IMAGE QUALITY

IMAGE QUALITY

The image quality is defined as the ability of the film/detector to record

each point in the object as a point on the film. It is used to describe

the visibility of diagnostically important detail in the radiograph.High-quality images are required to make accurate diagnosis. The image quality depends on (i) contrast, (ii) spatial resolution, (iii) noise, (iv) geometric factors, (v) detective quantum efficiency, and (iv) sampling and aliasing.

CONTRAST

The term contrast refers the difference in density between adjacent

areas in the radiograph. The contrast may be a subject contrast and

film contrast. The product of subject contrast and film contrast gives

the radiographic contrast. Subject contrast is the difference in X-ray

intensities transmitted through different parts of the patient. It depends

on patient thickness, tissue mass density, effective atomic number,

shape of the subject, and photon energy (kVp).

Thicker and thinner body part attenuates radiation differently, and varies the transmitted X-rays. The subject contrast is proportional to the relative number of transmitted X-rays. Tissues of equal thickness, having different mass density contribute to subject contrast. Photo-electric absorption varies with effective atomic number. If the adjacent tissues effective atomic number is different, they contribute subject contrast. Contrast media such as Barium (Z = 56), and Iodine (Z = 53) will enhance subject contrast. Shape of the anatomy, which coincides with X-ray beam increases the subject contrast. Shapes that have change in thickness for X-ray path may reduce subject contrast. High kVp gives lower subject contrast, where as low kVp gives higher subject contrast. The film contrast tells us how the film responds to difference in exposure. The film contrast depends on, characteristic curve, film density, screen or nonscreen exposure, and film processing. High contrast film is made of homogeneous size of silver grains, where as low contrast film is made of heterogeneous grains. Double emulsion films produce greater contrast than single emulsion films. The fog and scatter will reduce film contrast. They produce unwanted film density, which lowers final radiographic contrast.

RESOLUTION

Resolution is the ability to image two closely placed small objects,

as two independent images. There are three types of resolution, namely, spatial resolution, contrast resolution and temporal resolution. Spatial resolution refers the ability of the imaging system to record the objectin the two special dimensions (x, y) of the image. In other words,it is the ability to image small objects that have high subject contrast,e.g. bone-soft tissue interface. Contrast resolution is the ability todistinguish anatomical structures of similar subject contrast,e.g. liverspleen.In general, film-screen radiography has excellent spatialresolution. Temporal resolution is the ability of the imaging systemto localize the object in time, from frame to frame and follow itsmovement. Temporal resolution is high for fluoroscopy

NOISE

The noise (mottle) is the random fluctuation of film density about some

mean value following uniform exposure. It degrades image quality and

limits the ability to visualize low-contrast objects. Noise is mainly made up of screen, film, and quantum noise. The screen noise is caused by nonuniformities in screen construction. Film noise is caused by the grain structure of emulsions. Qdiscrete nature of X-ray photons and it is the most important source of noise in radiography. The adjacent areas of the film receive photons that differ from the mean value, which contribute to quantum mottle.

GEOMETRIC FACTORS

Magnification

Image quality is affected by geometric factors, such as magnification,

distortion and focal spot blur. All radiographic images are magnified

and the magnification (M) is the ratio between the image size and

object size. If SID is the source to image distance and SOD is the

source to object distance, then

M = SID/SOD

When the object is closer to the source, the magnification is larger.

When the object moves away from the source, magnification decreases.For chest radiography the SOD is about 180 cm, and the magnificationis unity. Lesser the magnification means image blur is less and higherthe resolution.

Distortion

Distortion is the result of unequal magnification of different parts of

an object. It may be caused by object thickness, object position and

object shape. Thick objects produce more distortion than thin objects.

Patient with irregular anatomy may contribute to distortion in a

radiograph. If the object plane and imaging plane are not parallel

then distortion occurs, due to positioning. The distortion is minimal

for object that is positioned at the centre. Object that is positioned

lateral to the center may have severe distortion. The objects that are

lateral may have unequal magnification than that at the centre. The

angle of inclination of the object also influences the degree of distortion.

Focal Spot Blur

The focal spot (F) of an X-ray tube is not a point and have a dimension

(0.6–1.8 mm), which produce penumbra at the edge of the field.

Penumbra is the region at the edges, where the radiation intensity

decreases laterally. It causes blurred region at the edges of the field

in a radiograph, which is called focal spot blur (f).

f = F(M–1)

The focal spot blur increases with large focal spot size and higher

magnification. It is small on the anode side and large on the cathode

side, due to Heel effect. To reduce blur, smaller focal spot size and

lesser magnification should be used. To have lesser magnification,

the patient–film distance is reduced by keeping them close to each

other.

OPTIMAL QUALITY IMAGE

Radiographs with quality images can be produced with proper patient

preparation, selection of imaging devices and correct exposure

techniques.The patient anatomy should be placed closer to the receptor/film.The axis of the anatomy should lie parallel to the receptor plane. The central X-ray beam should pass through the center 0 of theanatomical region. If multiple anatomy are to be imaged with equalmagnification, then all the anatomical structures must be positioned at equal distance from the film. The patient must be immobilized to avoid motion blur. Motion blur can be reduced by the following:

1. Short exposure time.

2. Providing instructions to the patient.

3. Keeping the source to image distance higher.

4. Keeping the object to image distance smaller.

Selection of exposure technique plays an import role in obtaining

a good quality image. The exposure time should be always shorter,

which will improve image quality. Shorter exposure time reduces motionblur. High frequency generator provides short exposure time than singlephase generators.The kVp controls the radiographic contrast, since it influences both quantity and quality of X-rays. As the kVp increases, higher amount of X-rays are transmitted through the patient. It reaches the film and affects the optical density. Compton interaction also increases, differential absorption is reduced, resulting reduction of subject contrast. The scatter radiation that reaches the film is higher, which increases noise in the image, resulting loss of contrast. As the contrast is low, the latitude is larger and the margin for error is increased. Hence, high kVp reduces contrast and the only advantage is the reduction of patient dose with wide latitude of exposures.

The mA controls optical density in the film, since it influences quantityof X-rays. As the mA increases, the number of X-rays reaching the film increases, resulting in higher optical density. The radiographic noise is lower but the patient dose is higher. Too low mA and high mA shift the optical density away from the straight line portion of the characteristic curve. Thus, it indirectly affects the radiographic contrast.