- Email: [email protected]
Pelvimetry by digital radiography
ClinicalRadiology (1985)36,327-330 © 1985RoyalCollegeof Radiologists
0009-9260/85/407327502.00
Pelvimetry by Digital Radiography Ph. ADAM, Y. A L B E R G E , S. CASTELLANO, M. KASSAB and B. ESCUDE Department of Radiology, CHU Rangueil, University of Toulouse, Toulouse, France We studied the ability of digital radiographs generated on a computed tomography scanner to replace conventional pelvimetry. Two digital radiographs were usually sufficient to measure the maternal pelvis. The antero-posterior (AP) digital radiograph allowed measurement of the transverse diameter of the pelvic inlet, the interspinous distance and the intertuberous diameter. These three distances were corrected for magnification after reference to the lateral digital radiograph. The lateral radiograph gives the AP diameter of the pelvic inlet and the low sagittal diameter. Dosimetry studies demonstrated that maternal skin doses and fetal gonad doses were very low. Pelvimetry using digital radiography is a simple procedure which offers the advantage of low radiation exposure to both fetus and mother and a high accuracy of measurement.
Pelvimetry is an examination frequently performed, sometimes in 20% of deliveries (Ramde et al., 1983). Every abnormal presentation, and in particular breech presentation, requires pelvimetry. Generally, in French departments of obstetrics, three views of the pelvis are demanded for pelvimetry: one lateral view and two antero-posterior (AP) views, the Thom's view and the Fernstrom view (Ram@ et al., 1983). Even with improvements in equipment and techniques, the child irradiated in utero by the three views of conventional pelvimetry has received at birth the equivalent of one or several years of natural irradiation (Ramee et al., 1983). It must also be emphasised that most pelvimetric measurements use more radiation than those performed according to the accurate methods of Russell et aI. (1980) or Ram6e et al. (1983)_ This examination is often carried out by inexperienced teams with difficulty, inaccuracy and little regard to radiation exposure. Federle et al. (1982) were the first to replace completely the conventional method with two lowdose digital radiographs generated on a computed tornography (CT) scanner (digital radiograph pelvimetry) and one computed tomographic section. They demonstrated that digital radiograph pelvimetry is an accurate, simple and low-dose procedure. The purpose of our work is to emphasise again the qualities of digital radiograph pelvimetry and, to improve the method of Federle et al. (1982), more numerous measurements of the maternal pelvis were made from the radiographs. (In most cases two digital radiographs are sufficient, without a CT section.) Correspondence to: Dr Ph. Adam, Department of Radiology, Chemin du Vallon, CHU Rangueil, 31054 Toulouse Cedex, France.
METHOD Digital radiographs were obtained in AP and lateral projections, using the scout view adaptation of the CGR CE 10 000 (130 kV, 90 mA, 13.3 ms pulse width, 1 mm spacing). For axial CT sections the radiological constants used were: 130 kV, 45 mA and 3.4 s. Phantom studies showed the errors of the distances measured on lateral and AP digital radiographs. The phantom used was a 20 cm-long scale. On the lateral radiograph this scale was measured, using the electronic cursor and computcr program, along a plane perpendicular to the table top and centred within the gantry opening. For the AP radiograph the scale was disposed transversely within the gantry opening and along a plane parallel to the table top; the scale was measured at each centimetre between 0 and 16 with regard to the table top. For the dosimetry studies we used a phantom and an ionisation chamber. The phantom consisted of unexpanded polystyrene square plates (40 cm ×40 cm x2 cm); its height was comparable with the mean thickness (34 cm) of a pregnant abdomen. For the determination of the maternal skin doses we measured the entrance skin dose and the exit skin dose. For the determination of the fetal gonad dose delivered by the AP digital radiograph, we took into consideration the variable position of the fetal gonads according to fetal presentation. Like Ramde et al. (1983), we considered that in vertex presentations the fetal gonads were located 7 cm under the maternal skin and that in breech presentations the fetal gonads were located close to the pelvic inlet (mean distance of 10 cm with regard to the table top). Russell et al. (1980) measured the fetal doses at the assumed centre of the fetus, and not the fetal gonad doses. They did not take into consideration the fetal presentation. For the measurements of fetal gonad doses delivered by the lateral digital radiograph, the fetal gonads were taken to be located in the middle of the phantom. To calculate the dose received by the fetal gonads when one CT section was performed through the ischial spines, we took into consideration the mean distance between the fetal gonads and the plane of section; this distance varies according to the presentation and is taken as about 30 cm for vertex presentation and about 6 cm for breech presentation. RESULTS
We have verified by phantom studies that the distances measured along a perpendicular plane to the table top, and centred within the gantry opening, were represented accurately by the lateral digital radiograph. On the lateral radiograph it was possible, using the electronic cursor and the CE 10000 computer
328
CLINICAL RADIOLOGY
program, to measure the A P diameter of the pelvic inlet and the low sagittal diameter of the pelvis, with no need to correct for magnification (Fig. 1). We were also able to determine the inclination of the plane of the pelvic inlet. Using the AP digital radiograph enabled us to measure the transverse diameter of the pelvic inlet, the interspinous distance (mid-pelvic diameter) and the intertuberous diameter (Fig. 2). F r o m the p h a n t o m studies, we knew that the distances measured on the AP radiograph along a plane parallel to the table top and centred within the gantry opening could be measured accurately using the cursor and computer program. But above or below the centre of the gantry, there were errors in distance m e a s u r e m e n t s by the cursor. The p h a n t o m studies gave us these variations and permitted us to draw a graph represented by a straight line (Fig. 3). This graph allowed us to correct the transverse diameters measured on the A P radiograph according to their level with regard to the plane of the table. From a construction derived from that of Ball and Marchbanks (1935), and also used by Bean e t al. (1967), we know from the lateral digital radiograph the levels of the transverse diameter of the pelvic inlet, of the mid-pelvic diameter and of the intertuberous distance with regard to the plane of the table top (Fig. 4). If
Fig. 2-The AP digital radiograph of the pelvis gives us the following uncorrected values: transverse diameter of the pelvic inlet (1), interspinous distance (mid-pelvic diameter) (2) and intertuberous diameter (3). 209,0
-~-- . . . . . . .
2°3.9+ !
~
191.1~--.
~
"
:
:
.
'..
:
.
1~5~4-:
:
oJ
183.4~--...
i
'
.
:.
:
,
: .
:
i
1,9.3-
. . . . . ~
'
q
-
~--
~e"
168.o
[.. 0
~.~'-
.
• I 1
.
• J- .. 2
~. J ~ "J - - .
"
:
3
.
.
5
:.
:
.
1 :" ~
i~
.
.
.
.
.
"
i
Level
7
.
"
"
:
-
.
.
.
: .
............
t
8
"
.
J~ . ..i.....i 6
"
: ,:
~
1 ............. 4
•
.
~ .
"
"
:
..
--
" :
.
.
:
~
: .
• .,.
~________,,~--:.
~ .
! :..!.f
:
i
~ .
;
. . ,
i j /
"175.7 ~ - -
X+167,7
?.!
:
: , -
°
170,6
. Y=2.491
.:..!.:,!.!.?
198.5
I_.
. .
9
10
of the scale/table
.
top
~ .
.
!......~... 11
:
12
"
: .
.
.
J. ...........
|
13
i4
.
l .....L 15
16
(cm)
Fig. 3 - M e a s u r e m e n t s of a 20 era-long scale transversely disposed within the gantry, on an AP digital radiograph performed at different levels with regard to the table top.
Table 1-Fetal gonad dose in mGy according to the method of pelvimetry and presentation
Method of pelvirnetry
Ramfe et al. (1983): rare-earth screens and three views Ramde et al. (1983): rare-earth screens and two views Russell et al. (1980): two views Antero-posterior and lateral digital radiographs Antero-posterior and lateral digital radiographs+one CT section
Fig. 1 A lateral digital radiograph of the pelvis allows measurement without correction of the AP diameter of the pelvic inlet (1), the low sagittal diameter of the pelvis (2) and the cord of the sacrum
(3).
Vertex presentation
Breech presentation
2.20
1.40
<0.10 0.35
0.70 0.35
0.37
0.17
0.40
0.40
the correction was low or negligible for the transverse diameter of the pelvic inlet, the corrections were necessary for the two other diameters. The dosimetry studies gave us the maternal skin doses and the fetal gonad doses. The entrance skin dose delivered by one digital radiograph was 0.4 m G y ; the exit skin dose was unmeasurable, The fetal gonad
329
PELVIMETRY BY DIGITAL RADIOGRAPHY
(a)
(b)
Fig. 4 - (a) Line 1 is the A P diameter of the pelvic inlet. Line 2 is the cord of the sacrum and is used to construct Line 3 parallel to Line 2 and passing through the ischial spines. The junction of Lines 1 and 3 represents the level of the transverse diameter of the pelvic inlet. Line 4 is the distance of the transverse diameter from the table top. (b) Line 1 is the plane of the table; Line 2 is the distance of the ischial spines above the table top; Line 3 is the distance of the tuberosities from the table top.
doses are given in Table 1:0.37 mGy for the two digital radiographs with vertex presentation; 0.4mGy for the two digital radiographs and one CT section with vertex or breech presentation; 0.17mGy for the two digital radiographs with breech presentation. DISCUSSION Like Federle et al. (1982), we think that digital radiography is a simple procedure, easy and rapid to perform_ The setting up of the maternal pelvis must be perfect, in the centre of the gantry opening, to get strictly antero-posterior and lateral views, as for conventional pelvimetry. Using the AP digital radiograph, Federle et al. (1982) measured only the transverse diameter of the pelvic inlet. We-find that the AP digital radiograph allows measurement of the interspinous distance (midpelvic diameter) and the intertuberous distance. According to Federle et al. (1982), the ischial spines are frequently difficult to locate on AP radiographs (digital or plain), so they recommend an axial CT section through this level to measure the mid-pelvic diameter (Fig. 5). It was empirically determined that the ischial spines usually lie at the same level as the fovea of the femoral head. We found that the ischial
Fig. 5 - T h i s axial CT section allows measurement of the interspinous distance (1) if the ischial spines are difficult to locate on the A P digital radiograph (L +130, W 800).
spmes are generally visible on the AP digital radiograph and that, in most cases, the axial CT section can be avoided. To know the true values of the diameters measured on the AP radiograph by the electronic cursor, corrections are necessary_ The lateral radiograph, by the determination of the level of
330
CLINICALRADIOLOGY
these diameters with regard to the table top, and with reference to our graph, allows the corrected values to be obtained. On the lateral digital radiograph we can also measure, without any correction, the AP diameter of the pelvic inlet, the low sagittal diameter of the pelvis and the inclination of the plane of the pelvic inlet_ Digital radiograph pelvimetry gives the pelvic diameter with a high degree of accuracy. In general use this method is likely to be more accurate than other methods because it is simple. On a dry cadaveric osseous pelvis, we compared direct measurements on the anatomical piece and measurements by the digital radiograph method and the conventional method. The error is less than 1% by the digital radiograph method and 10% by the conventional method. The greatest contribution of digital radiograph pelvimetry is certainly the reduction in radiation exposure. The International Commission on Radiological Protection has recommended that the radiation dose to the fetus should not exceed 1 cGy during pregnancy (Reekie et al., 1967). The fetal doses delivered by conventional pelvimetry are very variable according to the method but the mean gonad exposure has been estimated to be 2.23 mGy in male fetuses and 3.41 m G y in females, according to a recent survey (Wall et al., 1980). With some conventional methods the fetal doses can be smaller (Table 1). Russell et al. (1980) give us a fetal dose of 0.35 mGy with small fields for two views (lateral view and orthodiagraphic view). These 'fetal' doses are comparable with our 'fetal gonadal' doses. The fetal gonadal doses of Ramde et al. (1983) with three views are larger than with our digital radiograph: six times larger in vertex presentation and eight times larger in breech presentation. The fetal gonadal doses of Ramde et al. (1983), with two views (lateral view and Fernstrom view) are much smaller and comparable with our digital radiograph method. We can explain the difference between the twoviews and the three-views methods of Ramde et al. (1983) as follows. For the Thom's view, the patient is sitting with a 45 ° tilting of the trunk with regard to the horizontal plane; the beam is vertical and focused above the symphysis pubis. Thus, the pelvic inlet is parallel to the film. This view gives us the better appreciation of the morphology of the pelvic inlet, but
heavily irradiates fetal gonads, particularly with a vertex presentation. The Thorn's view should not now be used; two views often r e c o m m e n d e d are the lateral view and the Fernstrom view. For the Fernstrom view, the patient is supine with flexed and abducted thighs. The beam is tilted 20 ° cranially and centred on the symphysis pubis. This view is easy to perform, accurate and largely reduces the radiation exposure for the fetal gonads, which are located out of the direct X-ray beam_ We note that only a small additional dose is delivered by one CT section because of the tight collimation of the CT section and because of the distances between the section plane and the fetal gonads. Our entrance skin doses delivered by two digital radiographs (0.4 m G y x2) are smaller than the doses given by Federle et al. (1982) (1 mGy x2) and much smaller than the doses of Russell et al. (1980) (1 m G y + 2 . 2 mGy). Conventional pelvimetry is a radiological technique that requires practice to be perfect. We emphasise that digital radiograph pelvimetry is a more simple and more accurate procedure and gives a smaller radiation dose than conventional pelvimetry.
REFERENCES
Ball, R. P. & Marchbanks, S. S. (1935). Roentgen pelvimetry and fetal cephalometry: a new technique. Radiology, 24, 77-84. Bean, W. J., Cook, R. T., Eavenson, L. W. & Bristow, L. J. (1967). Pelvimetry. Radiologic Clinics of North America, 5, 29-46. Federle, M. P., Cohen, H. A., Roscnwein, M. F., Brant-Zawadski, M. N. & Cann, C, E. (1982). Pelvimetry by a digital radiography: a low-dose examination. Radiology, 143, 733-735. Ramde, A., Duvauferrier, R. & Grail, J. Y. (1983). Pelvimetrie. Soci~td Fran~aise de Radiologie M6diea/c, Paris, 7-9 November 1983. Reekie, D., Davison, M. & Davidson, J. K. (1967). The radiation hazard in radiology of the female abdomen and pelvis. British Journal of Radiology, 40, 849-854. Russell, J. G. B., Hufton, A. & Pritchard, C. (1980). Gridless (low radiation dose) pelvimetry. British Journal of Radiology, $3, 233-236. Wall, B. F., Fisher, E. S., Shrimpton, P. C. & Rae, S. (1980). Current Levels of Gonadal Irradiation from a Selection of Routine Diagnostic X-ray Examinations in Great Britain, NRPB-R105.
National RadiologicalProtection Board, Harwell, Didcot, Oxon.
0009-9260/85/407327502.00
Pelvimetry by Digital Radiography Ph. ADAM, Y. A L B E R G E , S. CASTELLANO, M. KASSAB and B. ESCUDE Department of Radiology, CHU Rangueil, University of Toulouse, Toulouse, France We studied the ability of digital radiographs generated on a computed tomography scanner to replace conventional pelvimetry. Two digital radiographs were usually sufficient to measure the maternal pelvis. The antero-posterior (AP) digital radiograph allowed measurement of the transverse diameter of the pelvic inlet, the interspinous distance and the intertuberous diameter. These three distances were corrected for magnification after reference to the lateral digital radiograph. The lateral radiograph gives the AP diameter of the pelvic inlet and the low sagittal diameter. Dosimetry studies demonstrated that maternal skin doses and fetal gonad doses were very low. Pelvimetry using digital radiography is a simple procedure which offers the advantage of low radiation exposure to both fetus and mother and a high accuracy of measurement.
Pelvimetry is an examination frequently performed, sometimes in 20% of deliveries (Ramde et al., 1983). Every abnormal presentation, and in particular breech presentation, requires pelvimetry. Generally, in French departments of obstetrics, three views of the pelvis are demanded for pelvimetry: one lateral view and two antero-posterior (AP) views, the Thom's view and the Fernstrom view (Ram@ et al., 1983). Even with improvements in equipment and techniques, the child irradiated in utero by the three views of conventional pelvimetry has received at birth the equivalent of one or several years of natural irradiation (Ramee et al., 1983). It must also be emphasised that most pelvimetric measurements use more radiation than those performed according to the accurate methods of Russell et aI. (1980) or Ram6e et al. (1983)_ This examination is often carried out by inexperienced teams with difficulty, inaccuracy and little regard to radiation exposure. Federle et al. (1982) were the first to replace completely the conventional method with two lowdose digital radiographs generated on a computed tornography (CT) scanner (digital radiograph pelvimetry) and one computed tomographic section. They demonstrated that digital radiograph pelvimetry is an accurate, simple and low-dose procedure. The purpose of our work is to emphasise again the qualities of digital radiograph pelvimetry and, to improve the method of Federle et al. (1982), more numerous measurements of the maternal pelvis were made from the radiographs. (In most cases two digital radiographs are sufficient, without a CT section.) Correspondence to: Dr Ph. Adam, Department of Radiology, Chemin du Vallon, CHU Rangueil, 31054 Toulouse Cedex, France.
METHOD Digital radiographs were obtained in AP and lateral projections, using the scout view adaptation of the CGR CE 10 000 (130 kV, 90 mA, 13.3 ms pulse width, 1 mm spacing). For axial CT sections the radiological constants used were: 130 kV, 45 mA and 3.4 s. Phantom studies showed the errors of the distances measured on lateral and AP digital radiographs. The phantom used was a 20 cm-long scale. On the lateral radiograph this scale was measured, using the electronic cursor and computcr program, along a plane perpendicular to the table top and centred within the gantry opening. For the AP radiograph the scale was disposed transversely within the gantry opening and along a plane parallel to the table top; the scale was measured at each centimetre between 0 and 16 with regard to the table top. For the dosimetry studies we used a phantom and an ionisation chamber. The phantom consisted of unexpanded polystyrene square plates (40 cm ×40 cm x2 cm); its height was comparable with the mean thickness (34 cm) of a pregnant abdomen. For the determination of the maternal skin doses we measured the entrance skin dose and the exit skin dose. For the determination of the fetal gonad dose delivered by the AP digital radiograph, we took into consideration the variable position of the fetal gonads according to fetal presentation. Like Ramde et al. (1983), we considered that in vertex presentations the fetal gonads were located 7 cm under the maternal skin and that in breech presentations the fetal gonads were located close to the pelvic inlet (mean distance of 10 cm with regard to the table top). Russell et al. (1980) measured the fetal doses at the assumed centre of the fetus, and not the fetal gonad doses. They did not take into consideration the fetal presentation. For the measurements of fetal gonad doses delivered by the lateral digital radiograph, the fetal gonads were taken to be located in the middle of the phantom. To calculate the dose received by the fetal gonads when one CT section was performed through the ischial spines, we took into consideration the mean distance between the fetal gonads and the plane of section; this distance varies according to the presentation and is taken as about 30 cm for vertex presentation and about 6 cm for breech presentation. RESULTS
We have verified by phantom studies that the distances measured along a perpendicular plane to the table top, and centred within the gantry opening, were represented accurately by the lateral digital radiograph. On the lateral radiograph it was possible, using the electronic cursor and the CE 10000 computer
328
CLINICAL RADIOLOGY
program, to measure the A P diameter of the pelvic inlet and the low sagittal diameter of the pelvis, with no need to correct for magnification (Fig. 1). We were also able to determine the inclination of the plane of the pelvic inlet. Using the AP digital radiograph enabled us to measure the transverse diameter of the pelvic inlet, the interspinous distance (mid-pelvic diameter) and the intertuberous diameter (Fig. 2). F r o m the p h a n t o m studies, we knew that the distances measured on the AP radiograph along a plane parallel to the table top and centred within the gantry opening could be measured accurately using the cursor and computer program. But above or below the centre of the gantry, there were errors in distance m e a s u r e m e n t s by the cursor. The p h a n t o m studies gave us these variations and permitted us to draw a graph represented by a straight line (Fig. 3). This graph allowed us to correct the transverse diameters measured on the A P radiograph according to their level with regard to the plane of the table. From a construction derived from that of Ball and Marchbanks (1935), and also used by Bean e t al. (1967), we know from the lateral digital radiograph the levels of the transverse diameter of the pelvic inlet, of the mid-pelvic diameter and of the intertuberous distance with regard to the plane of the table top (Fig. 4). If
Fig. 2-The AP digital radiograph of the pelvis gives us the following uncorrected values: transverse diameter of the pelvic inlet (1), interspinous distance (mid-pelvic diameter) (2) and intertuberous diameter (3). 209,0
-~-- . . . . . . .
2°3.9+ !
~
191.1~--.
~
"
:
:
.
'..
:
.
1~5~4-:
:
oJ
183.4~--...
i
'
.
:.
:
,
: .
:
i
1,9.3-
. . . . . ~
'
q
-
~--
~e"
168.o
[.. 0
~.~'-
.
• I 1
.
• J- .. 2
~. J ~ "J - - .
"
:
3
.
.
5
:.
:
.
1 :" ~
i~
.
.
.
.
.
"
i
Level
7
.
"
"
:
-
.
.
.
: .
............
t
8
"
.
J~ . ..i.....i 6
"
: ,:
~
1 ............. 4
•
.
~ .
"
"
:
..
--
" :
.
.
:
~
: .
• .,.
~________,,~--:.
~ .
! :..!.f
:
i
~ .
;
. . ,
i j /
"175.7 ~ - -
X+167,7
?.!
:
: , -
°
170,6
. Y=2.491
.:..!.:,!.!.?
198.5
I_.
. .
9
10
of the scale/table
.
top
~ .
.
!......~... 11
:
12
"
: .
.
.
J. ...........
|
13
i4
.
l .....L 15
16
(cm)
Fig. 3 - M e a s u r e m e n t s of a 20 era-long scale transversely disposed within the gantry, on an AP digital radiograph performed at different levels with regard to the table top.
Table 1-Fetal gonad dose in mGy according to the method of pelvimetry and presentation
Method of pelvirnetry
Ramfe et al. (1983): rare-earth screens and three views Ramde et al. (1983): rare-earth screens and two views Russell et al. (1980): two views Antero-posterior and lateral digital radiographs Antero-posterior and lateral digital radiographs+one CT section
Fig. 1 A lateral digital radiograph of the pelvis allows measurement without correction of the AP diameter of the pelvic inlet (1), the low sagittal diameter of the pelvis (2) and the cord of the sacrum
(3).
Vertex presentation
Breech presentation
2.20
1.40
<0.10 0.35
0.70 0.35
0.37
0.17
0.40
0.40
the correction was low or negligible for the transverse diameter of the pelvic inlet, the corrections were necessary for the two other diameters. The dosimetry studies gave us the maternal skin doses and the fetal gonad doses. The entrance skin dose delivered by one digital radiograph was 0.4 m G y ; the exit skin dose was unmeasurable, The fetal gonad
329
PELVIMETRY BY DIGITAL RADIOGRAPHY
(a)
(b)
Fig. 4 - (a) Line 1 is the A P diameter of the pelvic inlet. Line 2 is the cord of the sacrum and is used to construct Line 3 parallel to Line 2 and passing through the ischial spines. The junction of Lines 1 and 3 represents the level of the transverse diameter of the pelvic inlet. Line 4 is the distance of the transverse diameter from the table top. (b) Line 1 is the plane of the table; Line 2 is the distance of the ischial spines above the table top; Line 3 is the distance of the tuberosities from the table top.
doses are given in Table 1:0.37 mGy for the two digital radiographs with vertex presentation; 0.4mGy for the two digital radiographs and one CT section with vertex or breech presentation; 0.17mGy for the two digital radiographs with breech presentation. DISCUSSION Like Federle et al. (1982), we think that digital radiography is a simple procedure, easy and rapid to perform_ The setting up of the maternal pelvis must be perfect, in the centre of the gantry opening, to get strictly antero-posterior and lateral views, as for conventional pelvimetry. Using the AP digital radiograph, Federle et al. (1982) measured only the transverse diameter of the pelvic inlet. We-find that the AP digital radiograph allows measurement of the interspinous distance (midpelvic diameter) and the intertuberous distance. According to Federle et al. (1982), the ischial spines are frequently difficult to locate on AP radiographs (digital or plain), so they recommend an axial CT section through this level to measure the mid-pelvic diameter (Fig. 5). It was empirically determined that the ischial spines usually lie at the same level as the fovea of the femoral head. We found that the ischial
Fig. 5 - T h i s axial CT section allows measurement of the interspinous distance (1) if the ischial spines are difficult to locate on the A P digital radiograph (L +130, W 800).
spmes are generally visible on the AP digital radiograph and that, in most cases, the axial CT section can be avoided. To know the true values of the diameters measured on the AP radiograph by the electronic cursor, corrections are necessary_ The lateral radiograph, by the determination of the level of
330
CLINICALRADIOLOGY
these diameters with regard to the table top, and with reference to our graph, allows the corrected values to be obtained. On the lateral digital radiograph we can also measure, without any correction, the AP diameter of the pelvic inlet, the low sagittal diameter of the pelvis and the inclination of the plane of the pelvic inlet_ Digital radiograph pelvimetry gives the pelvic diameter with a high degree of accuracy. In general use this method is likely to be more accurate than other methods because it is simple. On a dry cadaveric osseous pelvis, we compared direct measurements on the anatomical piece and measurements by the digital radiograph method and the conventional method. The error is less than 1% by the digital radiograph method and 10% by the conventional method. The greatest contribution of digital radiograph pelvimetry is certainly the reduction in radiation exposure. The International Commission on Radiological Protection has recommended that the radiation dose to the fetus should not exceed 1 cGy during pregnancy (Reekie et al., 1967). The fetal doses delivered by conventional pelvimetry are very variable according to the method but the mean gonad exposure has been estimated to be 2.23 mGy in male fetuses and 3.41 m G y in females, according to a recent survey (Wall et al., 1980). With some conventional methods the fetal doses can be smaller (Table 1). Russell et al. (1980) give us a fetal dose of 0.35 mGy with small fields for two views (lateral view and orthodiagraphic view). These 'fetal' doses are comparable with our 'fetal gonadal' doses. The fetal gonadal doses of Ramde et al. (1983) with three views are larger than with our digital radiograph: six times larger in vertex presentation and eight times larger in breech presentation. The fetal gonadal doses of Ramde et al. (1983), with two views (lateral view and Fernstrom view) are much smaller and comparable with our digital radiograph method. We can explain the difference between the twoviews and the three-views methods of Ramde et al. (1983) as follows. For the Thom's view, the patient is sitting with a 45 ° tilting of the trunk with regard to the horizontal plane; the beam is vertical and focused above the symphysis pubis. Thus, the pelvic inlet is parallel to the film. This view gives us the better appreciation of the morphology of the pelvic inlet, but
heavily irradiates fetal gonads, particularly with a vertex presentation. The Thorn's view should not now be used; two views often r e c o m m e n d e d are the lateral view and the Fernstrom view. For the Fernstrom view, the patient is supine with flexed and abducted thighs. The beam is tilted 20 ° cranially and centred on the symphysis pubis. This view is easy to perform, accurate and largely reduces the radiation exposure for the fetal gonads, which are located out of the direct X-ray beam_ We note that only a small additional dose is delivered by one CT section because of the tight collimation of the CT section and because of the distances between the section plane and the fetal gonads. Our entrance skin doses delivered by two digital radiographs (0.4 m G y x2) are smaller than the doses given by Federle et al. (1982) (1 mGy x2) and much smaller than the doses of Russell et al. (1980) (1 m G y + 2 . 2 mGy). Conventional pelvimetry is a radiological technique that requires practice to be perfect. We emphasise that digital radiograph pelvimetry is a more simple and more accurate procedure and gives a smaller radiation dose than conventional pelvimetry.
REFERENCES
Ball, R. P. & Marchbanks, S. S. (1935). Roentgen pelvimetry and fetal cephalometry: a new technique. Radiology, 24, 77-84. Bean, W. J., Cook, R. T., Eavenson, L. W. & Bristow, L. J. (1967). Pelvimetry. Radiologic Clinics of North America, 5, 29-46. Federle, M. P., Cohen, H. A., Roscnwein, M. F., Brant-Zawadski, M. N. & Cann, C, E. (1982). Pelvimetry by a digital radiography: a low-dose examination. Radiology, 143, 733-735. Ramde, A., Duvauferrier, R. & Grail, J. Y. (1983). Pelvimetrie. Soci~td Fran~aise de Radiologie M6diea/c, Paris, 7-9 November 1983. Reekie, D., Davison, M. & Davidson, J. K. (1967). The radiation hazard in radiology of the female abdomen and pelvis. British Journal of Radiology, 40, 849-854. Russell, J. G. B., Hufton, A. & Pritchard, C. (1980). Gridless (low radiation dose) pelvimetry. British Journal of Radiology, $3, 233-236. Wall, B. F., Fisher, E. S., Shrimpton, P. C. & Rae, S. (1980). Current Levels of Gonadal Irradiation from a Selection of Routine Diagnostic X-ray Examinations in Great Britain, NRPB-R105.
National RadiologicalProtection Board, Harwell, Didcot, Oxon.