Ch. 20 Carlton

What was digitally computerized in the 1970s-1990s?

CT and Ultrasound (U/S).

What was the result of the creation of "electronic data sets"?

It was applied to all imaging modalities.

What became possible due to digital advancements?

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

What does Computed Radiography (CR) use?

Photostimulable imaging plates (PSP, IP).

What does Digital Radiography (DR) include?

Direct conversion (without scintillator) and indirect conversion (with scintillator).

What are DR receptors?

Flat panel detectors.

What do healthcare reimbursements encourage?

DR migration.

What format are electrical signals from receptors in?

Analog format.

What is required to convert analog signals to digital language?

Analog-to-Digital Conversion (ADC).

What is the requirement in binary machine language?

Two-symbol alphabet: 0 and 1.

What is the relationship between a bit and a byte?

8 bits = 1 byte.

What two important steps does ADC involve?

Sampling and quantification.

What does digital data consist of?

Bit values (binary digit).

How are bit strings connected?

As bytes.

How is computer memory expressed?

In total bytes (Megabytes, Gigabytes, Terabytes).

What is the matrix made of?

Pixels and Voxels.

What is Field of View (FOV) a part of?

The Matrix.

What does spatial resolution depend on?

Matrix size.

What is a pixel?

Picture element.

What is a voxel?

Volume element.

What is the relationship between Pixel Pitch and Size to spatial resolution?

Inversely related to spatial resolution.

How is sampling frequency expressed?

As pixels/mm.

What is sampling frequency dependent on?

Matrix size and image receptor size.

What does each pixel contain?

Bit-depth.

What does bit depth determine?

Number of assigned gray shades to pixel value.

What is greater bit depth yield?

Greater range of gray shades.

What does a 2^12 bit depth yield?

4,096 gray shades.

What is the effect of greater bit depth?

Increases data set size and volume data for image processing.

What affects image file size?

Pixel size, matrix, and bit depth.

What range does gray scale bit depth fall between?

8 to 32 bits.

What is DICOM?

Digital Imaging and Communications in Medicine is the digital standard for imaging.

What computation is by Histogram Characteristics?

Graphical computation (signal values of data set)

How are signal values arranged in a histogram?

From minimum to maximum.

What does the shape of a histogram depend on?

Anatomy and exposure (Short scale vs. long scale).

What is identified using a Histogram?

Pixel Values of Interest (VOI) and Exposure Index (EI).

What comparison does Histogram Rescaling perform?

Acquired & Reference histogram.

What does rescaling compensate for?

Under/over exposure by shifting histogram.

What does rescaling produce?

Consistent image appearance regardless of exposure.

What happens if there is excessive overexposure?

It can’t be compensated (Dose Creep).

What adjustments are made by a Look-Up-Table (LUT)?

image contrast.

Where are LUT values assigned?

Data points (pixels) in histogram.

What does a LUT produce?

Contrast-look according to reference contrast scale for exam view.

LUT’s vary between

exams and manufacturers.

What cannot be compensated with LUT’s?

Sub-optimum exposure values.

How is image display adjusted?

LUT (graph of processed pixel values).

What changes are allowed in LUT adjustments?

optical density or contrast.

When can pathologies be enhanced?

When difficult to visualize.

What is an adjustment similar to?

Changing DLOGE curve of film emulsion.

How does Spatial Resolution Increase?

Matrix size increases, pixel size decreases, spatial resolution increases.

How does Density Resolution Increase?

Gray scale bit depth increases, density resolution increases.

Low Contrast Resolution (LCR) is the ability to represent

small energy values

What is Noise?

Undesirable signal values impair diagnostic value.

What systems suppress noise?

electronic (system) noise and quantum noise.

How is noise measured?

As Signal-to-Noise Ratio (SNR).

What is Detective Quantum Efficiency (DQE)?

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

What is the formula for DQE?

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

What does a DQE of 1 indicate?

100% or no loss of information.

What is the DQE measurement of DR systems?

Between 30% and 70%.

What does a higher DQE mean?

Lower dose.

What information does Exposure Index (EI) provide?

exposure to image receptor.

How is the acceptable ranges for best image quality expressed?

Target Exposure Indicators (EIT).

What is calculated using histogram values and Pixel Values of Interest (PVOI) mid-points?

Exposure Index (EI).

Exposure Index varies between

Vendors

Exposure index is responsible for

EI values for image quality and ALARA compliance.

Exposure Index Systems has

No universal system; different manufacturers use different systems.

What is compared in EI (Exposure Index)?

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

AAPM and IEC use what to standardize exposure values between manufacturers using EI#’s.

Deviation Index (DI)

What values Uses department

EIT values.

Deviation index compares

Acquired exposure (KIND) and target exposure (KTGT).

DI expressed as

A scale of DI values.

Computed radiography & Digital radiography have

excellent Low Contrast Resolution and greater dynamic range.