Laser Alignment System for HighQuality Portable Radiography1 Heber MacMahon, MD NicholasJ. Yasillo Michael Carlin, RT

In portable radiography, image quality is degraded by scattered radiation. Use of an accurately aligned, antiscatter grid can provide consistently high image quality, but the necessary precision for grid alignment is difficult to achieve with conventional techniques. The authors developed a simple laser alignment method for bedside radiography. A compact laser device is mounted on the collimator housing of a mobile radiography machine, so that the laser beam is precisely parallel to the central x-ray beam. A small reflector device, which attaches to the edge of the grid cassette, indicates beam centering and alignment in a clear, intuitive way. In clinical use, the laser alignment technique provided uniformly high image quality, equivalent to that of fixed equipment. The system has the advantages of being simple to use and inexpensive to implement with existing equipment. INTRODUCTION In portable radiography,

U

usually

poor The

misunderstood.

of mobile diation,

x-ray

equipment;

which

fogs

the

image problem in fact,

radiograph,

quality

is widely

is commonly it is caused reducing

accepted, attributed

mainly contrast

but

by uncontrolled and

the

to intrinsic obscuring

causes

are

limitations scattered

diagnostic

rain-

formation (1) (Fig 1). Use of an accurately aligned antiscatter grid can provide consistently high image quality, but the necessary precision is difficult to achieve with conventional “eyeball” technique. A mechanical alignment system has been demonstrated, but currently it is not available commercially (2). For portable radiography, excellent results can be achieved with a 6: 1 or 8: 1 grid, provided that the beam energy is no greater than 90 kV and that the grid is accunately aligned. Use of a higher kilovoltage requires use of a higher grid ratio, with correspondingly more critical alignment requirements, to achieve adequate reduction of scattered radiation. Figure 2 demonstrates accurate beam alignment compared with inaccurate alignment. In the case of a conventional lead strip grid, align-

Index

terms:

Images,

1992;

RadioGraphics I From Laude

July C

quali

#{149} Lasers

23 and

received

bedside

#{149} Radiography,

technology

12:111-120

the Department ofRadiology, award for a scientific exhibit

R.SNA,

#{149} Radiography,

September

University at the 1990 12; accepted

ofChicago, 5841 S Maryland Ave. RSNA scientific assembly. Received September

13. Address

reprint

Chicago, IL 60637. Recipient ofa April 22, 199 1 ; revision requested

requests

Cum

to MM.

1992

111

.

:-

Figure

1. (a) Portable radiograph obtained with conventional nongrid technique (90 kV, OC film/Lanex medium screen [Eastman Kodak, Rochester, NYJ). Note poor contrast and detail, particularly in the mediastinum. (b) Portable radiograph of the same patient obtained the next day with an accurately aligned 6: 1 grid

and

the same

markedly strated.

technique.

improved

In addition

due

ment

is critical

across along

the grid lines. Moderate the direction of the grid

significantly

in only

impair

to differences

to reduced

one

image

scatter.

that

dimension,

angulation lines does

quality.

With

in positioning

Fine

pulmonary

is, not

a fo-

cused grid, it is also necessary for the beam to be centered accurately. Variations in focusfilm distance affect mainly film density, provided that the x-ray beam is properly centered and aligned. In this article, we illustrate the high image quality that can be achieved by using an accuratehy aligned, antiscatter grid in combination with

a moderately

wide-latitude

screen-film

system for portable radiography. We describe the systems we devised to verify and achieve accurate x-ray beam alignment, and we compare the actual alignment accuracy achieved with conventional techniques and a new laser system

in a series

ofchinical

cases.

and

detail,

U

lung

MATERIALS

inations.

RadioGraphics

U

MacMahon

et a!

lines,

is clearly

AND

METHODS

quality

is

demon-

Two

pairs

of small

radiopaque

tanta-

lum markers (1 x 3 x 0.7 mm) are inserted into the front and rear surfaces of several grid cassettes (Fig 3). Each pair of markers is precisely

positioned

(0. 1-mm

tolerance)

so that

when the x-ray beam is perfectly aligned and centered, the markers are exactly superimposed on the film. Misalignment on decentering of the beam results in misnegistration of the markers, which can be accurately quantitated with a calibrated magnifying loupe (Fig 4). Thus, alignment errors can be determined in both vertical and horizontal dimensions 1#{176}. A simple

been developed alignment and relative positions graph.

U

the image

septal

. Verification of Beam Aligmnent We devised a method by which the accuracy ofx-ray beam alignment can be determined in a large number of bedside radiographic exam-

within

112

expansion,

including

computer

program

that determines focus-film distance of the markers

Volume

has

the beam from the on the radio-

12

Number

1

A

B

2.

Correct versus grid alignment.

In position A, the x-ray beam is accurately aligned with the grid (shown sche-

I

I

/

r

Figure incorrect

matically in cross section). With the x-ray tube in position B, the beam strikes

the grid at an angle,

caus-

ing a much greater proportion of the primary x rays (ie, not scattered radialion) tO be absorbed by the

‘I I I

grid.

This

creased and

results image

in decontrast

density.

..

‘1.MM’ ,J ,.

.‘

4/

FRONT Figure tantalum

3.

Schematic markers

were

SIDE illustrates inserted

where into

radiopaque the

front

rear plastic surfaces of 6: i external grid cassettes. The markers were used to accurately determine x-ray beam angle in portable radiographs.

January

1992

and

Figure 4. Enlarged view of a portable chest radiograph shows the tantalum markers, which do not interfere with image interpretation due to their small size and peripheral location. The degree of misnegistration shown here indicates an alignment error of 3.5#{176}.

MacMahon

Ct

a!

U

RadioGraphics

U

113

Figure

5.

Prototype laser device aluminum housing

in a machined

on a mobile radiography be rotated on its pedestal parallel to the grid that the laser beam the

.

reflector

enclosed mounted

unit. The laser can within fixed limits,

lines, in order to ensure coincides with the area

of

device.

Laser

System

Alignment

To facilitate accurate alignment of the central x-ray beam with a focused grid in a clinical setting, a laser alignment system was devised. This

technique

laser

line

employs

projector

reflector on the mobile

device. collimator radiography

Medical

Systems,

and

a single,

low-power

a unique,

compact

The laser device is mounted housing of a conventional machine (AMX 4; GE Milwaukee)

(Fig

5).

The

cassette

[35

x 43-cm]

(which

holds

film

cassette)

a 14

x

when

17-inch the

x-ray

beam is accurately centered. An opaque line on the transparent front surface of the colhimaton housing appears as a dark shadow within limation

the

field light.

projected This

dark

by the line

serves

standard as a dis-

indicator, since it coincides with the reflection at a distance of48 inches (122 which is the focus distance of the grid (Fig 6). Angular beam misalignment across the grid

is indicated

by

proximately cm), which

3 x 2 x 2 inches) temporarily attaches

sible la-

sen is powered from the collimation light cmcuit and is automatically activated when the collimation light is turned on. The device is located so that the laser beam is projected parallel to the central x-ray beam, with a separation of8 inches (20.3 cm). The laser beam is converted from a spot to a line by a cylinder lens. The linear laser reflection aligns with the long side ofthe 16 x 18-inch (40 x 45-cm) grid

tance laser cm),

col-

corner

bright beam

RadioGrapbics

U

MacMahon

et a!

grid

device

(ap-

(7.5 x 5 x 5 to any acces-

cassette

by means

will

reflection on the mesh. is at an angle to the grid,

of

be

at a corresponding

If the x-ray the laser beam

angle

to the

box,

and the reflected laser beam will cause a second reflection on the mesh. The amount of separation of the two reflected lines on the mesh indicates the magnitude of alignment error. A separation between the reflections of 4 mm corresponds to approximately 3#{176} of beam angulation, which we regard as the maximum acceptable angle for optimal image quality ment

with errors

reflector rate, the producing

U

of the

reflector

aluminum brackets and a magnetic strip (Fig 7). The front of the reflector box is covered by a mesh material supported by transparent plastic material. At the bottom of the box is a mirror. The incident laser beam creates a

clearly

114

a compact

indicated

a 6: 1 grid.

that

degrade to the

Thus,

x-ray

image

beam

quality

technologist

align-

are on

the

box. When beam alignment is acculaser reflections are superimposed, a single line (Figs 7, 8).

Volume

12

Number

1

7a.

6.

I

7b.

Figures 6-8. (6) Diagram of the laser alignment The laser (straight arrow) projects a linear beam line)

that

aligns

the x-ray

‘vI

:

Iv]

I,

Alignment Error 8.

Correct Alignment

a long

side

is accurately

of the

centered.

grid

cassette

when

An opaque

line on the

transparent surface row) casts a shadow coincides with the

grid.

of the collimator housing (curved anwithin the collimator light field, which laser beam at the focal distance of the Inaccurate grid alignment causes two bright red (appearing white here) on the front surface of

(7a)

reflections

1

with

beam

system. (dashed

the reflector

device

(arrows)

tion oftube position, the cating accurate alignment.

cides

with

beam

is correctly

device, laser

amount

the edge

(7b)

After

appropriate

in cross Magnitude

of separation

(8)

section.

indicating

Schematic

between

mdicoin-

that the

of the

Dotted

of alignment

conrec-

are superimposed, reflection also

of the grid cassette, centered.

shown beam.

.

reflections The laser

reflector

line nepresents error

is shown

the laser

reflections

the by the

(an-

rows).

For portable chest radiography, the reflecton box is placed at an upper corner of the grid cassette (Fig 9). (Because ofx-ray beam divergence, the grid cassette must project above the patient’s shoulder in every case.) The laser can be tilted, within fixed limits, along the axis of the grid lines so that noncnitical errors in long-axis alignment can be accommodated. For broad, heavy-set patients, the grid and film cassette are placed transversely, and the reflector box is placed ventically. In this situation, the collimator housing, with the attached laser device, is rotated 90#{176}

January

1992

so

that

upper

the laser beam projects parallel edge of the cassette. However,

ticipate sary

that when

near

this a new

maneuver

may

design

is implemented

be

to the we an-

unnecesin the

future.

. Clinical We conducted

Applications

verification

and

three

experiments

laser

alignment

with

the

systems

de-

U

RadioGrapbics

scnibed.

MacMahon

et a!

U

115

Figure

9.

Laser

in operation.

and grid

alignment

In this

cassette

case,

system the

film

are oriented

verti-

cally, and the laser reflection is aligned with the lateral edge of the grid cassette. A shadow caused by

the distance with

the

correct

indicator

laser

coincides

reflection,

focus-film

confirming

distance.

In the first investigation, we sought termine the correlation between grid ment and image quality in a systematic We conducted an observer test using of 161 conventional chest radiographic nations

performed

with

portable

summarized

to dealignway. a series exami-

equipment

and 6: 1 grids with tantalum markers. Three experienced chest radiologists reviewed the radiognaphs, from which the alignment markers had been obscured. The radiologists scored each case for overall image quality, using

the

a 10-point

scale

in which

highest quality and i the In the second experiment,

estimate

focus-film clinical

the

range

of alignment

distance

errors

variations

practice

when

the

gardless

that

tional

to and

occur

in

conventional

eye-

ball technique is used. In a random series of 200 chest radiographs obtained with tantahum-marked grids and conventional portable equipment, we measured the amount of misregistration

of the

alignment

In the last experiment, racy of the laser alignment of 50 clinical cases. The nations

were

performed

markers.

we tested system radiographic with

the

the accuin a series examinew

system

and tantalum-marked grids, and we measured the amount of misnegistration of the alignment markers. In all cases, portable radiography was performed Lanex

at 90 kV, with medium screens

RESULTS The results of the which the accuracy ment was correlated

6: 1 grids, (Eastman

OC film, Kodak).

and

U

116

U

RadioGraphics

U

subjective observer test, in of eyebalhed grid alignwith image quality, are

MacMahon

et a!

10.

The

average

quality

of exposure.

The range of alignment errors in 200 portable chest nadiognaphs obtained with conven-

10 indicated

lowest. we attempted

in Figure

score for each case was plotted against grid alignment error. All cases with the highest quality ratings also had accurate grid alignment. Several cases in which the x-ray beam was accurately aligned received a low quality rating because the radiographs were overexposed. However, no case with a large grid alignment error had a high quality rating, ne-

eyeball

technique

is shown

in Figure

1 1 The accuracy of unaided x-ray beam alignment was quite variable. In many cases, excellent alignment was obtained partly due to intensive technologist training, augmented by feedback provided by the tantalum alignment .

indicators.

The range consecutive

of alignment portable chest

tamed

the

with

laser

error for the radiographs

device

is also

50 ob-

shown

in

Figure 1 1 With laser alignment, the range of error was markedly reduced. Major alignment errors, which significantly degrade quality, were essentially eliminated. .

DISCUSSION In most hospitals, for portable chest U

a nongrid radiography.

technique is used Because of the large amounts of scattered radiation reaching the film, contrast and detail are reduced, particuharly in heavy patients. To retain adequate contrast in this situation, a relatively high-contrast film is commonly used. This practice can provide acceptable detail in optimally exposed areas of the lungs; however, visibility of mediastinal anatomy and the retrodiaphragmatic portions of the lower lobes is limited. Although the benefits of using an antiscatter

Volume

12

Number

1

10

0. 45

40. 0

35

LI

30.

U CONVENTIONAL LASER ALIGNED

LI

I.

25. 20.

5

10

15

20

ANGULATION ERROR ( DEGREES

ANGULATION

10.

ERROR

(DEGREES)

11.

Figures 10, 11. (10) Subjective image quality ratings correlated ble chest radiographs obtained with 6: 1 grids and conventional alignment with conventional eyeball technique compared with laser-aligned cases are accurate within 3#{176}, which is the maximum image quality.

with grid alignment errors for eyeball technique. (1 1) Accuracy the laser alignment method. Note acceptable error with a 6: 1 grid

161 portaof grid that 95% for optimal

of

a. b. Figure 12. (a) Portable radiograph of a 3 1-year-old woman with mediastinal emphysema and diffuse interstitial infiltrates due to bronchiolitis obhiterans with organizing pneumonia. The radiograph was obtained at 90 kV with an accurately aligned 6: 1 grid (OC film/Lanex medium screen) and shows excellent lung and mediastinal

detail.

the radiology medium

p = pot-table.

department

screen).

The

(b)

with image

Posteroantenior

a dedicated

quality

radiograph

chest

is virtually

obtained

radiography

unit

indistinguishable

with

the

patient

at 125 kV with from

that

erect

a 12: 1 grid

ofthe

portable

2 days

later

in

(OC fllm/Lanex

examination.

s

standard.

grid

in radiography are well established, use in portable radiography frequently produces unsatisfactory results. Such poor results can be attributed to inaccurate grid alignment and to use of inappropriately high kilovoltage levels for a given grid ratio. Also, the additional weight and bulk of an external

of a grid

grid gists,

cassette

may

particularly

ceived

as marginal

January

1992

be

a deterrent

when

the

As demonstrated here, use of a wide-latitude film with a 6: 1 grid and a beam energy of 90 kV or lower can provide consistently high image

quality

in portable

the contrast and ally indistinguishable with

state-of-the-art

detail

radiography.

achieved from that fixed

In fact,

can be virtuproduced

equipment

(Fig

12).

to technolobenefits

are

per-

or inconsistent.

MacMahon

Ct

a!

U

RadioGraphics

U

117

14a.

14b.

Figures patient

13, 14. (13) was radiographed

A chest phantom at 90 kV with

of 0#{176} (a) and 3#{176} (b) are illustrated. these controlled conditions, with

with 1 inch (2.5 cm) ofLucite added to simulate a 6: 1 grid (OC film/Lanex medium screen). Grid

and

However,

relation

endotnacheal

accurate

of the grid in x-ray beam is critically This can be achieved fixed geometry de-

central important (Figs 13-15). either with a mechanical, vice

118

U

RadioGraphics

or with

the

U

laser

technique.

MacMahon

Portable density

with detail,

radiograph obtained with due to grid misalignment.

the grid vertically including

oriented

improved

the

and

visibility

of

tube.

alignment

to the

adult errors

The effect of an alignment error as small as 3#{176} is clearly visible under resulting diminished contrast and mediastinal detail. With a 6#{176} alignment

ennon (not shown), grid cutoffeffects were quite pronounced. (14a) grid and film transversely oriented shows poor contrast and decreased ( 14b) Portable radiograph of the same patient, obtained subsequently accurately aligned, shows improved overall contrast and mediastinal

the catheter

an average alignment

Although

et a!

the

fixed

geometry

advantages and film tive

to the

method

has

has certain unique the ability to support the grid in any desired position rela-

approach

(eg, cassette patient),

the

the

virtues

laser

of low

Volume

alignment

cost

12

and

sim-

Number

1

d.

b.

Figure 15. Series ofportable radiographs ofa 205-lb (76.5-kg) liven transplant recipient illustrate the effects of grid alignment on image quality. Examples with approximately equivalent density were selected to facilitate visual comparison. The beam alignment angles were determined from the tantalum markers in each case (arrow). Alignment errors of 20#{176} (a), 8#{176} (b), 3#{176} (C), and 0#{176} (d) are shown. Image quality, including lung

and mediastinal detail, improves as the alignment the image quality is high, even though the lungs failure to use a grid results in poor image quality,

phicity, easily

as well

as the

to existing

facility

equipment.

to be adapted The

devices

are

particularly

damage

or

loss

is not

a serious

we have reflector

been boxes

able to reduce substantially

cently, of the sacrificing

alignment

reflector

inexpensive,

accuracy.

error decreases. are poorly inflated.

so that concern.

Re-

the size without

In the

future,

plan to incorporate the reflectors into a specially designed grid cassette. The highly reliable, solid state laser and all optics are enclosed within a machined, solid aluminum housing for maximum strength and durability.

we

January

1992

particularly

When grid alignment Except in children

in the

mediastinum.

In addition from scattered sure is a major phy.

In the

is accurate, as in d, and very small adults,

case

to poor image quality radiation, incorrect problem in portable of conventional

resulting film exporadiogra-

nongrid

radi-

ography performed with high-contrast film, the range of acceptable exposure is quite nannow. Apart from the kilovoltage and milhiamperage settings, variations in the focus-film distance can have a marked effect on exposure. Although a retractable tape measure is supplied

a projected

with

portable

distance

radiography

indicator

MacMahon

machines,

(as provided

et a!

U

RadioGraphics

U

119

with

the

laser

alignment

practical

advantages

variation

in focus-film

system)

and

has

may

result

distance.

some

in less

More

tant, the reduction in scatter means of a laser-aligned grid

impor-

achieved by allows the use

of

a wide-latitude film without sacrificing excellent local contrast throughout the lungs and mediastinum. Thus, diagnostic quality images can be achieved over a relatively wide range of exposures, compared with the nongrid and high-contrast film alternative. Automated exposure control devices have been developed for portable radiography and are likely to be used more widely as the technology improves. However,

accurate

exposure

alone

provide high image quality in the presence of uncontrolled scatter. The prototype laser alignment device has been readily accepted by technologists, as it does not interfere with the normal operation of the equipment and provides distance and alignment information that can be easily assimilated.

Initially,

the

expose grid

alignment

eliminates

this ence,

cutoff of the

Because

sistent

and

grid

predictable

excellent

consistently,

with

laser with

can

device

beam

alignment the

to the

is con-

be obtained

in technically

difficult

(Fig

Laser-aligned,

patient. The tantalum markers confirming accurate alignment Image quality is similar to that achieved dard high-kilovoltage examination.

views tions

RadioGraphics

U

MacMahon

et a!

of

are super(arrow). in a stan-

such as lateral and decubitus projecfor which accurate grid alignment is pan-

ticularly

important

scatter

due

to the

large

amount

of

generated.

situa-

Acknowledgments: lowing server

MD, and U 1. 2.

We are grateful

radiologists tests: John

Steven

who participated J. Fennessy, MD,

M. Montnen,

to the fol-

in the obCarl Vyborny,

MD.

REFERENCES Barnes

and

aging.

GT. Contrast RadioGraphics MacMahon H, Yasillo

1991;

ography Thoracic graduate

U

radiograph

an adult imposed,

experience

120

portable

device, expeni-

16). Although primarily developed for chest radiography, the laser system is potentially adaptable to any other type of bedside radiographic examination, including nonstandard tions

16.

to over-

laser

x-ray

“

_&

F

Figure

accurate

maximum

corrected

results

even

the

allows

primary

is easily

the

with

and

accurate

problem

and

is a tendency

because

achieved

grid

transmission

film.

there

radiograph,

‘

cannot

with

scatter

in x-ray

NJ, LoughhinJJ.

a fixed

geometry

system. Proceedings Radiology, Annual Course, San Diego,

Volume

im-

11:307-323.

Clinical

mobile

of the Meeting 1989.

12

nadi-

Society of and Post-

Number

1