MOD 9 - Exposure and IQ in DR

Contrast Resolution

the ability of the imaging system to express slight attenuation differences (brightness) between similar adjacent tissues

High contrast resolution

• images will have sharp differences between structures but not be able to differentiate similar attenuating tissues (such as muscle, water and fat imaged together)

• smaller grey scale

Low contrast resolution

• images will have dull differences between structures but better at defining these similarly attenuating tissues

• extensive grey shades

Spatial Resolution

the measure of a system's ability to accurately demonstrate small objects as distinct, often considered high detail or high resolution

Spatial Resolution is dependent on

the fixed spatial resolution determined by the detector element size

Method of Spatial Resolution Measurement

• Line Pair Testing and Spatial frequency (determined from pixel pitch)

• Modulation Transfer Function (MTF)

Spatial Frequency

tests the system's ability to distinguish adjacent small objects as separate using the 'line pair' test tool that contains pairs of thin wires with increasing narrow spaces between the lines

spatial frequency =

line pair = defined as both the solid line and the adjacent space

higher spatial frequency =

higher spatial resolution

higher spatial frequency of the anatomy

smaller in size = more difficult to image

Modulation Transfer Function (MTF)

the ability of a system to accurately demonstrate small objects accurately

• most common method of describing spatial resolution

MTF range and meaning

MTF range = 0-1

0 = the object is not represented at all

1 = ideal, system expresses very small objects exactly as they exist

lower MTF =

high spatial frequency objects = harder to image and thus look blurrier

can MTF be 1

No, MTF can never be 1 in diagnostic imaging due to a variety of geometric factors and detector element size limitations

Image Noise

a blanket title that defines all destructive data being included in an image

Electronic Noise (dark current noise)

inherent noise in digital imaging systems due to the use of electrical devices

Quantum Mottle

another form of image noise, that is directly related to dose and receptor exposure

Quantum Mottle in images and dependent on what

shown as brightness fluctuations and is photon-dependent

quantum mottle with low exposure

incident beam lacks sufficient energy = high absorption (photoelectric effect) → insufficient beam transmission = increased noise and patient dose

signal-to-noise ratio

ratio of useful diagnostic image and noise

low-signal-to-noise ratio

noisy images that are produced when the detector pixels do not receive sufficient signal to produce an accurate sample

Dynamic Range

DR system's wide grey scale to express its deep contrast resolution

Exposure Latitude

a DR algorithm which is associated with a histogram of ideal exposure intensities that during processing can correct a wide variation in actual exposure

Exposure Latitude cannot “fix“

excessive noise caused by quantum mottle

Detector Saturation

when the detector elements are flooded with transmitted photons (high exposure) and the differential attenuation required for radiographic is lost

Detective Quantum Efficiency (DQE)

represents the detector absorption efficiency for a wide range of photon energies

DQE is dependent on

detector materials and design

DQE and kV

increased kV = low DQE that requires the extra kV to produce a good image

high DQE =

fewer x-ray photons are required to produce an image

which material provides the highest DQE

amorphous selenium (a-Se)

Windowing

a post-processing function which allows the viewer to alter the grayscale display to improve visualization of different densities