Histograms in Direct Radiography

Histograms

• Created by the computer during image acquisition.
• Graphical representation of the data set.
• Shows the number of pixel values vs the prevalence of those values.
• The x-axis represents the amount of exposure.
• The y-axis represents the incidence/count of pixels at each exposure
• Are specific to the anatomical part being imaged.
• Examining histogram data for each exposure allows for comparisons between them.
• Errors in the histogram can lead to masking errors at the processing screen.
• Ideally, all four edges of the collimated field are correctly recognised
• Values of interest (VOI) are indicated on the histogram.
• Analysis helps the computer system determine where the VOI should go, therefore where the edges of the image should be.
• Rescaling allows adjustment of values of interest, potentially improving the image appearance.

Extraction and Processing

• Direct Radiography (DR) systems have two indirect methods and one direct method.
• Indirect method uses a caesium iodide phosphor plate, or gadolinium oxysulfide, as the scintillator.
• This is coupled to a CCD by a fibre-optic bundle or optical lens, or photodetector, or amorphous silicon photodiode, and a TFT. CMOS can also be used.
• Direct method, uses amorphous selenium photoconductor and a TFT, without a scintillator

Direct Digital Radiography Image Capture

• Before exposure, an electric field is applied across the surface of the amorphous selenium layer.
• During exposure, x-rays are absorbed by the amorphous selenium, resulting in electric charges in proportion to the x-ray exposure.
• Charges are stored in storage capacitors attached to the TFTs where they are amplified and converted to digital signal.

Direct Digital Overview

• Exposure to x-ray photons causes a voltage applied, which triggers electronic engagement (electrons).
• Charges collect within pixelated components and migrate toward the TFT array.
• TFTs read the signal
• ADC converts and transmits it to the computer workstation

Indirect vs Direct Digital Systems

• Indirect DR involves X-ray to stored electrons to light photons to readout electronics
• Direct DR involves X-ray to charge to readout electronics.
• Direct DR advantages include higher spatial resolution and suitability for mammography and low-energy X-ray machines.
• Indirect DR advantages include lower cost, faster readout, suitability for general radiography due to possible large IR sizes, and easier repair/replacement.

Pre-Processing

• Pre-processing occurs automatically before the image is displayed.
• Involves checking the histogram.
• Automatic rescaling occurs to adjust the the image to the correct brightness
• Involves system-level pixel variation corrections.

Post-Processing Functions

• Window width and window level are discussed in another lecture elsewhere.
• Annotation feature where patient positioning, markers (L and R), radiographer initials/identifier number, and measurements may be added
• Image flip feature
• Image inversion feature
• Magnification feature
• Edge enhancement increases contrast but increases noise
• Smoothing suppresses noise.

Using Digital Receptors

• Carestream (Kodak) systems use El values as logarithmic expressions
• In these systems, 1500 to 1800 is a change of 300, which represents twice as much exposure to the image receptor.
• Selecting the anatomical part and position for x-ray assists the computer in selecting the correct histogram and expected exposure levels, and prevents histogram analysis errors.

Digital Systems and Dose Recording

• DAP (Dose Area Product) is a measure of exposure in air.
• Combines exposure in air with a computation to estimate absorbed dose to the patient.
• The DAP meter is embedded in the collimator.
• It depends on exposure factors and the field size.

Exposure Index

• El represents exposure at the detector.
• EIT is the target exposure, or what is anticipated for that body part.

Detective Quantum Efficiency DQE

• The expression of radiation level needed to create an optimum image is the DQE.
• Image converts x-ray exposure received into a radiographic image.
• A DQE of 100% or 1.0 indicates the image receptor is 100% effective in converting x-ray exposure into image quality with zero loss of information.
• A higher DQE means a more efficient system, lower values for exposure can be used
• Systems with high DQE can allow for lower patient exposures

Look-up tables

• LUTs impact the contrast displayed on the image.
• LUTs are Histograms of luminescence values used to assign the greyscale values
• A rescaling occurs at this step, the histogram is compared with the anticipated histogram and the LUT determines the contrast
• Overexposed IRIs rescaled lighter and vice-versa to display consistent contrast over a wide range of exposures
• Do not take this ability by increasing exposures to ensure sufficient signal as this breaks the ALARA principle.