Ch 16 Bushlong

Radiographic quality

The accuracy with which the anatomical structure that is being examined is imaged on the radiograph

Most important characteristic of radiographic quality

• spatial resolution

• contrast resolution

• noise

• artifacts

High quality image always demonstrates

• tissue

• structures

Effective focal spot is always…

smaller than actual focal spot

Spatial resolution

the ability to image/see small objects that have high subject contrast 

Chest

High pt/subject contrast

Breast

Low pt/subject contrast

Spatial resolution improves with…

Smaller pixel size and with smaller focal spot and therefore smaller effective focal spot

Smaller focal spot…

gives better detail

Use smaller focal spot for…

extremities

Contrast resolution

ability to distinguish anatomical structures of similar tissues

Ex: liver-spleen and gray matter-white matter

In x-ray imaging, contrast resolution improves at…

Lower kVp

Noise or quantum model

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

Lower noise improves____, resulting in ______

contrast resolution; better radiographic images

Quantum mottle

• Random nature by which x-rays interact with the IR

• most noise out of the 2 components

How to do you reduce quantum mottle?

Use high mAs, and low kVp

Scatter radiation

another type of image noise

Speed

loosely describes the sensitivity of film to x-rays

What goes hand in hand?

Speed, resolution, & noise

What happens if one of these characteristics are altered?

It changes the other two

Ex: Up in speed, down in noise & resolution

Medical image quality

• Fast IR have high noise and low contrast resolution

• Low noise accompanies slow IR and high contrast resolution 

• Spatial resolution is limited to pixel size

Fast IR

800 speed → high technical factors → more scatter

Slow IR

200 speed → down in technique

IR factors

• Pixle size

• Dynamic range

• Intensity response

• Signal to noise

• Postprocessing

Geometric factors

• Distortion (shape & size)

• Magnification

• Blur

Subject factors

Contrast

• thickness

• density

• atomic number

Motion

• voluntary

• involuntary 

Magnification

All radiographic images are actually larger than what they represent 

In most cases, we as radiographic should minimize…

magnification, but there are exams in which magnification is needed

Magnification factor

Depends on the geometric conditions of the examination

• MF=Image size/Object size

• MF= SID/SOD

Most imaging departments are changing their traditional ____ cm SID to ____ SID to reduce what?

100 cm; 120 cm; magnification, improved spatial resolution, and reduced pt dose

The image size can be…

Measured directly from the radiograph, but the object size can not

The magnification factor can be determined from the ratio of…

SID to SOD

MF= SID/SOD

Object size can be calculated as follows:

Magnification factor= Image size/Object size= SID/SOD

Object size= Image size (SOD/SID)

These relationships hold true for objects off the…

Central ray also, as long as the OID is the same, and the object is essentially flat

Large SID

Use as large as a SID as possible

Small OID

Place the object as close to the IR as possible

Does OID or SID has a greater effect on magnification?

OID because the OID is the major controlling factor of magnification

SID influences magnification 

Distortion

This is the unequal magnification of different portions of the same object

3 factors that affect distortion

• Object thickness

• Object postion

• 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…

Considerable distortion when radiographed off the central ray

Object postion

If the object plane and the image plane are not parallel, distortion occurs

Foreshortening

Which is the reduction in image size, increase 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

Objects positions shifted laterally from the…

CR will show more spatial distortion

Focal-spot blur

This is a blurred region of the radiograph that is caused by the effective size of the focal spot, which is large to the cathode side of the image

This is undesirable

The arrowhead positioned near the tube target creates…

a larger focal spot blur than the effective focal spot

The arrow much closer to the IR has a…

smaller focal-spot blur than the effective focal-spot

Minimizing focal-spot blur

• Use small focal spots

• Position the patient so that the anatomical part is close to the image receptor (decrease OID)

• Use as much of a large SID as possible

Bc of the heel effect, the focal-spot blur is…

small on the anode size & large on the cathode side of the image

Radiographic contrast=

IR contrast x subject contrast

Image receptor contrast

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

Subject contrast

is 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

Different body part thicknesses affect subject contrast — the thicker the anatomy, the fewer x-rays pass through, and the thinner the anatomy, the more x-rays pass through. The differences in x-rays exiting each section create the subject contrast.

Tissue mass density

different areas of the body that 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 # 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

kVp

This is the most important influence on subject contrast 

Low kVp results in…

high subject contrast or short gray scale contrast

High kVp results in…

Low subject contrast or long gray scale

Motion blur

This is the loss of radiographic quality due to the movement of the patient or the x-ray tube during exposure

Involuntary 

motion of heart and lungs

Voluntary

motion of the limbs and muscles

Ways to reduce motion blur

• use the shortest possible exposure time

• give breathing and patient instructions or use restraining devices

• use large SID

• use small OID

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 higher kVp, with a lower mAs)

• Use correct mAs

• add filtration to the x-ray tube

Thicker pt

throw off more scatter

SID has no affect on…

Finished image contrast