Ch. 20 Carlton

Historical Development

  • 1970s-1990s: Digital computerization of CT and Ultrasound (U/S).

  • Creation of “electronic data sets” applied to all imaging modalities.

  • Visualization in multiple planes and computer manipulation via post-processing.

Types of Digital Radiography Systems

  • Computed Radiography (CR): Uses photostimulable imaging plates (PSP, IP).

  • Digital Radiography (DR):

    • Direct conversion (without scintillator).

    • Indirect conversion (with scintillator).

    • DR receptors are flat panel detectors.

  • Healthcare reimbursements encourage DR migration.

Digital Image Formation

  • Electrical signals from receptors are in analog format.

  • Must be converted to digital language

    • Analog-to-Digital Conversion (ADC) is required.

  • Binary machine language:

    • Two-symbol alphabet: 0 and 1.

    • Bit versus Byte: 8 bits = 1 byte.

ADC

  • Involves two important steps sampling and quantification.

  • Digital data consists of bit values (binary digit).

  • Bit strings connected as bytes.

  • Computer memory expressed in total bytes (Megabytes, Gigabytes, Terabytes).

Digital Image Characteristics

  • Matrix: Made of Pixels and Voxels.

    • Field of View (FOV).

    • Spatial resolution depends on matrix size.

  • Pixel: Picture element.

  • Voxel: Volume element.

Pixel Pitch and Size

  • Inversely related to spatial resolution.

  • Sampling frequency expressed as pixels/mm.

  • Dependent on matrix size and image receptor size.

Bit-Depth

  • Each pixel contains bit-depth

  • Determines number of assigned gray shades to pixel value.

  • Greater bit depth yields greater range of gray shades.

  • 2^12 bit depth yields 4,096 gray shades.

  • Greater bit depth increases data set size and volume data for image processing.

Image File Size

  • Affected by pixel size, matrix, and bit depth.

Gray Scale Bit Depth

  • Ranges from 8 to 32 bits.

  • Digital Imaging and Communications in Medicine (DICOM) is the digital standard for imaging.

Histogram Characteristics

  • Graphical computation of signal values of data set.

  • Arranges signal values from minimum to maximum.

  • Shape varies depending upon anatomy and exposure. (Short scale vs. long scale)

  • Used to identify Pixel Values of Interest (VOI) and Exposure Index (EI).

Histogram Rescaling

  • Acquired histogram compared to reference histogram.

  • Rescaling compensates for under/over exposure by shifting histogram.

  • Produces consistent image appearance regardless of exposure.

  • Excessive overexposure can’t be compensated (Dose Creep).

Look-Up-Table (LUT)

  • Adjustments to image contrast.

  • LUT values assigned to data points (pixels) in histogram.

  • Produces contrast-look according to reference contrast scale for exam view.

  • LUT’s vary between exams and manufacturers.

  • Sub-optimum exposure values cannot be compensated with LUT’s.

LUT Adjustments

  • Image display adjusted from LUT (graph of processed pixel values).

  • Permits changes in optical density or contrast.

  • Can enhance pathologies; used when pathologies difficult to visualiz.

  • djustment similar to changing DLOGE curve of film emulsion.

Digital Image Quality

  • Spatial Resolution: Matrix size increases, pixel size decreases, spatial resolution increases.

  • Density Resolution: Gray scale bit depth increases, density resolution increases.

  • Contrast Resolution: Low Contrast Resolution (LCR) is the ability to represent small energy values; CR/DR receptors have excellent LCR and greater dynamic range.

Noise

  • Undesirable signal values impair diagnostic value.

  • Systems can suppress noise to a point, including electronic (system) noise and quantum noise.

  • Measured as Signal-to-Noise Ratio (SNR).

Detective Quantum Efficiency (DQE)

  • Measure of how sensitive and accurate incoming data is converted to output viewing.

  • DQE=(SNRo)2/(SNRi)2DQE = (SNRo)^2 / (SNRi)^2

  • DQE of 1 = 100% or no loss of information.

  • DR systems have a DQE between 30% and 70%.

  • Higher DQE means lower dose.

Exposure Index (EI)

  • Provides information about exposure to image receptor.

  • Acceptable ranges for best image quality expressed as Target Exposure Indicators (EIT).

  • Calculated using histogram values and Pixel Values of Interest (PVOI) mid-points.

  • Varies between vendors.

  • Responsible for understanding and applying EI values for image quality assessment and ALARA compliance.

Exposure Index Systems

  • No universal system; different manufacturers use different systems.

  • Comparing Sensitivity number (S number), Exposure Index (EI), or Log Median Exposure (LGM) values.

Deviation Index (DI)

  • Created by AAPM and IEC to standardize exposure values between manufacturers using EI#’s.

  • Uses department EIT values.

  • Comparison between acquired exposure (KIND) and target exposure (KTGT).

  • Expressed as a scale of DI values.