Radiographic Mammographic Donald

Exposure Dose

R. Jacobson,

PhD

Calculator Calculator’

A set of dose signed

calculators

to facilitate

culation dose)

has been

quick

of surface

and

phy

de-

easy

exposure

for routine

Users Guide (3), recdosage in mammogra-

Mammography-A

ommend

and

specfied

dose.

by

On

aver-

age glandutar dose in mammographic studies. Acceptance by technologists has been good, and in two inspections by the Joint Commission for Accreditalion of Health Care Organizations, the calculators were deemed adequate to satisfy the diagnostic radiology standards that doses be monitored.

employing a grid should deliver no than 8 mGy (0.8 rad) for a twoview study. Recently published federal guidelines governing Medicare reim-

bursement

for screening

zations

creditation (JCAHO)

breast

screen-film

mammogram

grid (4). We present

tic

radiology

ance

Commission of Health

standard

individual

specifies

Our

that

“monitors

a

doses

from diagnostic radiology procedures” (1). An analysis (2) of the new standards states that compliance with this standard requires that exposure values be determined for all routine procedures for

each

individual

calculation on

x-ray

system.

dose

must

of x-ray

calculators of x-ray

The

made from the at a variety of Tabulated standose values are not acceptable. standard, in its strictest interpreta-

This tion, means that the value delivered in a radiographic should be available for the quested. JCAHO

standard

specifies “with respect ards from equipment, ence

to

the

currently thority

radiation

dose

procedure patient if re-

DR.2.2.5 (2) further to radiation haz(there

be)

adher-

of any

recommendations

recognized on

of the

and hazards,

reliable such

auas

the

National

Council on Radiation Proteclion and Measurements (NCRP), and any requirements of appropriate licensing agencies or other governmental bodies.” The NCRP, in report no. 85,

From the Department of Radiology, Medical Physics Section, Medical College of Wisconsin, 8700 W Wisconsin Aye, Milwaukee, WI 53226. From the 1989 RSNA scientific assembly. Received July 9, 1990; revision requested August 8; revision received September 19, 1991; accepted September 23. Address reprint requests to the author. ( RSNA, 1992 I

578

#{149} Radiology

a

performing in compli-

NCRP

standards.

and Methods is based

device

of a circular

slide

rule.

on exposure-to-dose assuming

backscatter calibrated

be based

with

for dose,

and

(0.3

on the An

ex-

ample of a device that is used to calculate surface exposure is shown in Figure 1. With an appropriate scale factor, and

equipment in question kilovolt peak settings. dard

JCAHO

calculating

principle

based

measurements

actual

with

Materials

for diagnos-

(DR.2.2.4.2)

qualified

for AcOrgani-

Care

mammograms

specify a maximum dose of 3 mGy rad) per view for a 4.5-cm compressed

1992; 182:578-580

HE 1992 Joint

glan-

more

measurements

T

average

of an analysis publication also

and

Index terms: Breast radiography, radiation dose, 00.11 a Dosimetry #{149} Radiations, exposure to patient and personnel

Radiology

using

the basis ratio, this

dose,

a typical

distance

exposure (or skin peak employed for with the source-to-

(SOD)

used

for

amination. Next, the value pere seconds used is located

outer

scale,

provides

for the

factor, the device could be to indicate skin dose. To ar-

rive at the surface dose), the kilovolt the study is aligned

object

conversion value

the

result.

ex-

for milliamon the

and the adjacent desired

the

inner

scale

Measure-

ments of radiation output (free in air) performed over a range of kilovolt peak settings and made on the basis of system usage, for example, at every 5 or 10 kVp between 60 and 120 kVp, are used to calibrate each calculating device for a specific x-ray tube. In the event of a tube replacement, or if routine quality control measurements indicate a change in tube output, a new dose calculator can be quickly and easily established for that tube. The error in surface exposure for a mis-setting of kilovolt peak is typically 3%-4% per kilovolt peak between 60 and 120 kVp. The error due to ignoring

backscatter and

is dependent

on field

size

beam energy, but a typical value for the increase in surface exposure due to backscatter is 30% ± 10 (5). could

be

included

in

the

bration of the device. In mammography, the recommended dose specification is average glandular

is generally

on to Some exand theoretical work (6,7) average glandular dose to

cali-

to patient

has related

exposure

agreed

risk

in air. These

(3).

data are pre-

sented in the literature in graphic (3,6) and tabular (8) form, and these sources generally agree with each other to within 5%. The examination parameters required to calculate average glandular dose are the output (mR [2.58 x 10 mCi/kg]/mAs) and half-value layer (HVL) at the kilovolt peak used, milli-

ampere seconds, and compressed breast thickness. The calculation procedure is tedious and time consuming and is simplified by the use of a mammographic dose

calculator. The operation of this (Fig 2) also is based on the principle of a circular slide rule. Examination input parameters are kilovolt peak, milliampere seconds, and compressed

device

breast

thickness.

In use,

the wheel

is rotated

until

the

relevant kilovolt peak and milliampere second values are aligned. The average glandular dose is then read adjacent to the compressed thickness. A correction for fatty or dense breasts, which varies between 5% and 10%, depending on compressed thickness (8), is also incorporated into the device. The calculator must be individually calibrated for a particular tube on the basis of measurements of radiation output and HVL at a variety of kVp settings. Average glandutar dose (Dg) is defined by the simple relationship D = Dg X Xa, where Dg is the average glandular dose in centigrays (rads) per roentgen exposure (without backscatter) and X, is the mcident exposure that would be measured free-in-air for the kilovolt peak and mitliampere second values used in the examination (3). The graphic data in reference 3 relating Dg to HVL and compressed breast thickness for a motybdenum target tube was found empirically to bedescribed by the following equation: Dg = HVL x lIT x 2.2, where T is the compressed breast thickness. This equation describes the graphic

tween pressed 5 cm.

x-ray

This factor

which

correspond penmentat

of risk-benefit specifies maximum recommended dose values; for example, mammography performed with a screen-film system

(or skin

radiography

that

be

dular

cat-

and

data 0.3 and

breast Therefore,

within

2%

for

HVL

be-

0.4 mm

Al and comthickness between 3 and with

the

equation

X,,

=

mAs x mR/mAs for the relevant kilovolt peak, the average glandular dose for a molybdenum tubetarget can be computed as follows: Dg = mAs x mR/ mAs x HVL x lIT x 2.2. The numerical constant divided by the compressed breast thickness is in-

February

1992

1.

2.

Figures

1, 2.

Mean

(1) Radiographic

Glandular

kVp

mR/mAs

;i. Zk_ L

Dose

x HVL

i .:.ll:

---

9L

-

Figure 3. Back calculator.

corporated calculator ing

to the

=

X

calculator

exposure

1/x

Calculator

z

100

=

MARK

E .3’

3/

iL

.2Z

22.

ifl

LZ

side

of mammographic

into

.

dose

of the dose factor correspondthickness. The

the design

as a single appropriate

of measured radiation output (mR/mAs) and HVL at a given kilovolt peak are combined into a single parameter that is dependent on kilovolt peak and is unique for each mammographic x-ray system. The back of the mammographic dose calculator (Fig 3) provides a worksheet for recording of these measurements and calibration of the device. The reciprocal of the product of output product

and

HVL,

multiplied is

Volume

They are fixed for each radiographic x-ray tube. The devices are easy to use, and acceptance by technologists has been good. Radiation dose is not presently being determined for each examination, but the calculators are in place if room-to-room comparisons are desired or a patient requests the dose for a particular study. Both hospitals underwent JCAHO inspections in 1989, and in each case, the inspector was satisfied with the calculator as a method of monitoring diagnostic doses. In the mammography imaging area, the reaction of the technologists to the ments

of two

near

the console

dose

182

exposure calculators have in the radiology departNumber

#{149}

calculator

To evaluate ments made

hospitals.

has

calculator,

have

been compared

puted

values

also been

positive.

the validity of measurewith the mammographic

dose

results obtained

obtained

with

with

it

hand-com-

by using

pub-

data (3,8) for several patient studies covering a wide range of breast sizes and tissue compositions. Agreement between the two methods was within a few percentages in all cases. lished

Discussion

In view of the general consensus that average glandular dose is the preferred method to specify patient dose in mammography, and because of the NCRP recommended dose limits, compliance with the JCAHO standards should be accomplished in mammography by measuring average glandular dose and assuring

Results The surface been installed

dose calculator.

by 100 (designated

the value that must be transferred to the wheel of the calculator. With the wheel in the calibration position, the MARK value for each kilovolt peak is located on the numerical scale of milliampere seconds, and an arrow is drawn by hand on the wheel and labeled with the appropriate kilovolt peak.

“MARK”)

and (2) mammographic

2

that

it is within

the

recom-

mended maximum limits. Currently, in diagnostic radiology other than mammography, there is no consensus as to

how tissue dose should be specified to reflect patient risk. Consequently, surface entrance exposure (or skin dose) was and

chosen as the index to be measured monitored. The JCAHO will accept

a methodology that is state-of-the-art (personal communication, 1989). Skin dose or surface exposure will be acceptable until a consensus is reached within the radiology community as to how diagnostic doses should be measured and monitored.

Alternatively, a computer program can be used to obtain surface exposure or skin dose data from radiologic examinations. If a personal computer or terminal is available in the x-ray department, a program could be written that would give the desired result on the basis of input of room number, kilovolt peak, milliampere seconds, and SOD. The drawback of a single terminal would be inconvenience and possible multiuser conflicts. Alternatively, a spreadsheet-type

output

could

be gen-

erated for each x-ray tube. To cover the range of 60-120 kVp and 1-500 mAs for a 50-100-cm SOD, with an incremental accuracy of 10%, between 10 and 20 pages

of numbers

would

be required

per tube. A larger increment between entries would reduce the required number of pages, but manual interpolation between

values

In summary, calculators

cept

would

then

be required. of these dose on the conrule, and the cat-

the design was

patterned

of a circular slide culators are easily calibrated for a particular x-ray system. Surface exposure is determined for general diagnostic procedures, and average glandular dose Radiology

579

#{149}

can be calculated for mammographic examinations. The calculators are inexpensive and easy to use, and have been found by JCAHO inspectors to satisfy the requirement that diagnostic doses be monitored. U

References 1.

Accreditation brook Terrace,

manual

Ill: Joint

for hospitals. Commission

Verification Identification

Oakon Ac-

identify

Index Spine,

terms: anatomy

Radiology

lumbosacral

of L-5 and of S-i).

nine

182:580-581

transitional

vertebrae

quent

are common congenital a reported prevalence (1-6). Since magnetic

structural

anomalies

have

often

been overlooked. of disk disease

Incorrect localization due to lumbosacral transitional vertebrae may account for inconsistency between clinical presentation and MR image analysis. We attempted to ensure verification of disk disease location and presence of a transitional body by obtaining cervicotho-

From the University of Nebraska College Medicine, Omaha (P.Y.H.) and the Department of Radiology, University of Nebraska Medical Center, 600 5 42nd St, Omaha, NE 68198-1045 I

0.15,

F.J.H.). Received July 15, 1991; revision requested August 9; revision received Septemben 4; accepted September 17. Address reprint requests to F.J.H. V RSNA, 1992

580

Radiology

#{149}

along

lumbar and by counting

with MR verte-

of

hundred consecutive patients for evaluation of the tumbosa-

was

applied

of view.

with Other

a large image

(48-cm)

enhancing

techniques such as flow compensation, saturation, and no phase wrap can also be used. Each patient was positioned with the isocenter at the iliac crests. Six sagittal images (scouts) of the cervicothoracic spine were obtained with a section thickness of 5 mm. These images included C-2 through T-12. Acquisition of the six images required approximately 2 of additional imaging time. The image that best demonstrated the odontoid process of C-2 was chosen for vertebrae counting purposes. By means of the cross-hair cursor on the monitor and beginning at C-2, the cervical and minutes

scout

thoracic vertebrae were counted caudad to T-11 or T-12. T-11 or T-12 was then marked with the cursor, and the distance between this point and the isocenter appeared at the bottom of the monitor arrow). an “5” denoting

L, Villafana

T, Day JL, Lightfoot DA. in mammography. Radiology 1984; 150:577-584. Hammerstein CR, Miller DW, White DR. Masterson ME, Woodard HQ, Laughlin JS. Absorbed radiation dose in mammography. Radiology 1979; 130:485-491. Handbook of glandular tissue doses in mammography. Publication no. FDA 858239. Rockville, MD: Department of Health and Human Services, 1985.

Dosage evolution

7.

8.

screen (case 1, Fig la, curved The distance was indicated by (superior) followed by a number

the distance

superior

to the isocenter.

The (S 95 case 1) was recalled on a routine lumbar spine image by means cursor (Fig lb, curved arrow). was localized, lumbar vertebrae the T-12

indicating

level

number mm in sagittal of the After T-12 could

be easily counted in a caudal direction. For patients who move between ob-

and Methods

in this study. Imwith a 1.5-T superconducting magnet (Signa; GE Medical Systems, Milwaukee) and 4.6 version software. Imaging parameters for the cervicothoracic sagittat scout image included a multiple-echo, multiplanar (MEMP) pulse sequence with repetition time (TR) of 400 msec, echo time (TE) of 10 msec (400/10), matrix of 256 x 192, and two signal averages. An Oxford magnet with a G2 body coil (Oxford Superconducting Technology, Carteret, field

anomalies, of 3% to 21% resonance (MR) imaging has become the diagnostic study of choice for evaluating disk disease of the lumbar spine, these fre-

with

images

in routine

cral spine participated aging was performed

NJ) UMBOSACRAL

scout

angle.

Two referred

Stanton

Vertebrae’

obtained

Materials

6.

on MR Images:

imaging studies brae caudad from C-2 rather than cephalad from the presumed lumbosacral

Spine, abnormalities, 331.131 a Spine, MR. 331.1214

1992;

Segments

those

standard images used in magnetic resonance imaging studies of the lumbar spine, and vertebrae were counted down from C-2 rather than up from L-5. In 200 patients, these techniques reveated 24 transitional vertebrae (15 of sacratization of tumbarization

5.

racic sagittat

transitional vertebrae and disease localion, cervicothoracic sagittat scout images were obtained in additon to the

cases cases

4.

of Transitional

Y. Hahn, BA J. Strobel, MD Francis J. Hahn, MD

accurately

3.

ofLumbosacral

Paul John

To

2.

creditation of Health Care Organizations, 1992;10. Tolbert DD, Hubbar LB. Bushong SC, Khan FM, Payne JT. Quality control issues governing the radiation physicist. Admin Radiol 1989; 8(9):46-52. Mammography-a user’s guide. Report no. 85. Bethesda, Md: National Council on Radiation Protection and Measurements, 1986. 42 CFR 494 (d). Federal Register. December 31, 1990; 251:53525. Harnisor RM. Backscatter factors for diagnostic radiology (1-4 mm Al HVL). Phys Med Biol 1982; 27:1465-1474.

in miffimeters

tainment of a routine lumbar image and a cervicothoracic

spine MR sagittal scout, another thoracolumbar sagittal scout (case 2, Fig 2a) can be obtained by use of the license plate (5 x 11-cm) surface coil and imaging parameters of 400/10, 48-cm field of view, 256 x 192 matrix, and two signal averages. The position of the diaphragm, liver, retroperitoneum, and aorta can also be used as supporting criteria for accurate localization of lower thoracic vertebrae.

Representative Case woman

Cases

1.-A 32-year-old presented with

right-handed occasional back-

ache (during the previous 6 months) radiating to the right buttock area and extending into the back of the thigh. The pain was associated with numbness, tingling, and weakness, and was aggravated by sitting or standing upright for long periods. The pain was relieved by non-weight-bearing rest. The cervicothoracic sagittal scout and vertebrae count from C-2 revealed six lumbar vertebrae, indicating lumbarization of 5-1 (Fig 1). Mild spinal stenosis at L5-L6 (arrow) was also evident. Case 2.-A 36-year-old man presented with recent symptoms of a backache radiating down the lateral thigh to the right foot. The backache typically improved with rest. The clinical presentation pointed to a diagnosis of L-5 radiculopathy. Routine lumbar MR sagittat images (2,000/30) showed a bulging disk at L5-S1 when the level of pain was determined by the conventional method of counting cephalad from the presumed lumbosacral angle. Cervico-

February

1992