1 Acta Radiologica ISSN: (Print) (Online) Journal homepage: MR Pelvimetry - A Practical Alternative A. R. Wright, P. T. English, H. M. Cameron & J. B. Wilsdon To cite this article: A. R. Wright, P. T. English, H. M. Cameron & J. B. Wilsdon (1992) MR Pelvimetry - A Practical Alternative, Acta Radiologica, 33:6, To link to this article: Published online: 04 Jan Submit your article to this journal Article views: 171 Full Terms & Conditions of access and use can be found at
2 Acru Rudiologicu 33 (1992) Fux. 6 FROM THE MAGNETIC RESONANCE IMAGING UNIT, NEWCASTLE GENERAL HOSPITAL, THE UNIVERSITY DEPARTMENT OF RADIOLOGY, ROYAL VICTORIA INFIRMARY, AND THE RESEARCH UNIT, PRINCESS MARY MATERNITY HOSPITAL, NEWCASTLE UPON TYNE, GREAT BRITAIN. MR PELVIMETRY - A PRACTICAL ALTERNATIVE A. R. WRIGHT, P. T. ENGLISH, H. M. CAMERON and J. B. WILSDON Abstract Pelvimetry remains a useful technique as part of the assessment of the term breech pregnancy where vaginal delivery is planned. MR pelvimetry is accurate, well tolerated and shows soft-tissue structures as well as bone. It avoids the potentially carcinogenic effects of ionising radiation and is thought to be completely safe for mother and fetus. A technique of MR pelvimetry is described which uses gradient-echo sequences. This quick, practical method makes minimal inroads into valuable scanning time, and may therefore be considered a potentially cost-effective alternative to conventional pelvimetry. Key words: Pelvis, measurements; -, MR; magnetic resonance (M R). rapid imaging. Although genuine indications for pelvimetry are limited, conventional radiologic pelvimetry is still a commonly performed examination (1 I, 15). Drawbacks include radiation exposure to fetus and mother, potential inaccuracy and inability to visualise soft tissue stuctures. Pelvimetry using CT is a partial solution to some of these problems, although it uses ionising radiation and has its own specific difficulties. MR pelvimetry has the considerable advantage over both conventional radiographic and CT pelvimetry of not using ionising radiation, and is thought to be free from harmful effects on the fetus (17). The method is accurate with good demonstration of soft-tissue structures as well as bony landmarks, and patient acceptability is high (19). A major obstacle to widespread adoption of MR pelvimetry is the relative lack of machines, and the pressing demands already being made on them by other clinical specialties. A quick, practical method of MR pelvimetry is described which makes minimal inroads into machine time. This uses TI :weighted gradient-echo sequences enabling a full pelvimetry examination in less than 5 min of scanning time. Material and Methods Twenty-seven female patients with a mean age of 27.8 years (range years) were scanned in order to evaluate the method. Twenty-one were pregnant; 8 of these had breech pregnancies and the remainder had cephalic presentations. All pregnant patients were at or near term, and gave informed consent to the examination. Scans were carried out on a General Electric MR Max 0.5 T machine with the patient supine entering the scanner feet first. No patient was too large to fit into the bore of the scanner. TI -weighted gradient-echo sequences were used throughout, and the sequence parameters for pilot and definitive scans are shown in the Table. A few patients had more detailed spin-echo TI -weighted scans (TRITE ms) where there was an additional clinical problem relating to the uterus, placenta or fetus. With initial centring at the level of the anterior superior iliac spines, a set of 3 axial pilot scans was taken. From one of these, a set of 5 sagittal scans was prescribed (Fig. 1 a). The median sagittal image thus obtained was used to measure the sagittal inlet (true conjugate) and sagittal outlet as with conventional lateral pelvimetry (Fig. 1 b). The curvature of the sacrum is easily assessed on this view. Using the median sagittal image, a set of axial oblique scans was then prescribed parallel to and centred upon a plane from the S1 /S2 disc space to the superior border of the symphysis pubis (Fig. 2 a). This plane is parallel to the true pelvic inlet, and the scans obtained allow assessment of the overall shape of the inlet as well as measurement of the transverse diameter (Fig. 2 b). The median sagittal image was again used to prescribe Accepted for publication 12 May
3 MR PELVIMETRY 583 a b Fig. 1. Gradient-echo pelvimetry scans. An axial pilot scan (a) is used to prescribe a set of sagittal scans. From these, the median sagittal image (b) is used to measure the sagittal inlet and outlet. Fetal and maternal anatomy are also well displayed. Table Sequence parameters used for MR pelvimetry TR TE Flip angle Field of view Slice thickness Slice interval Matrix size Signal averages (NEX) Number of slices Scan time 180 ms (100 ms for pilot) 14 ms x 42 cm 10 mm II mm 224 x 224 (160 x I28 for pilot) 2 or I (I for pilot) 5 (3 for pilot) I min 30 s or 45 s (I9 s for pilot) a set of axial oblique images parallel to a line from the sacrococcygeal junction to the inferior border of the symphysis pubis (Fig. 3 a). One of these scans invariably shows the ischial spines, allowing measurement of the bispinous distance in the mid-pelvis (Fig. 3 b). All measurements were made from the exterior surface of the appropriate bony cortex using the standard measurement cursor built into the machine. The accuracy of the cursor system was calibrated indepe ndently using a water phantom of known dimension. This was scanned in the axial, coronal, and sagittal planes using the same machine parameters as in the study. Measurements were taken in various planes similar to those used when measuring patients scans. Overall error (a combination of machine and operator error) was less than 1% and was not affected by scan plane. Where patients had both MR and standard radiographic pelvimetry, results from the 2 techniques were compared. Results All patients were scanned without difficulty, and good quality images were obtained in all cases allowing straightforward measurement. There were no unsuccessful examinations due to claustrophobia, patient size, or discomfort. All patients except one had satisfactory pelvic dimensions on MR pelvimetry. Ten patients had both MR and conventional pelvimetry. In these cases, there was good concordance between the 2 techniques for sagittal inlet and sagittal outlet measurements. However, for both transverse inlet and bispinous distance, MR pelvimetry gave either the same or bigger measurements than conventional pelvimetry in all cases. These differences between the 2 techniques were statistically significant (using paired t-testing). For the transverse inlet, the differences were small (range cm, mean 0.3 cm, p < 0.02). For the hispinous distance, differences were larger and potentially clinically important (range cm, mean 1.1 cm, p < 0.01). In 2 patients the bispinous distances were difficult to assess on radiographs due to poor contrast, but were easily measured on the MR images. Of the 8 patients with breech pregnancies, all had satisfactory pelvic measurements with MR pelvimetry. Five proceeded to vaginal deliveries, including one with twins. The other 3 patients had Caesarean section due respectively to
4 584 A. R. WRIGHT ET AL. a b Fig. 2. The median sagittal image is used to prescribe a set of scans in a plane parallel to the pelvic inlet (a). These scans allow measurement of the transverse inlet and assessment of the overall shape of the inlet (b). footling breech with failure to progress in late first stage; footling breech twins in a subfertile patient; and failure to progress in the second stage. In none of these cases was the Caesarean section considered on clinical grounds to have been related to an inadequate pelvis. Discussion Indications. Numerous workers have examined the role of pelvimetry in obstetrics in an attempt to identify precisely where it is likely to affect management and therefore potentially improve clinical outcome. As a result of this reappraisal, the number of indications for pelvimetry is now felt to be limited. Pelvimetry is no longer considered to have any value in the obstetric management of the cephalic presentation (7. 10). However, it is generally accepted that pelvimetry is an important element in the selection of breech fetuses for vaginal delivery when used in conjunction with the clinical picture (4, 8), and this is its main indication today. There may also be a minor role for pelvimetry where there is pelvic deformity; for example, post-traumatic or related to metabolic bone disease (5). Virtually all of the work assessing the clinical utility of pelvimetry has been based on conventional pelvimetry, and much of the impetus for these studies has come from concern over possible harmful effects to the fetus from irradiation. However, the principles for selecting patients for pelvimetry should apply regardless of which imaging technique is used (9). Radiation dose. The association of fetal radiography exposure with childhood malignancy by STEWART et al. (20, 21) in the mid-50 s led to efforts to reduce fetal dose during pelvimetry. The number of views in each examination was reduced, and the pelvic brim view with its high fetal dose was abandoned (3, 15). Subsequently, the use of strict collimation, fast film-screen combinations and gridless technique has made a further contribution to dose reduction in conventional pelvimetry (18). Against this background, recent reports have suggested that pelvimetry is not being used with optimum selectivity. KACZMAREK et al. (1 1) showed a substantial increase in the annual number of pelvimetry examinations in the USA over an 18-year period. MOLE (15) has shown that the initial reduction in requests for radiography by obstetricians in the UK following the publications of STEWART et al. (20, 21) was not maintained. In fact, by the 70 s. the rate of antenatal radiographs reached the same level as in 1954 to This paper also reaffirms the hypothesis that carcinogenesis by ionising radiation has no threshold, and challenges the widely held view that the fetus is less radiosensitive in the 3rd trimester than at earlier stages of pregnancy (15). Although it may not be possible to extrapolate the above trends into current radiological practice, it is clear that there is no reason for complacency over the issue of fetal irradiation. CT pelvimetry is a relatively low-dose technique compared with conventional pelvimetry. The method of FEDERLE et al. (6) uses an a.p. and lateral scout view and requires a
5 MR PELVIMETRY 585 a Fig. 3. The median sagittal image is again used to prescribe a set of axial oblique images in the plane shown (a). Measurement of the bispinous distance is made from one of these (b). b single axial slice for the bispinous distance. Other radiologists have not used the axial slice in all patients, but only when necessary (1, 22). Estimates of fetal radiation dose from CT pelvimetry vary. A study by MOORE & SHEARER (16) suggests that the fetal dose from 2 scout views and an axial slice is between 2.5 and 3.8 mgy. Further dose reductions may be possible with CT pelvimetry by using a p.a. projection for one scout view and reducing mas to the lowest possible level (23). Around 90% of the overall dose arises from the axial slice. FEDERLE et al. (6) suggest using the foveae as a landmark for the axial slice, but in fact this does not always give the correct level for the ischial spines (2, 22). In this case, one or more additional slices would be needed to locate the spines, and overall dose would thus be markedly increased. The proportion of this dose received by the fetus is highly variable, depending on presentation and position in the pelvis, and may not be minimal as suggested by some authors (6, 16). MR pelvimetry uses no ionising radiation, and there is no evidence to date that exposure of the fetus to the static fields, gradient fields, or radiofrequency fields used in current clinical MR scanners has any adverse effect. The view of the National Radiological Protection Board of the UK is that...it is prudent, until further information becomes available, to exclude pregnant women during the first 3 months of pregnancy. However, they state that MR should be considered for this group when required for diagnosis that would otherwise involve a radiographic procedure (17). Accuracy. There is considerable potential for error with conventional pelvimetry. This is caused by a number of factors including poor definition of bony landmarks due to low contrast (a particular problem with large patients); rotation of the patient, and other projectional anomalies; inaccuracy in correcting for magnification; and operator error in making and recording the measurements (12). The above sources of error also apply to CT pelvimetry although to a lesser extent. Difficulties due to low contrast will arise in the obese.patient, and patient positioning is important. Although sagittal measurements made from the lateral scout views are accurate, transverse measurements from the a.p. scout views may be inaccurate by up to 10% as a result of magnification error. Inaccuracy in this plane can only be avoided by careful positioning of the points to be measured in the plane of the centre of rotation of the scanner, or by applying a correction formula (23). Pelvimetry using MR has a clear advantage in accuracy over radiographic techniques. Tissue contrast is high in all patients and landmarks are easily recognised, even in obese patients. Good quality images were obtained from 2 patients in this study who weighed over 100 kg. These patients were easily accommodated in the bore of the scanner. Patient positioning is not critical with MR pelvimetry as the scan plane can be obliqued if the patient is not straight. A major benefit with MR is the ability to make measurements without the need to correct for magnification since measurements are accurate in all planes. There is a potential source of error with MR pelvimetry in the measurement of the sagittal outlet due to difficulty in identifying the junction between the sacrum and the first
6 586 A. R. WRIGHT ET AL. Fig. 4. Poor definition of the sacrum is due to susceptibility artefact from rectal gas. mobile coccygeal segment. This measurement is most likely to be inaccurate with conventional pelvimetry and CT pelvimetry for the same reasons (12). However, the degree of inaccuracy involved is unlikely to be clinically important with any technique. A further problem noted with 2 patients was poor definition of the sacrum due to susceptibility artefact from rectal gas (Fig. 4). This artefact, which is common with gradient-echo sequences, did not cause any major problems with measurement of the sagittal dimensions. The interspinous distance was always easily assessed with MR, whereas this important measurement can be difficult with conventional pelvimetry due to low contrast, and requires an axial slice with CT pelvimetry. In the patients who had both MR and conventional pelvimetry, statistically significant differences in the transverse measurements were noted between the 2 techniques. Although these differences are not likely to be clinically relevant in the case of the transverse inlet measurements, the differences in the bispinous distance measurements were large enough in some cases to be of clinical importance. We believe that the differences are due to underestimation of the bispinous distance on conventional pelvimetry. This gives some cause for concern as the potential consequences of inaccurate pelvimetry, namely inappropriate Caesarean section or inappropriate trial of vaginal delivery, have obvious clinical and possibly medicolegal repercussions. In general, MR pelvimetry is considered more accurate than conventional pelvimetry due to better contrast resolution, and because measurements are made directly with no need to correct for magnification. These important factors, as well as the ease and simplicity with which measurements are made compared with radiographic techniques, make it likely that operator error also will be reduced with MR. Scans obtained by using the technique described above show the overall shape of the pelvic inlet as well as the transverse diameter (Fig. 2 b). This feature can be useful in patients with pelvic deformity from whatever cause. Other considerations. Another important advantage of MR pelvimetry over other techniques is superior visualisation of soft-tissue pelvic structures. This may be useful in evaluating potential causes of soft-tissue dystocia, or where there is an additional clinical problem relating to the placenta, uterus, or fetus (13, 14). In this situation, however, better quality, spin-echo sequences may be necessary with an associated increase in scan time. There was a high degree of patient acceptability of MR pelvimetry in this series. No patient refused the study, and no studies were abandoned due to claustrophobia, or difficulty lying supine during the short scanning period. MR pelvimetry is quickly and easily carried out using the method described and all measurements can be performed by technical staff after brief instruction. Radiological input is, therefore, only necessary in difficult cases, with a consequent improvement in cost-effectiveness. Scans with 2 signal averages were of better quality than those with one signal average at the expense of an increase in scan time from 45 s to 1 min 30 s for each block of scans. However, all scans with one signal average were perfectly adequate for pelvimetry. Depending on scan parameters, total scanning time is between 2.5 and 5 min, with an overall examination time of less than 15 min in most cases. This is comparable with other forms of pelvimetry. If costed on a pro-rata basis related to scan time, MR pelvimetry would not be prohibitively expensive, although it clearly is more costly than other techniques at present. With increasing availability of scanners, however, there is potential for cost-effectiveness. Conclusion. MR pelvimetry has no known adverse effects and gives good quality, easily interpretable images with a high degree of patient acceptability. Using the gradient-echo technique outlined in the study, this can be accomplished simply and quickly with minimal incursion into scanning time. Provided that indications for pelvimetry examinations remain within the present guidelines, MR pelvimetry should be considered an effective, safe and potentially cost-effective alternative to other forms of pelvimetry. Requestfor reprints: Dr. Andrew R. Wright, Department of Radiology, Royal Infirmary of Edinburgh, Lauriston Place, Edinburgh EH3 9YW, Great Britain. REFERENCES 1. ADAM P., ALBERGE Y.. CASTELLANO S.. KASSAB M. & ESCUDE B.: Pelvimetry by digital radiography. Clin. Radiol. 36 (1985) ARONSON D. & KIER R.: CT pelvimetry. The foveae are not an accurate landmark for the level of the ischial spines. AJR 156 (1991) CLAR K. C.: Positioning in radiography, 8th edn. Heinemann Medical, London 1964.
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