RADSCI 202 Exam #2

What are the three fundamental steps of digitizing an image?

Scanning, sampling, and quantizing

What occurs during the scanning step of digitization?

The image field is divided into an array (matrix) of small cells or pixels. In a CR reader, the laser beam scans across the plate to retrieve the data.

What occurs during the sampling step of digitization?

The intensity of the radiation or light is measured from each designated pixel area.

What occurs during the quantization step of digitization?

The analog measurements are rounded out to discrete numerical values (gray levels) based on the system's dynamic range. (Note: The conversion to binary is completed by the Analog-to-Digital Converter as part of this step.)

What is a pixel?

A picture element (pixel) is the smallest element of a digital image, assigned a numerical value that corresponds to a specific brightness or gray level.

What is pixel pitch?

The distance measured from the center of one pixel to the center of an adjacent pixel.

What is the formula for calculating the computer file size of a medical image?

Matrix Size × Bit Depth

What is the formula for calculating dynamic range (number of available gray levels)?

2^bit depth. For example, an 8-bit depth produces 2^8, or 256 shades of gray.

What does increasing the window level do to the image?

Increasing the window level makes the overall image darker (controls brightness).

What does adjusting the window width do to the image?

Adjusting the window width changes the length of the gray scale, effectively controlling the image contrast.

What is the concept of "decoupling of brightness" in digital imaging?

The final image brightness and contrast are completely decoupled from the original acquisition. Brightness is primarily controlled by computer postprocessing algorithms (rescaling), not solely by the original kVp or mAs exposure to the detector.

What is Detective Quantum Efficiency (DQE)?

A measurement of the overall efficiency with which a detector can absorb input x-ray exposure and convert it into a useful output image.

How does a higher DQE affect patient dose?

A higher DQE (such as ~77% in indirect DR systems) provides a superior signal-to-noise ratio. Because it absorbs photons so efficiently, the required radiographic technique can be lowered, saving patient exposure.

What is a dexel (DEL)?

In DR systems, the individual hardware detector elements that make up the image receptor are accurately called dexels or dels.

What is the Active Matrix Array (AMA)?

A layer of microscopic dels laid out in columns and rows, controlled by data lines and gate lines that manage the readout sequence.

What components are contained in every del, and what are their functions?

Every del contains a microscopic capacitor (which stores the electrical charge generated from x-ray exposure) and a thin film transistor/TFT (which acts as a switching gate that opens to release the stored charge during image readout).

How does direct-conversion DR work?

Direct-conversion systems use a semiconductor layer made of amorphous selenium (a-Se) to absorb x-rays and convert their energy directly into an electrical signal.

How does indirect-conversion DR work?

Indirect-conversion systems first use a scintillator layer (typically cesium iodide or gadolinium oxysulfide) that converts x-ray energy into light. This light then strikes an active matrix array of amorphous silicon, which converts the light into an electrical signal.

What are F-centers in CR systems?

Electronic holes in the crystal lattice (also called metastable sites) where ionized electrons become trapped when the photostimulable phosphor (PSP) plate captures the latent image.

What causes ghosting artifacts in CR systems?

If the PSP plate is not fully erased after reading, residual information from previous exposures can be left behind, causing ghosting artifacts.

How is a CR plate erased after image reading?

The PSP plate is moved into an eraser section where it is flooded with bright white light to flush out any remaining trapped electrons.

What is the function of the photomultiplier tube (PMT) in CR systems?

During scanning, the laser forces trapped electrons to emit a dim light. The PMT detects this light, converts it into an electrical current, and amplifies the signal so it can be digitized.

What is the primary limiting factor for spatial resolution in digital systems?

The size of the pixels (or dels). Smaller pixels or dels yield higher spatial resolution.

What is the Nyquist criterion, and why is it important?

The sampling frequency must be at least double the highest spatial frequency of the image being recorded. This prevents destructive aliasing (Moire) artifacts.

What is fill factor?

The percentage of a del's square area that is actively devoted to the semiconductor detection layer (photon absorption).

What happens to fill factor when dels are made smaller, and what are the consequences?

Because the microscopic TFT and capacitor cannot be reduced in size, making the overall del smaller causes the fill factor to plummet. A lower fill factor reduces the detector's DQE and contrast resolution, necessitating an increase in radiographic technique and a higher radiation dose to the patient.