4 clinical applications of basic x ray physics principes

If the anatomic areas are overexposed, the optical densities fall within the Use of high tube voltage results in a results in a decrease in the visibility of detail in a much lower radia

Clinical Applications of Basic X-ray Physics Principles1

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LEARNING

OBJECTIVES

After reading tbis article

and taking the test, the

reader will:

selection of x-ray exposure

factors influences image

density and contrast.

. Be able to identify

causes of unsharpness in a

radiograph and learn how

the visibility of detail is

affected by technique and

equipment factors.

. Be able to identify the

four basic types of x-ray

generators and to select

the appropriate type for a

particular radiographic

application.

various scatter reduction

mechanisms with respect

to image quality and

patient dose

current, and exposure time-determine the basic characteristics of radiation

I #{149} INTRODUCTION

An understanding of basic x-ray physics includes knowledge of the principles of x-ray

principles to produce an image that allows the radiologist to visualize the internal

tech-nique factors and equipment design for a particular clinical examination

gener-ally has a detrimental effect on the other factors Image evaluation must also include

Index terms: Physics ‘ Radiography

RadloGraphics 1998; 18:731-744

‘From the Department of Diagnostic Radiology, Mayo Clinic, 200 First St, SW, Rochester, MN 55905 From the AAPM/ RSNA Physics Tutorial at the 1996 RSNA scientific assembly Received March 1 3, 1998; revision requested May 27 and re-ceived November 24; accepted November 28 Address reprint requests to the author.

©RSNA, 1998

732 #{149}Imaging & Therapeutic Technology Volume 18 Number 3

demon-strate how radiographic density varies with different milliampere-second values The normal radiograph (b)

was acquired at 70 kVp and 16 mAs With the kilovoltage unchanged, the underexposed radiograph (a) was

ra-diographic density varies with different kilovoltage values The normal radiograph (e) was acquired at 70 kVp

and 16 mAs With the miffiampere-seconds unchanged, the underexposed radiograph (d) was acquired with a

kVp The 10-kVp rule is demonstrated by observing that the densities of the underexposed radiographs (a, d) are similar and the densities of the overexposed radiographs (c, f) are similar

expo-sure Therefore, it is important to consider

patient exposure as tow as possible

size, x-ray generator type, and scatter rejection

addi-tional information on principles of basic x-ray

physics and image quality (1-7)

FACTORS

den-sity and contrast of the radiograph and the

pa-tient exposure

current and exposure time, expressed as

also affect density, but in this case increasing

rays through the patient As a result, small

mdli-ampere-seconds and tube voltage that results

known as the 10-kVp rule: An increase of 10 kVp is equivalent to doubling the milliampere-seconds

density similar to that achieved by reducing the

milliampere-seconds from 16 mAs to 8 mAs,

32 mAs It should be noted that the 10-kVp rule does not apply for radiographs acquired at <60

the extremities

contrast is defmed as the difference in

relative radiation intensities of the x-ray beam

exiting the patient The subject contrast is

adjacent background tissue The penetrability,

ef-fective energy of the x-ray beam: Higher-energy

tow-energy beams do Because x-ray beam energy is

directly affected by changing the tube voltage, the latter is a major factor in determining radio-graphic contrast

a d.

b.

f.

-c.

-4

Shoulder

C,)

0

C)

C.

0

log Relative Exposure

Figure 2 Effect of tube voltage on contrast and dose (a) High-contrast radiograph of a skull

phan-torn was acquired at 70 kVp and 60 mAs (b) Low-contrast radiograph of a skull phantom was

ac-quired at 100 kVp and 9 mAs In addition to a reduction in contrast, the increase in kilovoltage results

in a large reduction in patient exposure The entrance skin exposure for the low-contrast radiograph

is 1 1 5 mR (0.297 x 10’ C/kg), whereas the skin exposure produced in the high-contrast radiograph is

Figure 3 Characteristic curve for a screen-film

represented by the solid and dashed lines A larger

dii-ference in optical densities between the two areas

indi-cates higher contrast is present in the image When the

anatomic areas are properly exposed, the optical

densi-ties fall within the linear portion of the characteristic

curve and the contrast is greatest If the anatomic areas

are overexposed, the optical densities fall within the

Use of high tube voltage results in a

results in a decrease in the visibility of detail in

a much lower radiation exposure to the patient

compared with that needed in the 70-kVp

beam is absorbed by the patient

Milliampere-Seconds Selection

.

May-June 1998 Schueler #{149}RadioGrapbics U 735

C.

film radiography, both underexposure

characteristic curve (or Hurter and Driffield

[H&D] curve) describes the relationship

has three regions that correspond to different

exposure levels For low- and high-exposure

levels, the slope of the curve is relatively small

steep slope Within the straight-line portion,

)‘

Figure 4. Loss of contrast due to improper exposure Underexposed radiograph of a skull phantom acquired at 70 kVp and 30 mAs (a) and the overexposed radiograph acquired at 70

kVp and 1 20 mAs (c) are lower in contrast corn-pared with the normal exposure acquired at 70

kVp and 60 mAs (t)

anatomic areas wilt be the largest for the

ex-posure results in densities that lie in the toe or

Un-der- and overexposure on contrast in a clinical

The choice of focal spot size primarily

also influences the amount of motion blur in an

tube current and tube voltage settings, thereby

affecting the exposure time In addition, the

may influence the focal spot size, radiation

pro-vided by the x-ray tube

rays Instead, it is a rectangular region of finite

size This causes a point in an object to appear

SOD

SID

OlD

1

6a.

Object Plane

Bf

blurred on the image The amount of blur in

Bf = F x (OlD/SOD),

where F = focal spot size, OlD = object-image

compare the focal spot blur to the size of the

object itself, we calculate the blur in the plane

of the object (Bf) Bf0 is determined by

The effect of focal spot size and magnifica-tion on blur in a clinical image is demonstrated

fo-cal spot blur wifi increase as the focal spot size

mar-gins are indistinct and some fme structures

Focal Spot

5.

Figures 5, 6 (5) Focal spot blur Diagram illustrates how the focal spot blur in the image _

closer to the focal spot OlD = object-image distance, SID = source-image distance, SOD = source-object distance

(6) Effect of focal spot size and magnification on blur (a, b) Radio-graph of the sella turcica, obtained with a small focal spot of nominal size 0.3 mm (measured size, 0.5 mm) (a), exhibits greater detail than the radiograph obtained with a large focal spot of nominal size 1.0

mm (measured size, 1 8 mm) (b) Both a and b have the same

nominal size 1 0 mm (measured size, 1 8 mm) but with the object in

corn-pared with b, even though a large focal spot was used

ing Bf by the magnification M (M = SID/SOD,

Bf0 = Bf/M

1 1.2 1.4 1.6 1.8 2 2.2 2.4

Magnification

0.1

0

a.

Figure 7 Blur in the object plane as a function of magnification (a) A radiographic system with a 1 0-mm

fo-cal spot and a high speed screen has a minimum composite blur at a magnification of 1.5 (b) A system with a

1 0-mm focal spot and a detail screen has a minimum composite blur when no magnification is used

b.

Magnification

magnification (Fig 6c). When there is no

magni-fication (M = 1), the focal spot blur is zero if

focal spot closer to the object, the focal spot

blur will increase

contribute to the total image blur in a

in-teracting with the intensifying screen Because

thickness of the screen phosphor layer A

(Br) of approximately 0.7 mm, whereas the

blur from a thin, detail screen is 0.2-0.3 mm

As with focal spot blur, it is more clinically

size of the object itself The receptor blur in

magnification:

Br = Br/M.

mag-nified

recep-tor blur It is calculated as the square root of

re-lationship between total image blur and

mag-nification is used, receptor blur dominates

the total image blur For this system, a magnifi-cation of 1.5 will produce the sharpest radio-graph Figure 7b demonstrates the

1.0-mm focal spot are used The detail screen

this system, the sharpest radiograph wifi be

ills-tance

Anode

Focal Spot Track

Anode

Angle

I

Cathode

I I I

I

I Effective Focal

I

to-tal image blur is patient motion Motion blur is

possible However, the selection of technical

may result in an increase in focal spot blur We

the anode and results in a tower heat capacity

be-tween geometric unsharpness and motion blur

small focal spot should be used to reduce focal

large focal spot with higher tube current and

with respect to the central axis of the x-ray

angles that range from 7#{176}to 20#{176}.This

minimiz-ing the effective focal spot size, which is the

the filament as the anode rotates As a result,

rays

For a given effective focal spot size, the

provide the highest heat capacity, but radiation

Figure 8. Diagram depicts the side view of a rotat-ing anode assembly of an x-ray tube The angle

be-tween the anode surface and the central axis is

de-fined as the anode angle The effective focal spot size is the length and width of the x-ray beam pro-jected down the central axis

a larger area, but the rate of heat dissipation is

spot track In clinical practice, the choice of

radio-graphic examination For applications requiring

high heat capacity and small field coverage,

small anode angles are used General radiogra-phy requires large field coverage, so large

an-ode angles are needed

U GENERATOR SELECTION

qual-ity and patient exposure Just as with x-ray

com-promise of various factors depending on the

basic types of generators are single phase,

po-Track Width Track Width

/

I I I

/

/ / / I

Field Coverage Field Coverage

-0

Figure 9. Comparison of small and large anode angles For a given effective focal spot

size, the choice of the anode angulation is a trade-off between heat capacity and field

cover-age (a) Diagram depicts the side view of an anode with a small angle The small angle

pro-vides a large focal spot track width for a high heat capacity, but the resultant radiation field

coverage is limited (b) Diagram depicts the side view of an anode with a large angle The

large angle provides larger radiation field coverage, but the rate of heat dissipation is low

because of the small width of the focal spot track

I fsJ#{149}SstJ#{149}SsJ#{149}CsJ\ Single phase

(two pulse)

(twelve pulse)

High frequency

Constant potential

Figure 10 Diagram illustrates representative

volt-age waveforms for single-phase, three-phase

potential x-ray generators

tential Selection criteria include minimization

low unit cost

Time

percentage difference between the maximum

single-phase generator exhibits a 100% voltage

genera-tor has a lower voltage ripple of 1 3%-25%.

Three-phase, 12-pulse generators have a ripple

of 3%- 10%, which is similar to the voltage

radiograph at a certain kilovoltage selection

low-energy x rays that do not contribute to the

Primary X-rays

Image Receptor

con-stant potential generator provides the most

substantial reduction in patient dose

Generator types with large voltage ripple

also require longer exposure times, which

re-sults in greater motion blur This is because the

con-stant potential generator is capable of the

shortest exposure pulses of approximately 0.5

msec

An additional generator property affecting

times can be shorter for a desired

milliampere-seconds

reduce the number of retakes Reproducibility

is also critical for digital subtraction

angiogra-phy because differences in tube voltage

in-complete subtraction

High-frequency and constant potential

superior to that available with single- or

three-phase power output depends directly on

limits the ability to compensate for sudden line

correct for variations from the desired settings

grid placed between the patient and image receptor

helps reduce the amount of scattered radiation that reaches the receptor

con-stant potential generators provide the lowest

size and high cost of the system

size and relatively less expensive In addition,

high-frequency generators can be designed to run from either single- or three-phase line

for mobile radiographic units

A large fraction of the x rays entering a patient

undergo Compton interactions, which produce

emit-ted in all directions, but they tend to be

of the primary beam is increased When the

tissue, the radiographic density change between

the soft tissue and bone should be very large

scat-tered x rays, which strike the image receptor

May-June 1998 Schueler U RadioGraphics #{149}741

rays that reach the receptor Two of these

meth-oils are the use of grids or an air gap

Collima-tion to limit the volume of irradiated tissue also

Grids

level of scattered radiation reaching the image

receptor is use of grids (9) A grid is constructed

of alternating strips oftead and nonabsorbing

ar-ranged to transmit only those x rays directed in

a line from the x-ray source (Fig 1 1) X rays

di-rected at an angle are preferentially absorbed by

large fraction of the scattered radiation is

entrance skin exposure for the radiograph

sub-stantially tower at 33 mR (0.085 x 10 C/kg)

ra-tio grids and higher energy exposures

scat-tered radiation that reaches the image receptor

is to place a gap between the patient and the

Primary X-rays

Patient

primary beam, a large fraction will not strike

the receptor if it is separated from the patient

by a sufficient distance (Fig 13) However,

pri-mary x rays directed in a line from the x-ray

distance is 15-45 cm, which wifi also

Figure 14

Both grid and air gap techniques are effective

results in less tube loading In addition, the

the source-to-patient distance used

air gap technique is used in cerebral

angiogra-phy, the geometric magnification is generally

adjusted to 1 5- 1 8 Some radiologists prefer

posi-tioning the patient closer to the x-ray source is

this case, because a large source-image

and the resulting magnification is slight

Air

Gap

$

Figure 13 Cross-sectioned diagram shows how

an air gap placed between the patient and image

re-ceptor helps reduce the amount of scattered radia-tion that reaches the receptor

x-ray beam Object characteristics include thick-ness, physical density, and effective atomic number (Z). Two tissue areas that have either

different thicknesses, densities, or Z will

characteristics include tube voltage and the

gen-erator, plus filtration (ie, added and inherent

attenuating material in the path of the x-ray

kilovolt-age has a substantial effect on image contrast

In addition, factors such as filtration and volt-age waveform shape also alter the distribution

of x-ray energies in the beam

to-gether to produce subject contrast is

Substance

Effective Atomic Number

Physical Density (g/cm3)

tions of x rays in tissue The probability of

pri-manly on tissue density, with very little

occurrence of photoelectric interactions has a

strong dependence on atomic number and

x-ray energy The probability of photoelectric

in-teractions increases as Z increases and

de-creases as energy increases

Imaging of soft tissue requires special attention

to radiographic technique Muscle, tissue

flu-ids, and fat have relatively low effective atomic

ton interactions are more likely to occur when

a patient is imaged with a high-energy x-ray beam (>40 keY) However, Compton interac-tions are not as effective as photoelectric inter-actions for distinguishing tissue types To

x-ray beam must be used An example of soft-tissue radiography is mammography in which technique factors of 25-30 kVp are used with a special x-ray tube anode and filter to produce

low-energy x rays

Contrast agents can be introduced into the body to change the attenuation of the object being imaged so that the subject contrast is

materials contain iodine and barium

den-sity Because the size of the structures normally

usually small, we need to maximize the differ-ential absorption Use of 60-75 kVp is prefer-able because it produces many x rays just