Digital Imaging Characteristics

Flashcards covering digital imaging characteristics, spatial resolution, CR vs DR, pixel/voxel data, and hardware components based on lecture notes.

What are the dimensions of a voxel?

A voxel is measured in the $X$, $Y$, and $Z$ direction representing height, width, and depth.

How is a pixel measured?

A pixel is flat or two-dimensional ($2D$) and measured in the $XY$ direction.

What systems are included in the ARRT definition of Digital Radiography?

Digital Radiography includes both computed radiography (CR) and direct radiography (DR).

What is the function of storage phosphors in Computed Radiography (CR)?

They are used to temporarily store energy representing the image signal.

How are electronic image signals captured in Direct Radiography (DR) systems?

Detectors directly capture and readout an electronic image signal.

What determined the fixed spatial resolution in Direct digital systems (DR)?

The detector element (DEL) size of the TFT.

How does fill factor (%) affect spatial resolution?

The larger the fill factor, the greater the spatial resolution.

Between a $90\%$ and $80\%$ fill factor, which provides greater spatial resolution?

A $90\%$ fill factor.

What is the effect of laser beam width on resolution in Indirect digital systems (CR)?

A narrow laser beam results in greater resolution.

How does phosphor crystal size relate to resolution?

Smaller phosphor crystal size results in greater resolution due to less light diffusion.

What is the relationship between monitor matrix size and spatial resolution?

A larger matrix size of the monitor equals greater spatial resolution.

What are the specific active layer materials in CR (Computed Radiograph) systems?

Europium Barium Flourohalides.

Describe the two steps of Scintillator-based (indirect acquisition) systems.

$1st$ converts x-rays to light and then converts light to an electronic signal.

Describe the one-step process of Non-Scintillator based (Direct acquisition) systems.

Converts x-rays directly to an electronic signal.

How is pixel pitch measured?

It is measured from the center of one pixel to the center of an adjacent pixel.

What is the formula to calculate approximate pixel size?

Divide the field of view (FOV) by the matrix size, ensuring units match.

Calculate the pixel size for a $2048 \times 2048$ matrix with a $25\,cm$ FOV.

$250\,mm$ divided by $2048 = 0.122\,mm$.

What do detector elements (DELs) collect in flat-panel devices?

Electrons given off by Amorphous Selenium or Amorphous Silicon.

How is image receptor spatial resolution related to TFT DEL size in DR?

It is fixed and inversely related; a decrease in DEL size results in an increase in spatial resolution.

What are the three component parts of a detector element (DEL)?

The TFT, Fill Factor (sensing area), and the capacitor.

What is the function of the capacitor in a DEL?

It stores electrical charges.

Define 'Fill Factor'.

The ratio of a pixel's total size to its actual collection area, expressed as a percentage.

What is the consequence of an $80\%$ fill factor?

$20\%$ is covered by electronics and $80\%$ is the sensing area.

What is the effect of the 'dead zone' on DELs?

It limits how small the DEL can be, which limits spatial resolution.

What does a photodetector do?

It converts light into an electronic signal (photo-conductor or photodiode).

Which material is used as a photodiode in indirect flat-panel devices?

Amorphous Silicon.

Which material is used as a photo-conductor in direct acquisition flat-panel devices?

Amorphous Selenium.

Why do newer DR units allow for less patient dose (less mAs)?

Because they utilize direct acquisition (x-ray photons directly to electronic signal).

Compare CCDs and CMOS regarding resolution and contrast.

CCDs typically provide better resolution and contrast.

What is a primary advantage of CMOS over CCDs?

CMOS has a much greater speed and is less expensive.

How does a lower sampling frequency in CR affect pixel size and spatial resolution?

It equates to a larger pixel size and lower spatial resolution.

What is the relationship between pixel size and resolution?

Pixel size is inversely related to resolution; as pixel size decreases, resolution increases.

True or False: FOV and matrix size are dependent on one another.

False; FOV and matrix size are independent of one another.

Define Modulation Transfer Function (MTF).

A measurement of recorded detail, sharpness, and resolution that measures the accuracy of an image compared to the original object.

What does an MTF of $0$ and an MTF of $1$ represent?

$0 = \text{no image}$; $1 = \text{signal with high fidelity (perfect image)}$.

How does spatial frequency affect MTF?

An increase in spatial frequency results in a decrease in MTF.

How is the number of shades of gray determined in digital imaging?

By the gray scale bit depth.

What is the formula for bit depth?

Bit depth is equal to $2^n$, where $n$ is the number of bits.

How many shades of gray are represented by $10$ bits?

$1,024$ shades of gray.

Define Contrast to Noise Ratio (CNR).

The ability to differentiate between brightness levels based on signal (mAs) versus noise.

What is the result of a low SNR?

The image will be grainy or mottled, and low-contrast soft tissue detail cannot be seen.

Define Detective Quantum Efficiency (DQE).

The percentage of incoming x-ray photons that are detected and absorbed by the receptor.

What is the average DQE range for digital systems?

$0.3 - 0.7$.

What happens to the image if the DQE is too high?

The image will be noisy due to low mAs utilized.

Define 'Speed Class'.

The operational exposure level at which a digital imaging system is operated.

If sampling frequency is $10\,\text{pixels/mm}$, what is the pixel size?

$100\,\text{micron}$.

What limits contrast resolution in digital receptors?

Quantization (number of bits per pixel) and dynamic range.

What primarily controls the amount of scatter in digital imaging?

The amount of exposure (intensity) expressed in mAs.

At what percentage of underexposure does quantum mottle/noise typically appear?

$50\%$ or greater.

What are some vendor names for Grid Replacement Software?

Skyflow, SmartGrid, Scatter Correction, Virtual Grid, and Intelligent Grid.