Radiographic image quality
Image quality depends on
Geometric factors
Subject factors
Image receptor factors
Geometric factors
Magnification
Distortion
Unsharpness/ penumbra (shaded region)
All images are magnified in radiography
SID/SOD to calculate magnification
Preferred- large SID and have the object as close to the image receptor as possible
If object is off axis and flat then magnification does not change the object (distort it)
Occurs due to unequal magnification to different parts of object- depends on the shape and position of the object
focal spot not in point source
focal spot blur= effective focal spot x OID/SOD
Geometric unsharpness- loss of image quality (definition) due to geometric factors (magnification/ distortion)
2 types of unsharpness: motion and absorption. Motion unsharpness: movement of the patient (respiration). Absorption unsharpness: blurring due to change in x-ray absorption across the edge of the object (insignificant)
preferred: angled anode, width of penumbra greater on anode side of tube, max sharpness towards the anode side of the tube
Subject Factors
Tissue density
Patient thickness
Kvp
Effective atomic number
Object shape (minor factor)
Subject contrast- contrast reduces with higher Kvp as linear attenuation coefficient is energy dependent
Subject Contrast
Contrast reduces with higher Kvp since the linear attenuation coefficient is energy dependent
if we lower Kvp- patient dose must be increased and reduces penetration of x-ray beam