Chapter 16 Bushong

Spatial Resolution

the ability to image small objects that have high subject contrast

example: bone-soft tissue interface, breast microcalcification, calcified lung nodule

• improvs with smaller pixel size and with smaller focal spot and therefore smaller effective focal spot

Contrast resolution

ability to distinguish anatomical structures of similar tissues

example: liver-spleen and gray matter-white matter

• improves at lower kVp

Noise

grainy or uneven appearance of an image caused by an insufficient number of primary x-rays 

• lower noise improves contrast resolution, resulting in better radiographic images

2 Components of Radiographic Noise 

1) quantum mottle

2) scatter

Quantum mottle

• random nature by which x rays interact with the image receptor 

• contributes the most noise out of the 2 components

• to reduce quantum mottle use high mAs, low kVp

Scatter Radiation

x rays scattered back in the direction of the incident x ray beam

Speed

• loosely describes the sensitivity of film to x-rays

• speed, resolution, and noise go hand in hand. if one of these characteristics is altered, it changes the other two

Medical Image Quality Rules 

1) fast IR have high noise and low contrast resolution

2) low noise accompanies slow IR and high contrast resolution 

3) spatial resolution is limited to pixel size

Image Receptor Factors (5)

• pixel size

• dynamic range 

• intensity response

• signal to noise

• post processing 

Geometric Factors (3)

• distortion

• magnification 

• blur 

Subject Factors (4)

• contrast thickness

• density

• atomic number

• motion 

Magnification (geometric factor) 

• all images are actually larger than they appear

• most cases, magnification should be minimized, however there are exceptions

• magnification factor depends on the geometric conditions of the exam

  • MF = image size/ object size

  • MF = SID/SOD

Minimizing Magnification

Large SID: use as large of an SID as possible 

Small OID: place the object as close to the IR as possible 

Which has a greater effect on magnification, OID or SID?

OID, because it is the major controlling factor of magnification

• SID influences magnification 

Distortion

the unequal magnification of different portions of the same object

3 Factors that Affect Distortion

1) object thickness

2) object position

3) object shape

Object Thickness

• thick objects are more distorted than thin objects, due to the OID changing across the object

• irregular shaped anatomy or objects can cause a considerable distortion when radiographed off the central ray 

Object Position

• if the object plane and the image plane are not parallel, distortion occurs

• foreshortening- the reduction in image size, increases as the angle of inclination increases

• when inclined objects are positioned lateral to the central ray, distortion may be elongated or foreshortened

Spatial Distortion

• occurs when multiple objects are positioned at various OIDs

• this is the misrepresentation in the image of the actual spatial relationships among objects 

• object positions shifted laterally from the CR will show more spatial distortion

Focal Spot Blur

a blurred region that is undesirable on the radiograph, causes by the effective size of the focal spot, which is larger to the cathode side of the image

Minimizing Focal Spot Blur

• use small focal spots

• position the patient so that the anatomical part is close to the image receptor

• use as much of a large SID as possible

* because of the heel effect, the focal spot blur is small on the anode side and large on the cathode side of the image

Radiographic Contrast is the product of which 2 factors?

image receptor contrast

subject contrast

IR Contrast

selectable with post processing and depends on bit depth, and window/level selection by technologist

Subject contrast

determined by the size, shape, and x ray attenuating characteristics of the anatomy that is being examined and the energy of the x ray beam 

Patient thickness (subject factor) 

different anatomical thicknesses contribute to the subject contrast; the degree of subject contrast is directly proportional to the relative number of x rays leaving those sections of the body

Tissue Mass Density (subject factor)

different areas of the body may have the same thickness, but different masses 

Effective Atomic Number

when the effective atomic number of adjacent tissues is very much different, subject contrast is high; contrast media with its high atomic number causes enhanced photoelectric absorption, causing high subject contrast

Object Shape

anatomical structures that coincide with the x ray beam have maximum subject contrast

• other anatomical shapes have reduced subject contrast bc of the change in thickness that they present across the x ray beam 

Tools for improved radiographic quality

• Good patient positioning

• Use the correct type Image receptors

• Use the shortest exposure time possible

• Use the correct kVp selection

• Best to use a higher kVp, with a lower mAs

• Use the correct mAs

• Add filtration to the x-ray tube