Claussen et al., Pelvimetry 287

J. Perinat. Med.13 (1985) 287

Pelvimetry by digital radiography and its dosimetry

C. Claussen, D. Köhler, R Christ*, G· Golde, B. Lochner

Departments of Radiology, (Director: Prof. Dr. R. Felix) and ^Obstetricsand Gynecology (Director: Prof. Dr. G. Kindermann) University HospitalsCharlottenburg, Free University of Berlin, Fed. Rep. Germany

1 IntroductionSince the introduction of sonography into ob-stetrical diagnostic methods fetus and placentacan be measured and localized without expo-sure to radiation. However, the questionswhether pelvic dimensions are adequate for thepassage of an infant or whether there is a me-chanical obstruction have not been satisfactori-ly answered by ultrasound which has not achie-ved the accuracy of x-ray pelvimetry [1, 12, 13].The principal disadvantage of conventional ra-diography lies in its relatively high radiationdose. Digital radiography, an accessory to allmodern computer tomographs, offers an alter-native.

2 MethodsComputer tomographic equipment of the thirdand fourth generations has an additionalprogram which allows the acquisition of digitalradiographs. They are chiefly needed to localizethe suitable transverse cut.With digital radiography summation imagescorrelating to the conventional radiograph areobtained. During the exposure for the digitalradiograph tube and detector of the computertomograph remain stationary while the patientis moved along the long axis of her body. Thevariable positions of the image generating (x-

Curriculum vitaeCLAUS CLAUSSEN, born1945 in Harneln, Germany.Medical School in Erlan-gen and Heidelberg. Grad-uation Heidelberg 1971.Radiology Residency, Uni-versity of Heidelberg;Board Certification in Ra-diology 1978. Since 1979on the staff of the Depart-ment of Radiology, Char-lottenburg University Hos-pitals, Free University of Berlin. Areas of interest: Dy-namic computer tomography, nuclear magnetic resonancetomography.

ray tube) and image processing (detectors) com-ponents in relation to the long body axis allowa. -p., p. -a., and lateral projections on therecumbent patient. By a mathematical imagereconstruction process a large spectrum of atte-nuation values is obtained and made visible ona television monitor in a 20-step gray scale.Integrated software allows direct measurementof the distance between any two points. Bymanipulating the gray scale, bony contours canbe shown on the screen which do not show upas clearly on the subsequently obtained hardcopy of the radiograph because of its lessersensitivity. Because of the central projection,there is a position-dependent distortion for

1985 by Walter de Gruyter Co. Berlin · New York

288 Claussen et al., Pelvimetry

measurements at right angles to the movementof the topogram ("topogram" is the manufact-urer's designation for the accessory equipmentfor digital radiography). We determined experi-mentally a formula to correct this scale distor-tion.We used third generation computer tomogra-phy equipment (Somatom 2, Siemens). On pa-tients in the supine position we obtained APand lateral topograms.

3 Radiation dose

H BENER [4] and FEDERLE et al. [2] indicate asskin dose for digital radiography a maximalentry dose of lOOmrad (1.0 χ ΙΟ'3 Gy) and askin exit dose of less than lOmrad (10~4 Gy).FEDERLE et al. obtained an absorption dose of22mrad (2.2 χ ΙΟ'4 Gy). These values differfrom those obtained by us on the Somatom 2on a 30 cm wax phantom simulating absorptionproperties of human body tissue. We used ioni-sation dosimeters. For a topogram length of256 mm the computer selected 80 mAs and125 kV. We found a skin entry dose of 153 μGy

(15.3 mrad) and an exit dose of 3.6 μΟγ. In thecenter at 15 cm depth 24 μGy (2.4 mrad) weremeasured. Thus, a radiation dose of about48 μGy (4.8 mrad) exists for the fetus becausea lateral and a.-p. topogram were obtained ineach case.In addition, we measured radiation doses withhigh amplifying screens for comparison [3, 6,9]. We used a Quanta III screen (Dupont) anda Cronex IV (Dupont) film. The phantommeasurements at 125 kV and 4.5 mAs (filter 0.1Cu 1 Al) showed a sufficient skeletal image.The skin entry dose was 750 μGy (75 mrad).The exit dose was 17 \iGy. In the center a doseof 177 μGy was measured. In our experiencethese values are optimal for conventional pelvi-metry. In comparison the radiation dose fordigital radiography is 7 times less.

4 Patients and indications

From May 1982 to October 1983 twenty-fivepregnant women underwent digital pelvimetry.The following dimensions were determined:transverse (Fig. 1) and obstetric conjugate

Fig. 1. Transverse diameter D = 11.2cm (normal 13.5cm); corrected distance 11.5cm (The correction resulted fromthe formula determined by us).

J. Perinat. Med. 13 (1985)

Claussen et al., Pelvimetry 289

diameters (Fig. 2) as the two most important the oblique diameters of the pelvic inlet, themeasurements and the interspinous diameter diagonal conjugate (promontory to lower rim(Fig. 3) as the measurement for the narrowest of symphysis) or the distance from the tip ofpart of the pelvis. Other measurements such as the coccyx to the lower rim of the symphysis

Fig. 2. Obstetric conjugate diameter. D = 11.5cm, (normal 11 cm). No correction necessary because the error isneglibly small.

Fig. 3. Interspinous distance. D = 8.2cm (normal 10.5cm). The determination of the real distance yielded the samevalue as the corrected one.

J. Perinat. Med. 13 (1985)

290 Claussen et al., Pelvimetry

(antero-posterior diameter of the pelvic outlet)can be determined at any time, even afterwards,without additional exposure. Statements aboutthe obstetrically important sacral curvature areonly indirectly possible.This technique also depicts the fetal skeletonand thus allows the diagnosis of fetal malforma-tions or abnormal positions of the fetal head(e. g., hyperextension in breech presentation).An additional indication for the topogram arebreech presentations and a relative dispropor-tion after a previous operative delivery.

\ 5 Discussion

As recently as several years ago there wereabout 20 indications for pelvic radiography du-ring pregnancy [1]. This has changed since theintroduction of sonography. Two reasons caus-ed us to perform x-ray pelvimetry only rarelyduring the last few years: First, it is well knownthat knowledge of the dimensions of the bonypelvis allows only an uncertain statement aboutthe subsequent delivery; except for gross devia-tions from the norm, the size of the fetus andits adaptation for delivery mechanics are themore important parameters. Secondly, the ra-diation to the fetus in late pregnancy has asmall but potential risk of somatic or geneticdamage [5, 10]. Radiation exposure should notexceed 1 rad (10~2 Gy) during pregnancy [8],The gonad dose for conventional radiographyduring pregnancy is dependent on techniqueand is assumed to be between 150 mrad (1.5 χΙΟ'3 Gy) [11] and 885 mrad (8.85 χ ΙΟ"3 Gy)[7].Because the radiation dose with digital radio-graphy is only 15% of that of conventional x-

ray pelvimetry we see a renewed possibility forpelvimetry.

6 Conclusion

Digital radiography in comparison to conven-tional radiographic technique has the followingadvantages [4]:

1. Very low exposure of the body to ionizingradiation. Because the detectors practicallyabsorb all the emitted and penetrating raysonly a low entry dose is required.

2. Extremely low scatter radiation because ofthe collimation of the beam to 1.5mmwidth and favorable tube and dectector dis-tances to the patient.

3. High contrast resolution.4. Direct readout of the distance between two

points.5. Simple electronic image manipulation

(measurement of absorption values andothers) and data storage.

6. Avoidance of erroneous exposures.

Digital radiography has the following disad-vantages when compared to conventional ra-diography.1. Poor spacial resolution.2. Poor image quality in obese patients for

lateral images because of insufficient num-bers of photons. This cannot be compensat-ed for by altering the output of the x-raytube.

We conclude that digital radiography is anexact and simple method with a low radiationdose which may replace conventional radiogra-phy for x-ray pelvimetry.

Summary

Sonography is firmly established in obstetrics as themethod of choice for the localization of fetus andplacenta. However, for the determination of pelvicdimensions sonography does not suffice. Thus, for a fewstrictly selected indications conventional x-raypelvimetry is used. This has the disadvantage of therelatively high radiation dose for the fetus and the

maternal gonads. Since the introduction of digitalradiography, there is the alternative of digital pelvimetry.We performed dosimetry comparing digital and con-ventional radiography with the new high amplifyingscreens (Quanta III, Dupont)r using an ionizationdosimeter and a wax phantom. The radiation dose ofdigital radiography was 24 μΟγ. This corresponds to about

J. Perinat. Med. 13 (1985)

Claussen et ah, Pelvimetry 291

15% of a comparable conventional exposure (177 ).A further advantage of digital radiography is a stan-dardized and rapid exposure technique; furthermorepelvic dimensions can be measured directly.

The disadvantages are the poorer spacial resolution andpoor image quality in obese patients. Digital radiog-raphy because of its lower radiation dose is suitable forpelvimetry for special indications.

Keywords: Computer tomography, digital radiography, dosimetry, high amplifying film-screen combinations,pelvimetry, radiation dose.

Zusammenfassung

Pelvimetrie mit Hilfe der digitalen Radiographie unterbesonderer Berücksichtigung der StrahlendosisDie Sonographie hat sich in der Geburtshilfe als Metho-de der Wahl zur Lokalisation von Fötus und Placentafest etabliert. Zur Bestimmung der Beckenmaße ergibtdie Ultraschalluntersuchung aber keine ausreichendenErgebnisse, daher wurde bisher bei wenigen streng aus-gewählten Indikationen eine konventionelle Röntgenpel-vimetrie durchgeführt. Nachteilig ist hier die relativ hoheStrahlenbelastung des Feten und der mütterlichen Go-naden. Mit der Einführung der digitalen Radiographiebesteht jetzt die Alternative einer digitalen Pelvimetrie.Dosismessungen wurden deshalb zum Vergleich bei derdigitalen Radiographie und konventionellem Röntgenmit neuentwickelten hochverstärkten Folien (Quanta

III, Dupont) vorgenommen. Verwendet wurde ein loni-sationsdosimeter und ein Wachsphantom. Die Strahlen-belastung der digitalen Radiographie betrug 24 . Dasentspricht etwa 15% einer vergleichbaren konventionel-len Aufnahme (177 ). Als weiterer Vorteil der digita-len Radiographie ist der standardisierte und schnelleAufnahmemodus anzusehen, weiterhin können die Bek-kenmaße direkt bestimmt werden.Nachteilig sind ein geringeres räumliches Auflösungs-vermögen der digitalen Radiographie und die ein-geschränkte Bildqualität bei adipösen Patientinnen. Ins-besondere wegen der geringen Strahlenbelastung bietetsich die digitale Radiographie zur Pelvimetrie bei Pro-blemfällen an.

Schlüsselwörter: Computertomographie, digitale Radiographie, Dosismessung, hochverstärkende Film-Folien-kombination, Pelvimetrie, Strahlenbelastung.

Resume

Pelvimetrie par radiographie digitale — dosimetrieL'echographie a solidement etabli sä place en obstetriquecomme la methode de choix pour la localisation dufoetus et du placenta. Cependant, I'echographie n'est passüffisante pour determiner les dimensions du bassin.C'est ainsi que effectue des radiopelvimetriesconventionnelles dans quelques indications strictementselectionnees. Cette attitude offre le desavantage d'uneirradiation relativement elevee pour le foetus et les gona-des maternelles. Depuis Introduction des radiographiesdigitales, la pelvimetrie digitale offre une alternative.Nous avons realise des dosimetries en comparant radio-graphie digitale et radiographie conventionelle avec lesnouveaux ecrans ä haut pouvoir d'amplification (quanta

HI, Dupont) en se servant de dosimetrie a ionisation etde fantömes de cire. La dose d'irradiation de la radiogra-phie digitale est de 24 micro Gy. Cela correspond au tauxde quinze pour cent d'une radiographie conventionellecomparable (177 micro Gy). Un autre avantage de laradiographie digitale est sa technique standardisee etd'exposition rapide; bien plus, les dimensions pelviennespeuvent etre mesurees directement.Les inconvenients en sont la pauvre resolution dansl'espace et la mauvaise qualite des images chez les patien-tes obeses. La radiographie digitale en raison de sa faibledose d'irradiation est adequate pour la radiopelvimetriepour des indications speciales.

Mots-cles: Couple film ecran a haute sensibilite, dose d'irradiation, dosimetrie, pelvimetrie, radiographie digitale,tomographie computorisee.

Bibliography

[1] BEAN, J., B. A. RODAN: Pelvimetry Revisted. Semin.Roentgenol. 17 (1982) 164

[2] FEDERLE, M. P., H. A. COHEN, M. F. ROSENWEIN,M. N. BRANT-ZAWADZKI, C. E. CANN: Pelvimetryby Digital Radiography: A Low-Dose Exam-ination. Radiology 143 (1982) 733

[3] FREYSCHMIDT, J., D. SAURE, W. UJAK, F. NÜSSLIN:Zum Problem der Dosiseinsparung durch Einsatzhochverstärkender Seltene-Erden-Folien oderdurch Erhöhung der Aufnahmespannung. Fortschr.Roentgenstn 129, l (1978) 113

J. Perinat. Med. 13 (1985)

292 Claussen et al., Pelvimetry

[4] HÜBENER, K. H.: Digitale Radiographie-Mög-lichkeiten und Perspektiven einer neuenradiologischen Technik. Habilitationsschrift 1980

[5] MACMAHON, B.: Prenatal x-ray exposure and child-hood cancer. JNCI 28 (1962) 1173

[6] MEYDAM, K., M. RÖMER, K. G. HERING: Do-sismessung an Patientinnen mit derRastermammographie. Roentgenpraxis 36 (1983)237

[7] OSBORN, S. B.: Variations in the radiation dose re-ceived by the patient in diagnostic radiology. Br. J.Radiol. 36 (1963) 230

[8] REEKIE, D., M. DAVISON, J. K. DAVIDSON: The radi-ation hazard in radiography of the female abdomenand pelvis. Br. J. Radiol. 40 (1967) 849

[9] REITHER, M., P. BROZIK, B. SCHORN: Dosismessungbei Ausscheidungsurographien im Kindesalter unterVerwendung einer neuen Film-Folien-Kom-bination. Roentgenpraxis 36 (1983) 170

[10] STEWARD, A., G. W. KNEALE: Radiation dose effectsin relation to obstetric x-ray and childhood cancer.Lancet 1 (1970) 1185

[11] UNITED NATIONS SCIENTIFIC COMMITTEE ON THEEFFECTS OF ATOMIC RADIATION (UNSCEAR):Sources and effects of ionizing radiation, report tothe General Assembly* with annexes UnitedNations, New York 1977

[12] SZOKE, B., D. Kiss: Ultrahang pelvimetriajelentösege a szülezetben. Orv. Hetil. 118 (1977)1233

[13] VACLAVINKOVA, V.: Ultrasonic Pelvimetry. Amethod for preliminary estimation of the pelvicoutlet. Ultrasonics 14 (1976) 133

Received April 19, 1984. Revised August 30, 1984.Accepted September 10, 1984.

Priv.-Doz. Dr. C. ClaussenUniv.-Klinikum CharlottenburgAbt. RadiologieSpandauer Damm 1301000 Berlin 19, Fed. Rep. Germany

1 1 !

J. Perinat. Med. 13 (1985)