RAD 254 Chapter 16Image Quality

Film factors

Geometric factors

Subject factors

Radiographic Quality• Resolution = ability to image two or more

objects and distinguish them on the radiograph– Spatial resolution = imaging small objects with

high subject contrast that are close together (breast micro calcifications in breast tissue)

– Contrast resolution = ability to distinguish items with similar subject contrast (white-gray matter)

– Related terms: detail; recorded detail; visibility of detail

“Noise”Film graininess, structure mottle,

quantum mottle, scatter • Film graininess = distribution, size, space

of silver halide in the emulsion (mammo)

• Structure mottle = as above, but involves the intensifying screen crystals (mammo)

• Quantum mottle = random way x-ray photons interact with film/receptor (increased speed receptor= higher QM)

• Scatter

Quantum mottle reduction

• High mAs

• Lowest optimum kVp’s

• Slowest possible film (receptor) screen speeds

Film factors

• Characteristic curve : density, contrast, speed latitude– Relationship of OD and radiation exposure– Also called H & D curve, Hortchkof curve,

sensitometric curve, – Terms: base + fog, toe, average

gradient/straight line portion, shoulder, solarized portion

• Processing: Time, temperature

Sensitometry/densitometry• Sensitometry = exposing the film to

radiation or light in step form exposures

• Densitometry = measuring the amount of light transmitted through each exposed step– Plotting the optical density (OD) verses “log

relative exposure” (LRE) = the curve– Increments in LRE of .3 = double the exposure– Useful range of OD is .2 – 2.5

Contrast

• Film/image receptor contrast (inherent in the film-screen combination) – reflected in the ave. gradient portion of the H & D curve (more vertical curve = higher contrast)

• Subject contrast: size, shape and attenuating characteristics of the body part imaged

Film Characteristics

• Speed = sensitivity and responsiveness of the emulsion of the film to x-rays/light

• Latitude = range of exposures to create blackness– “wider” latitude (responds more slowly to

light/x-rays) = lower contrast. – Latitude and contrast are INVERSLY

PROPORTIONAL

Geometric factors

• Distortion– Depends on object thickness, position, shape

• Magnification– To minimize magnification: use long SID and

short OID

• Blur (focal spot blur) – caused by the effective focal spot – larger on the cathode end of the emission spectrum

Subject Factors

• Contrast : thickness, tissue mass density, atomic number of tissue being imaged– Can be somewhat modified by varied kVp’s

• Motion

Primary factors

• kVp = primary control of radiographic contrast

• mAs = primary control of optical density (OD)

Four Patient Factors

Four Image-quality Factors

Three Types of Technique Charts

Three Types of AEC’s

Patient Factors

• Body habitus : – Sthenic – “average sized patient”– Hyposthenic – thin but healthy– Hypersthenic – big “frame” usually overweight– Asthenic – small, frail, emaciated, usually

elderly

• Thickness of part• Body composition• Pathology

Body Composition

• Atomic mass density of body parts to be imaged– Wide range of atomic mass densities = higher

kVp’s to be used (CXR)

• Pathology – radiolucent (destructive) or radiopaque (additive)

Radiolucent – Destructive Pathology

• Active TB• Atrophy• Bowel obstruction• Cancer (for the most part destructive)• Degenerative arthritis• Emphysema• Osteoporosis• Pnuemothorax

Radiopaque – Additive/constructive

• Aortic aneurysm• Ascites (abnormal accum. Intraperitoneal fluid)• Atelectasis (collapse of alveoli)• Cirrhosis• Hypertrophy• Metastases (for the most part)• Pleural effusion• Pneumonia• Sclerosis (hardening of tissue)

Image quality factors

• Optical Density – primarily controlled by mAs (20-30% change to see any difference) and SID

• Contrast – shades of gray – primarily controlled by kVp (hi/low; long/short scale)

• Image Detail – visibility of detail (contrast resolution) and sharpness of detail (spatial resolution)

• Distortion – improper alignment of part

Technique Charts

• Variable kVp – increasing kVp’s with increased body part thickness

• Fixed kVp – (Arthur Fuch’s) highest OPTIMUM kVp – atomic mass density driven

• High kVp – over 100 kVp – barium studies and CXR’s

Automatic Exposure Control

• Usually 3 sensors – Sensors shut off x-rays when a pre-set

amount of OD is achieved (pre-set kVp’s)

• 600 mAs over-ride (if sensors do NOT shut off x-rays, the machine automatically terminates the exposure at 600 mAs) – some units have variable “back up timers” for this

Anatomically Programmed Radiography

• Microprocessor technology based upon anatomical area to be imaged – Uses anatomic illustrations for the tech to

select the anatomical part to be imaged and the unit does the mA, kV and AEC selection