Unsharpness is the loss of
spatial resolution
In physics and geosciences, the term spatial resolution refers to distance between independent measurements, or the physical dimension that represents a pixel of the image. While in some instruments, like cameras and telescopes, spatial resoluti ...
in a
radiographic
Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical radiography ("diagnostic" and "therapeuti ...
image. There are generally considered to be three types of unsharpness:
geometric unsharpness, motion unsharpness and photographic or system unsharpness.
Motion unsharpness is caused by movement of the patient, the detector or the source of
X-ray
X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...
s, during the exposure. Movement of the patient, either voluntary or otherwise, is the most common cause and this can be minimised in a number of ways: immobilizing the patient, asking the patient to keep still or to hold the breath and keeping exposure time short, and thereby giving them less time in which to move, are the most obvious.
System unsharpness (previously called photographic unsharpnesss) is the result of the detector system employed. Every detector type has a limiting factor which determines its maximum spatial resolution. In film systems it is the size of the grains of photographic chemical. In
computed radiography systems it the size of the laser used to read the phosphor plate in the cassette reader. In
digital radiography
Digital radiography is a form of radiography that uses x-ray–sensitive plates to directly capture data during the patient examination, immediately transferring it to a computer system without the use of an intermediate cassette. Advantages inclu ...
systems it is the size of the individual
thin film transistors
A thin-film transistor (TFT) is a special type of field-effect transistor (FET) where the transistor is thin relative to the plane of the device. TFTs are grown on a supporting (but non-conducting) substrate. A common substrate is glass, because ...
. Since each type has a maximum capability, there is no way to minimise this system other than using a better and probably more expensive machine.
Geometric unsharpness is caused by aspects of the geometry of the X-ray beam. Two principal factors play simultaneously: the apparent focal spot size and the ratio between object-film distance (OFD) and focus-film distance (FFD). Fine focal spot sizes will minimise geometric unsharpness, and therefore give more detailed images, but it is often impossible to employ them due to the tube loading necessary in the exposure. Keeping the ratio FFD:OFD high will minimise geometric unsharpness. This is most easily done by keeping the OFD to a minimum, i.e., keeping the part of the body being X-rayed as close to the detector as possible. If this is not possible however, then increasing FFD beyond the normal 100–110 cm will be necessary to keep the unsharpness level acceptable.
References
{{reflist Wilks R: 1983: Principles of Radiological Physics; Churchill Livingston. Edinburgh
Radiography