Medical Ernest J. Wiesen, BSEE #{149}Jeffrey Graham G. Ashmead, MD .Alan

Improvement

R. Crass, MD M. Cohen, MD

Radiology

1991;

M. Bellon,

MD

#{149}

in CT Pelvimetry’

When computed tomographic (CT) digital radiography is used for pelvimetry, measurement error may occur. Geometric distortion in the latera! direction of the CT digital radiograph can lead to an error in any measurement of the transverse pelvic inlet. The authors measured the magnitude of this error on two scanners and present a general method for correction of this potential error. The authors also showed that an additional dose reduction is possible if the patient is imaged in the posteroanterior rather than anteroposterior projection. Index terms: Dosimetry 87.1211 #{149} Pelvis, measurement, Pregnancy, irradiation in

Errol

#{149}

#{149} Pelvis,

CT, 85.1281,

85.1211, 87.1281

.

W

have been using computed tomography (CT) to perform digital pelvimetry after the method of Federle et al (1), with use of two digital radiographs and one axial scan. The electronic cursor and the internal computer program supplied with the CT system are used to measure the distance. When clinical colleagues raised questions regarding the accuracy of the method, we analyzed a group of studies after the method of Ball and Marchbanks (2) and Beam et al (3). We found that although the sagittal measurements from the digital radiographs were very accurate, errors in the transverse measurements of over 10% were possible.

E

Our

clinical

staff

believes

that

this possible error is unacceptable. The inaccurate measurements were due to geometric distortion along the transverse plane of the CT digital madiograph. Along the direction of table travel (the longitudinal direc-

178:259-262

tion), very

the

x-ray

beam

is narrow,

with

small divergence. The longitudinal accuracy depends on the accuracy of measuring and displaying table position. However, the x-ray beam across the CT table (transverse direction) greatly diverges and causes geometric distortions. Most manufacturers

I

From

J.R.C., necology ter,

Case

the

Departments

E.M.B.,

A.M.C.)

(G.G.A.), Western

of Radiology

and Obstetrics

MetroHealth Reserve

Scranton Rd. Cleveland, March 16, 1990; revision vision received August Address reprint requests © RSNA, 1991

Physics

(E.J.W.,

and Gy-

Medical

University.

OH 44109. requested 1; accepted to E.J.W.

Cen3395

Received April 26; reAugust 7.

of CT

scanners

cali-

brate the system to make transverse measurements accurate in the plane perpendicular to the x-ray beam and passing through the CT center of rotation (COR). Therefore, any measurement made in this plane is very accurate in every direction. Any transverse measurement made in any other plane above or below the COR in the direction of the x-ray beam divergence will be in error. The amount and direction of the error depend on the distance and direction of the plane from the COR. Federle et al (1) reduced the error, a priori, by positioning the patient within the CT gantry so that the

transverse

measurement

points

were

at the COR. Adam et al (4) suggested a corrective method based on their experience. Their method references measurements to the table top and requires either a fixed standard table height for scanning or some comrection

for

table

height.

Most

currently

available CT scanners can perform digital radiographic imaging at vanous table levels, and the table level used cannot be ascertained from the resulting image, a factor resulting in possible

unacceptable

errors.

The

goals of the present study were to develop a generally applicable solution to improve accuracy in CT pelvimetry independent of manufacturer, table position, or patient habitus. Our method references all measurements to the COR plane of the scanner, the location with the least error. Previous reports (1,4-6) have shown that fetal doses are low in this examination. A recent review showed that it was acceptable to perform digital radiography in the anteropostemior (AP) projection. Because the fetus is anterior in the matemnal abdomen, potential fetal dose is maximized.

We

thought

that

sig-

nificant possible scanning,

dose reduction would be with posteroanterior (PA) as well as lowering the xray exposure. The x-ray exposure can be reduced in both of the digital madiographs and the axial scan until the images become too noisy for accurate identification of the anatomic landmarks necessary to make the measurements.

In this

study,

both

the

lowering of fetal dose with use of PA digital radiography and correcting inaccuracies in the transverse measumement were considered.

Abbreviations: center of rotation,

AP = anteroposterior, PA posteroanterior.

COR

=

259

MATERIALS

AND

Geometric

were

made

METHODS

distortion

measurements

by scanning

a wooden

phan-

tom (Fig la) in which lead beads cated 20 cm apart in the transverse tion

at the

COR

and

were lodirec-

at distances

of

10 cm

and 5 cm above and below the plane the COR. For accuracy measurements the direction direction),

lead

of table beads

travel were

of in

(longitudinal located 10 cm

apart when projected onto the COR plane at varying distances from the COR. A more detailed drawing of the phantom is shown in Figure lb. A 3/4-inch plywood board was perpendicular to and angled 25#{176} relative

to the

center

line

of the

b.

a.

hori-

zontal 3/4-inch plywood board. Lead beads were placed on the slanted vertical board. The horizontal board of the phantom was placed in the exact center of the CT scanning

table.

The

center

of the

aligned with the longitudinal of the phantom. The table until

the

center

the COR scan was was used

was

center line was elevated

phantom

was

in

of the scanner. A lateral digital obtained. The electronic cursor to measure distance (Fig lc, id).

Two scanners nique: tion,

of the

table

were

tested

the Picker 1200 Highland Heights,

SX

with

this tech-

(Picker IntemnaOhio) and GE

9800 (GE Medical Systems, Milwaukee). On the basis of these measurements, curves can be obtained and correction tors

error fac-

calculated.

To determine simply

correction

divided

each

factors,

of the

five

plot could

by the at each

measure-

from

the

ners

COR.

tested

Dose use

magnification factor was distance. We chose to

a curve so that the be easily obtained

The

are

correction at any

curves

shown

in

measurements

of a female

Research,

Rando

and fluoride

simeter

chips

with

all

patient

(Alderson

The

phantom water-filled similar

was

inbags. to that used we

measurements,

the

measured

provide a constant tube was located phantom

was

doses

scanned

tion for the PA view AP view. Normally, with the tube under patient

also

distance, under the

used docali-

in the and prone pelvimetry the patient

supine

To

the posi-

for the is done and the

RESULTS scanners tested showed a in accuracy of measurement

of approximately timeter With

260

the

above or below patient supine,

Radiology

#{149}

1.5%

error

per

the COR. the mea-

depending

cen-

too

large

in the

AP

(OR

or

on whether

is scanned

the

or PA

of magnification factors found

the error is approximately +9%. measurement can be multiplied correction

factor

(.91) and the error pected considerable

supine.

Both variation

either

for

this

reduced. variation

small

errors

with

()

/

This by

Curve (t%t,

distance

-10

2

.5

change

the

through the tal radiograph

The

center of the represents

2

-4

()

2

4

6

10

8

of the of the

scans.

-6

Figure 2. To successfully use curves, one must pay attention to the position of the xray tube, the COR, and the measured point to be corrected.

scanners. The measurement that should be corrected is shown in Figure 3. The localization of the transverse inlet on the lateral scan is via the methods of Bean et al (3). The table height and the patient position should not

two

between

‘.-Corettu.n Fator

We exin the

either

Magn.t.(atu)n

are

geometric error between the two scanners tested. However, the variation was small, and we used Figure to find the correction factor, with

very

1.14

1.

shown in Figure 2. If the transverse diameter of the pelvic inlet is 6 cm closer to the x-ray tube than the COR,

the

change.

the x-ray table, and

be

plane. The degree and the correction

maxi-

brated with a 660 system (Victoreen, Cleveland). If the distance changes between the x-ray source and the patient’s skin,

may

too small,

2.

Shearer (5), except thermoluminescent for

surement

scan-

made

Conn).

mum thickness of the creased to 23 cm with The method used was by Moore lithium

Figure

phantom

Stamford,

surement.

factor distance

for the

were

d.

Figure 1. (a) Photograph of the wood phantom used for the test. (b) Detailed drawing shows the location of all lead beads. Beads in the central row are 10 cm apart when projected on the plane of the COR. All dimensions are in centimeters. (c) Lateral CT scan of the phantorn. The outside lead beads are 20 cm apart. The effect of geometric distortion is shown by the varying distance between beads. Lines 1, 2, and 3 are the actual measurements, with the measured distance shown at lower right. (d) Same image as c, except measurement is in direction of table travel. All beads in that direction are equally spaced. Line I is an actual mea-

we

ments by the measurement at the COR (20 cm). This would yield the magnification at -10, -5, 0, +5, and +10 cm from the COR. To obtain the correction factors,

1 divided corrected

c.

line lateral digithe plane

graph.

COR

for

and

the

y coordinates

COR is represented of the y coordinate between the COR inlet of

measurement

lateral the

known

PA

CT scanners

Our

digital

radio-

display

the

x

the cursor. The when the value is 0. The distance and the transverse of

can

be

located

on

scan

(Fig 4a). The location tube and detector must be to determine which correc-

January

1991

0.

3.

(a) PA digital

radiograph

C.

used

to measure the transverse the measured distance shown at lower right. (b) Lateral view shows table, this measurement should not need correction. Diagonal white shows measured distance between ischial spines. Horizontal white Figure

inlet diameter. Horizontal white line is the actual measurement, with measurement of AP diameter. If the patient is accurately centered on line shows actual measurement. (c) Axial section obtained at 40 mAs line shows actual measurement.

the

COR

. - .

#

1 1

-P,

b. Figure

,

4. (a) Diagram shows method for locating the level of the transverse inlet diameter. The COR is located by drawing a horizontal line (dashed line) at the center of the lateral digital radiograph. The AP inlet (line 1) is drawn. Line 2 is drawn at the cord of the sacrum. Line 3 is constructed, parallel to line 2, to pass through the ischial spine. The intersection of lines 1 and 3 (point A) is the level of the transverse inlet diameter. The perpendicular distance between the COR and point A is the distance (d) to be used to find the correction factor in Figure 2 and corrects the measurement made when the image in Figure 3 is used. (b) Measurements as made on the CT scanner display. Lines are the same as those in a. The only distance displayed at lower right (28 mm) is the distance between point A and the COR.

Figure

5.

Five lithium fluoride dosimeter chips were location (1-12) for both AP

each aging.

tion

factor

must

that

dardized,

with

the

supine To locate

verse

and

inlet

draws

lateral

be applied.

the the

x-ray

nec-

be stantube

patient. the level

of the

diameter,

one

a line

digital

We

procedure

under

at the

first

COR

radiograph.

locates

on the Line

1 (Fig

3 is constructed tangent to the ischial spine and parallel to line 2. The intersection of lines 1 and 3 (point A) is the level of the transverse inlet diam-

Volume

The

perpendicular 178

Number

#{149}

the

distance 1

be-

COR

the distance rection factor

correct trans-

4a) is drawn at the AP inlet. Line 2 is drawn at the cord of the sacrum. Line

eter.

tween

the

the image 4b shows

The

a connection

measurement in Figure a clinical

distance

inlet and 2, 2.8 cm

line and point A is (d) used to find the conin Figure 2 that will

between

made 3a is used. measurement.

the

transverse

the COR is 2.8 cm. toward the detectors

factor

when Figure

In Figure yields

of 1 .04. Therefore,

the transverse measurement is multiplied by 1.04. Our measured doses did not deviate significantly from those obtained in previous work (5,6). The location

within the phanin Figure 5. The methe total dose from and either the PA or

of the dosimeters tom are shown suits

represent

both

the

lateral

AP digital

ommend

thermoluplaced in and PA im-

minescent

radiograph

but

not

the

axi-

al scan (Table). was used. Moore

Only and

one axial Shearer

scan (5) me-

ported

fetus

was

enough

that

the

low

to be included in the axial scan in only 55% of these examinations. We did not include the axial scan because only a single section of the fetus was

exposed in

only

ceives whole-body

to direct 55%

most

radiation

of the

cases.

of the

radiation

digital

and The

then

fetus

from

me-

the

scanning.

It is difficult to find the exact dose difference between AP and PA views because the exact fetal location within the abdomen is not known. If the dosimeter locations 3 and 5 in Figure

Radiology

261

#{149}

5 are

assumed

to more

accurately

rep-

resent the location of the fetus, the fetus receives from one-third to onehalf the radiation dose in PA digital radiography. If the average of the eight dosimeter locations is used, the dose is about half. Peak doses also showed about a 50% reduction. Briefly,

the

fetus

would

receive

approxi-

mately half the radiation dose if PA digital radiography were used instead of AP, if all other positioning conditions are kept constant. DISCUSSION To find the minimum acceptable exposure for the axial scan, a female pelvic phantom was scanned several times, starting at 240 mM and at decreasing milliampere-second values until it was thought that the image was usable despite the noise. Although a 30-mAs setting is generally used for digital scanning, the technologist will increase this setting to yield an acceptable image if the patient is unusually thick. A typical low-dose axial scan is shown in Figure 3c. Although any table height can be used, it is important to remember that neither the patient nor the pa-

262

Radiology

#{149}

tient table can be moved between the lateral and PA digital radiographic studies. The scans must be obtained at right angles to each other. When done properly, there is a horizontal line on the lateral scan that represents the plane of the COR on the PA scan. On most CT scanners, this will be a horizontal line in the center of the lateral image. It is possible that the CT system may be out of calibration on that some manufacturers shift the image. The COR should be yenfied by obtaining a digital radiographic scan of a wire passing through the COR. The wine should appear in the center of the image on both the PA and lateral digital radiographs. The correction factors for CT scanners made by other manufacturers should be determined. Possible modification of the geometry of the tested CT scanners indicate that it would be safer if correction factors were detemmined for every scanner used for digital pelvimetry. The correction and magnification factors can be determined as described previously. The technique we used combines the techniques of Federle et al (1) and Beam et al (3), except that we reference all measurements to the COR instead of the table top. Use of this technique will increase the accuracy

of the measurement of the transverse diameter of the pelvic inlet. The accuracy of the AP diameter of the pelvic inlet is already good if the patient is centered properly on the table. An additional reduction in fetal dose is possible if PA, instead of AP, digital radiography is used. N Acknowledgments: technologists MetroHealth tance.

The authors of the Department Medical Center,

thank

the CT

of Radiology. for their assis-

References 1.

2.

3.

4.

5.

Federle MP, Cohen HA, Rosenwein MF, Brant-Zawadski MN, Cann CE. Pelvimetry by digital radiography: a low-dose examination. Radiology 1982; 143:733-735. Ball RP, Marchbanks 55. Roentgen pelvimetry and fetal cephalometry: a new technic. Radiology 1935; 24:77-84. Bean WJ, Cook RT, Eavenson LW, Bristow U. Pelvimetry. Radiol Clin North Am 1967; 5:29-46.

Adam Y, Alberge S. Castellano M, Kassab M, Escude B. Pelvimetry by digital radiography. Moore mates

Clin Radiol 1985; 36:327-330. MM, Shearer DR. Fetal dose for CT pelvimetry. Radiology

esti1989;

171:265-267.

6.

Claussen C, Kohler D, Christ F, Golde G, Lockner B. Pelvimetry by digital radiography and its dosimetry. J Perinat Med 1985; 13:287-291.

January

1991

Improvement in CT pelvimetry.

When computed tomographic (CT) digital radiography is used for pelvimetry, measurement error may occur. Geometric distortion in the lateral direction ...
727KB Sizes 0 Downloads 0 Views