British Journal of Radiology (2005) 78, 245-251
© 2005 British Institute of Radiology
doi: 10.1259/bjr/33540239
Techniques and trouble-shooting in high spatial resolution thin slice MRI for rectal cancer
G Brown, MD, MRCP, FRCR
1
I R Daniels, FRCS
2
C Richardson, DCRD
1
P Revell, DCR
3
D Peppercorn, FRCR
2 and
M Bourne, DMRD, FRCR
4
1 Department of Radiology, Royal Marsden Hospital, Fulham Road, London SW3 6JJ, 2 Pelican Cancer Foundation, Pelican Centre, North Hampshire Hospital, Aldermaston Road, Basingstoke, Hants RG24 9NA, 3 Siemens Medical, Siemens House, Oldbury, Bracknell, Berkshire RG12 8FZ and 4 Department of Radiology, University Hospital of Wales, Heath Park, Cardiff CF14 4XW, UK
Correspondence: Dr Gina Brown, Consultant Radiologist and Honorary Senior Lecturer, Department of Radiology, The Royal Marsden Hospital NHS Trust, Downs Road, Sutton, Surrey SM2 5PT, UK
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Abstract
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MRI is increasingly advocated as an optimal method of staging rectal cancer. The technique enables depiction of the relationship of tumour to the mesorectal fascia and may thus identify tumours at risk of positive circumferential margin involvement at surgery. Depth of extramural spread may also be accurately measured and tumour deposits within the mesorectum are shown. It is important that a high spatial resolution technique is used in order to accurately depict these features and care should be taken in ensuring that images acquired cover the entire rectal tumour and mesorectum. This paper describes the technique of high spatial resolution rectal cancer imaging and the potential technical pitfalls in acquiring good quality images. Important factors to consider include: adequate scan duration to achieve high spatial resolution images with sufficient signal to noise ratio, careful positioning of the pelvic phased array coil, use of T2 weighted turbo spin-echo rather than T1 weighted imaging and careful planning of scans to ensure that images are obtained perpendicular to the rectal wall.
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Introduction
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Colorectal carcinoma is the second most common cause of cancer death in the UK. Rectal cancer comprises up to 50% of all cases and accurate pre-operative staging is assuming increasing importance with the availability of effective pre-operative neoadjuvant therapies. The international TNM classification [1] is the most widely used staging system based on the depth of tumour into and beyond the bowel wall, the number of nodal metastases and the presence or absence of distant metastases. In addition, pre-operative assessment of the relationship of tumour or tumour deposits in relation to the potential circumferential resection margin (the mesorectal fascia) is of importance as tumour involvement of this margin in the resection specimen predicts for local recurrence [2] and poor survival [3]. Demonstration of these features pre-operatively is of importance in selecting patients for neoadjuvant therapy prior to surgery.
Three principal imaging techniques, endoluminal ultrasound, endorectal and pelvic phased array MRI, have been studied in the staging of rectal cancer. All of these modalities have been subject to attention because of their ability to distinguish tumour from rectal wall. However, unlike endoluminal ultrasound and endorectal MRI, high spatial resolution MRI consistently depicts the mesorectal fascia [4] and anatomical structures that relate to total mesorectal excision surgery for rectal cancer [5]. The technique enables accurate measurement of depth of extramural spread [4], identification of tumour deposits within the mesorectum [6] and prediction of tumours with positive circumferential margin involvement [7]. It is important that a high spatial resolution technique is used in order to accurately stage tumours and measure depth of extramural spread. Care should be taken in ensuring that images acquired adequately cover the rectal tumour and mesorectum and that overstaging is prevented by ensuring that images are obtained perpendicular to the rectal wall. The aim of this paper is to describe the technique of high spatial resolution rectal cancer imaging and the potential technical pitfalls in acquiring good quality images.
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Method
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Indications and patient preparation
The examination should be performed for all patients with histologically proven rectal carcinoma who present for disease staging. The referring surgeon should indicate the tumour position (in terms of height above the anal verge) and any history of past pelvic pathology and surgery. Ideally the imaging should be performed before the multidisciplinary meeting to aid in the pre-operative decision making process. Ensuring that the patient is comfortable and pain free will ensure a good quality examination free from unwanted motion artefact. Patients need to be fully informed about the length of time of the scans and be positioned comfortably in the supine position within the scanner. There is no role for purgative bowel preparation or enemas. Small bowel movement is not a problem in our experience and therefore anti-peristaltic agents are not indicated. The patient is placed supine on the table and the flexible multi-element phased array body/pelvic coil is placed firmly around the pelvis to ensure good compression and to minimize the possibility of motion. A full bladder is unnecessary and is uncomfortable with the compression from the body coil.
Hardware
Magnet
A 1.0 T/1.5 T system can be used. All of the images presented here have been produced with a 1.5 T machine. The main consequence of using a 1.0 T magnet is the longer image acquisition times. Equally good images can be produced on these machines.
Coil choice
Surface coils:
Surface coils, such as the endorectal coil, are designed to maximize signal return from the small area being imaged [8–12]. These coils comprise an oval receive only loop coil mounted on the inner surface of an inflatable balloon and have the advantage of placement against the surface of the tissue being imaged such as the rectal wall. These coils can produce images of very high signal with little unwanted signal from tissues around but their principal limitation is their small volume of sensitivity. The area that can be imaged using such coils amounts to a total distance of one coil diameter away from the coil with a very rapid drop in signal intensity beyond the immediate vicinity of the coil (Figure 1
). A gap of several millimetres is present between the surface of the coil and the surface of the lesion and the balloon needs to be distended in order to maintain the position of the loop coil close to the primary tumour. Although the coil design permits improved signal to noise ratio, allowing image acquisition using a smaller field of view and thinner slices it is of limited value in the routine staging of rectal cancer. This relates to the use of any endoluminal technique wherein stenosis, stricturing, pain and discomfort, bowel wall motion, difficulty placing the probe in the upper rectum and coil migration all hamper image acquisition. These are significant limitations of using an endoluminal technique and therefore such devices are not recommended in the routine staging of rectal cancer.
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Figure 1. (a) Endorectal coil and (b) T2 weighted image using the endorectal coil. The bowel wall layers are shown but there is insufficient detail from the surrounding mesorectum to adequately stage rectal tumours.
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Phased array coils:
These coils gain the advantages of the surface coil by obtaining higher signal but with greater coverage than a single surface coil and improved homogeneity.
Sequences recommended in the local staging of rectal cancer
Initial localization images, in the coronal and sagittal planes are needed to plan the high resolution images of the pelvis and rectum as follows:
Sequence 1
- The first series is the sagittal T2 weighted fast spin echo (T2W-FSE), which enables identification of the primary tumour (Table 1).
Sequence 2
- The second series is the large field of view axial sections of the whole pelvis from the iliac crest to the symphysis pubis (Table 1
).
These first two sequences give a crude visualization of the primary tumour, possible sites of nodal involvement and direction of the rectal wall to enable the crucial high spatial resolution sequences 3 and 4 to be planned to enable characterization of nodes and detailed staing of the extent of the primary tumour.
Sequence 3
While the second sequence is being acquired, the high-resolution images can be planned (Table 2). The sagittal T2 weighted images obtained are used to plan T2 weighted thin-section axial images through the rectal cancer and adjacent perirectal tissues. It is critical that these images are performed perpendicular to the long-axis of the rectum (Figure 2). The images are obtained by using a 16 cm field of view and 3 mm section thickness.
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Figure 2. Planning the high resolution images. Images are acquired orthogonal to the long axis of the rectum. Note that for a low tumour, an additional oblique coronal scan (top right image) is also acquired, which is planned along the long axis of the anal canal.
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Sequence 4
The rapid change in calibre of the rectal lumen at the level of the anorectal junction limits the usefulness of oblique axial imaging alone. At this level axial images may not show the rectal wall in its entirety and clear delineation between the outer edge of the rectal wall and the levator muscle may not be possible. This can potentially lead to overstaging. It is therefore useful to utilize a high spatial resolution coronal imaging sequence, for lower third rectal tumours, which will show the levator, the sphincter complex, the intersphincteric plane and the relationship to the rectal wall most optimally (Figure 2).
Potential factors that may impair the quality of images
Coil positioning
In order to prevent poor signal to noise from the anorectal junction (Figure 3), it is important that the phased array coil is centred optimally to ensure adequate coverage of the rectum, mesorectum and anal sphincter complex. Thus there should be adequate coverage from the level of the sacral promontory to below the symphysis pubis (Figure 4).
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Figure 3. Poor coil positioning. The images shown (top left and right) have been obtained with the lower edge of the pelvic coil placed at the level of the symphysis pubis. Consequently there is poor signal to noise ratio from the lower rectum and anal sphincter region. Repositioning the coil restores adequate signal to noise ratio.
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Figure 4. It is important to centre the coil correctly to the whole rectum. Thus the lower edge of the coil needs to be placed so that it lies at least 10 cm below the symphysis pubis in order to ensure adequate signal is obtained from the lower rectum and anorectal junction. The upper most limit for coil placement is the sacral promontory.
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Choice of sequences
T1 weighted imaging:
Although the availability of short repetition time/echo time (TR/TE) volume imaging can provide images of high spatial resolution, images obtained fail to depict either tumour or the layers of the bowel wall due to similar relaxation rates of tumour and bowel wall (Figure 5).
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Figure 5. A T1 weighted image (repetition time (TR) 500 ms, echo time (TE) 10 ms) has been obtained with a high spatial resolution 1 mm3 voxel but the bowel wall layers cannot be distinguished.
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Fat saturation and contrast enhancement:
Contrast enhancement has not been shown to be an effective method for the local staging of rectal cancer [9, 13]. A contrast enhanced technique requires the high signal from surrounding perirectal fat on T1 weighted images to be suppressed to permit visualization of high signal enhancement of tumour. This results in a further reduction in signal to noise ratio and potential overstaging of tumours due to enhancement of adjacent non-tumour tissue namely vessels, desmoplastic reaction and normal nodes (Figure 6).
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Figure 6. T1 fat saturated image with contrast enhancement. The bright fat signal intensity from perirectal fat (asterisk) has been suppressed and is of low signal intensity, there is florid enhancement of the entire bowel wall (arrow) and perirectal vessels (arrowheads) which would not be readily distinguishable from tumour if it were present.
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T2 weighted fast spin-echo (T2 FSE):
T2 weighted images of the rectal wall and pararectal tissues result in visualization of individual bowel wall layers and tumour returns a brighter signal than the muscle coat but lower signal intensity than perirectal fat and thus superior depiction of tumour when compared with T1 weighted images (Figure 7).
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Figure 7. (a) T1 weighted (repetition time (TR)=500 ms, echo time (TE) 10 ms) and (b) T2 fast spin echo (FSE) (TR>3000 ms, TE 105 ms) axial images of rectal tumour. The bowel wall layers cannot be seen on the T1 weighted images. Vessels (arrowhead), lymph node (arrow), tumour (open arrow) and muscle coat (curved arrow) all return very similar signal intensities making distinction between structures difficult on the T1 weighted image. This compares with the T2 weighted image, which shows clear separation of tumour and bowel wall layers due to high inherent tissue contrast.
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Thus T1 fat saturated or short tau inversion recovery (STIR) imaging of the rectum and perirectal tissues offers no additional staging information and should not be used.
Cross-talk:
This varies with machines and patients and is manifested on the images obtained as loss of signal and unexpected loss of detail (Figure 8). This is overcome by interleaving slices during the high resolution acquisition or increasing the slice gap.
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Figure 8. Cross-talk. The image has been degraded by noise resulting from cross-talk. This varies with machines and manifests as loss of signal. This is overcome by interleaving slices during the high resolution acquisition.
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Tumour not seen on initial sagittal sequences:
On occasion, lack of clinical detail or the presence of a small tumour prevents the tumour being seen on the sagittal images. In these instances, tumour may only be visible on the high resolution images. In order to ensure that the tumour has not been missed it will be necessary to perform high resolution scans along the entire length of the rectum (as shown in Figure 2).
Patient unable to tolerate long scan:
Some patients (<5%), either due to co-existing medical conditions or claustrophobia, find the scan impossible to tolerate; a combination of patient discomfort, excessive motion of the anterior abdominal wall may result in motion artefact. This is seen as horizontal bands across the image (Figure 9). Of all of the sequences the oblique high resolution scans are the most important. The sagittal views can be shortened by altering the parameters; the large field of view axials are performed last and may even be omitted if the patient is in considerable discomfort.
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Figure 9. Motion artefact. (a) This characteristically produces a series of bands that degrade the image. (b) This can often be overcome by ensuring the patient is not in discomfort, by repositioning the patient and ensuring adequate abdominal compression and repeating the sequence again after swapping phase and frequency directions.
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Discussion
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High spatial resolution MRI is gaining acceptance as a method of determining the local stage of rectal cancer [4, 7, 10, 14, 15]. In comparison with endoluminal techniques such as endorectal ultrasound, MRI offers inherent advantages [16], namely the ability to assess all tumours including bulky obstructing or stricturing tumours. The large field of view afforded by high spatial resolution MRI using the surface coil permits assessment of the entire mesorectum as well as the relationship of tumour to the mesorectal fascia which forms the radial margins in rectal cancer surgery using total mesorectal excision. High contrast resolution using T2 weighted fast spin-echo sequences enables perirectal fibrosis to be readily distinguished from tumour by virtue of signal intensity characteristics and its ability to depict the outer muscle coat as a distinct layer on MR images permits accurate assessment of the depth of extramural spread from the outer muscle coat [4, 7]. However, as yet there is no role for MRI in the assessment of early tumours (T1), adenomatous polyps, or resolving the staging of T1 and T2 tumours in patients being considered for local excision of their tumour. Endorectal techniques are likely to be more useful in such patients and high frequency probes have the potential to identify superficially invasive T1 tumours suitable for cure by local excision [17].
The introduction of the pelvic phased array coil has enabled a major advance in the detail and reproducibility of rectal cancer imaging. It has now been shown to be of high accuracy in assessing the depth of extramural spread and predicting the relationship of tumour close to the mesorectal fascia and is thus a clinically effective method of locally staging all patients with the disease.
Received for publication March 29, 2004.
Revision received October 6, 2004.
Accepted for publication November 25, 2004.
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References
|
|---|
- Sobin L, Wittekind C, editors. TNM classification of malignant tumours (5th edn). New York: John Wiley & Sons, 1997.
- Adam IJ, Mohamdee MO, Martin IG, Scott N, Finan PJ, Johnston D, et al. Role of circumferential margin involvement in the local recurrence of rectal cancer. Lancet 1994;344:707–11.[CrossRef][Medline]
- Hall NR, Finan PJ, al-Jaberi T, Tsang CS, Brown SR, Dixon MF, et al. Circumferential margin involvement after mesorectal excision of rectal cancer with curative intent. Predictor of survival but not local recurrence? Dis Colon Rectum 1998;41:979–83.[CrossRef][Medline]
- Brown G, Richards CJ, Newcombe RG, Dallimore NS, Radcliffe AG, Carey DP, et al. Rectal carcinoma: thin-section MR imaging for staging in 28 patients. Radiology 1999;211:215–22.[Abstract/Free Full Text]
- Brown G, Kirkham A, Williams GT, Bourne M, Radcliffe AG, Sayman J, et al. High-resolution MRI of the anatomy important in total mesorectal excision of the rectum. AJR Am J Roentgenol 2004;182:431–9.[Abstract/Free Full Text]
- Bartram CI, Brown G. Endorectal ultrasound and magnetic resonance imaging in rectal cancer staging. Gastroenterol Clin North Am 2002;31:827–39.[CrossRef][Medline]
- Brown G, Radcliffe AG, Newcombe RG, Dallimore NS, Bourne MW, Williams GT. Preoperative assessment of prognostic factors in rectal cancer using high-resolution magnetic resonance imaging. Br J Surg 2003;90:355–64.[CrossRef][Medline]
- Schnall MD, Furth EE, Rosato EF, Kressel HY. Rectal tumor stage: correlation of endorectal MR imaging and pathologic findings [see comments]. Radiology 1994;190:709–14.[Abstract/Free Full Text]
- Indinnimeo M, Grasso RF, Cicchini C, Pavone P, Stazi A, Catalano C, et al. Endorectal magnetic resonance imaging in the preoperative staging of rectal tumors. Int Surg 1996;81:419–22.[Medline]
- Blomqvist L, Machado M, Rubio C, Gabrielsson N, Granqvist S, Goldman S, et al. Rectal tumour staging: MR imaging using pelvic phased-array and endorectal coils vs endoscopic ultrasonography. Eur Radiol 2000;10:653–60.[CrossRef][Medline]
- Drew PJ, Farouk R, Turnbull LW, Ward SC, Hartley JE, Monson JR. Preoperative magnetic resonance staging of rectal cancer with an endorectal coil and dynamic gadolinium enhancement. Br J Surg 1999;86:250–4.[CrossRef][Medline]
- Joosten FB, Jansen JB, Joosten HJ, Rosenbusch G. Staging of rectal carcinoma using MR double surface coil, MR endorectal coil, and intrarectal ultrasound: correlation with histopathologic findings. J Comput Assist Tomogr 1995;19:752–8.[Medline]
- Okizuka H, Sugimura K, Yoshizako T, Kaji Y, Wada A. Rectal carcinoma: prospective comparison of conventional and gadopentetate dimeglumine enhanced fat-suppressed MR imaging. J Magn Reson Imaging 1996;6:465–71.[Medline]
- Beets-Tan RG, Beets GL, Vliegen RF, Kessels AG, Van Boven H, De Bruine A, et al. Accuracy of magnetic resonance imaging in prediction of tumour-free resection margin in rectal cancer surgery. Lancet 2001;357:497–504.[CrossRef][Medline]
- Radcliffe A, Brown G. Will MRI provide maps of lines of excision for rectal cancer? Lancet 2001;357:495–6.[CrossRef][Medline]
- Brown G, Davies S, Williams GT, Bourne MW, Newcombe RG, Radcliffe AG, et al. Effectiveness of preoperative staging in rectal cancer: digital rectal examination, endoluminal ultrasound or magnetic resonance imaging? Br J Cancer 2004;91:23–9.[CrossRef][Medline]
- Brown G, Richards CJ, Bourne MW, Newcombe RG, Radcliffe AG, Dallimore NS, et al. Morphologic predictors of lymph node status in rectal cancer with use of high-spatial-resolution MR imaging with histopathologic comparison. Radiology 2003;227:371–7.[Abstract/Free Full Text]
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Top
Abstract
Introduction
