the ERC group and 74% for the non-ERC group (p = 0.30) (pooled specificities of 71% and 80%, respectively) (p = 0.16).

For the second major indication—prostate cancer staging—the findings are similar. Heijmink et al. [3] and Futterer et al. [4], both in 2007, also indicated better performance for staging PCa using the ERC approach compared to non-ERC option. However, with improvements to MRI systems, recent studies [19, 28,29,30,31] have consistently indicated no significant difference between the two approaches. The recent meta-analysis of Tirumani et al. [32], designed to compare the performance for staging T3 lesions (or lesions with local extraprostatic involvement) between ERC versus non-ERC approaches, indicated no significant difference, with an AUC of 0.741 for the ERC group and 0.711 for the latter approach. Similar results had been reported in a previous meta-analysis of De Rooji et al. in 2016 [33]. Also, an important common point in these two meta-analyses was the limited sensitivity, in general, of MRI for assessing PCa staging. As an attenuating factor favoring the use of MRI, both meta-analyses included studies performed with old systems, for instance, from 1994 [34], which has undoubtedly impacted these results.

A third specific situation relevant to the use of ERC is for performing MR spectroscopic imaging (MRSI). Although, not currently included in the majority of clinical protocols, MRSI [mainly proton based (H+)] is used mostly for research purposes [35, 36]. In this particular situation, the SNR is even more fundamental than in conventional imaging and, although some clinical studies have shown that this difference is non-significant for 3T systems [37], clinical and experimental studies have demonstrated that improved SNR with ERC is essential for optimizing spectral data in MRSI, particularly when using a 1.5 system [38,39,40]. Table 1 summarizes the main studies in this topic.

Table 1 Summary of main studies in the literature comparing MRI protocols with and without endorectal coil (ERC), for detection and staging purposes
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Patient preparation

The decision to use ERC or surface coils affects the patient preparation for undergoing a prostate MRI, regardless of the indication. If a surface coil is used alone, some studies have advocated bowel preparation prior to prostate MRI [41, 42], although there is no clear consensus on this [43]. The distended rectum is associated with increased DWI distortion and consequently impaired DWI image quality due to susceptibility artifacts [42]. Generally, DWI involves echo-planar imaging (EPI), a sequence historically known for its susceptibility artifacts that can be greater at 3T magnets and around air-soft tissue interfaces, as the rectum/posterior aspect of prostate (Fig. 5), which is troublesome as the majority of prostate cancers arise around this location [44]. In addition to degrading DWI image quality, rectum distention also affects T2 images. The overdistended rectum might increase motion artifacts on T2-weighted images, as demonstrated by Padhani et al. [45].

Fig. 5
figure 5

a This T2-w, sagittal image, acquired in a 3T system with a surface receiver coil only, demonstrates an overdistended rectal ampullae (R). b Axial DWI (b = 1400 s/mm2) image is heterogenous and the visualization of left lobe (*) is impaired due to susceptibility artifacts; c In the corresponding ADC map, from the same patient, a curved white stripe is seen, at left (arrows), also derived from susceptibility artifacts

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Some approaches have been described for bowel preparation prior to prostate MRI performed only with surface coil. Some of these are less invasive, including switching patients to a prone position to induce air displacement for a non-dependent position, away from the prostate-rectum interface, or even asking patients to defecate prior to MRI examination [46]. Another option is to use a rectal enema. Griethuysen et al. [42] described an important reduction of both incidence and severity of the susceptibility artifacts in patients who self-administered a bowel enema immediately before an MRI exam in a 1.5 T scanner. In their study, clinically relevant gas artifacts were over six times more common in patients without bowel cleansing compared to those who have used it. However, although some pieces of evidence favor the use of bowel preparation when ERC is not used, there is no specific recommendation for that in PI-RADS v2.1 [14].

On the other hand, if the choice is the ERC approach, an important issue is how to fill and correctly position the coil. Filling with air was the first option described in the literature. However, several studies have shown that perfluorocarbon or barium sulfate might significantly reduce susceptibility artifacts, which can be very helpful for DWI images and MRSI [47, 48].

When used, correct positioning of ERC is essential for optimizing image quality (Fig. 6). The coil should be inflated with 60–80 ml of air, perfluorocarbon, or barium sulfate and the receiver’s face of the coil placed towards the anterior rectal wall/posterior prostate, in such a way to ensure coverage of the whole prostate. The correct placement can be assessed on a sagittal scout image. Scout images are also useful for verifying that there is no excessive tilt of the coil (more than 20o) relative to the prostate in the axial plane [39, 49].

Fig. 6
figure 6

Endorectal coil positioning. a T2-w sagittal view showing the distended endorectal coil adequately positioned. The whole prostate (red lines) lies within the limits of the coil; b Axial T2-w image. The receiver face of the coil where detectors can be inferred by the more intense signal in the anterior face (arrows) towards to anterior rectal wall and peripheral zone of the prostate

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Conclusion

The decision of whether to use an ERC is multifactorial. Users should make this decision not only based on a complete knowledge of the MRI system and type of surface coil available, but also on essential patient information, such as the indication for the exam, body habitus, and presence of metallic artifacts in the pelvis. Although, some degree of improvement in image quality is achieved with the use of an ERC, modern MRI systems provide high-quality images using only a surface, phased array coil. Also, important, this choice should be followed by specific details on patient preparation, either for using surface coil solely or for the combined use, with an ERC, as well as a dedicated protocol for optimizing imaging in both circumstances.