Medical Image Viewing Notes

Photometry

Photometry is the science of the human eye's response to visible light. It provides a way to measure and control brightness, contrast, and color of the display, ensuring that what is seen on the screen corresponds to the original light levels.

Photopic vs. Scotopic Vision

Photopic Vision: Bright-light vision using cones; day vision.
Scotopic Vision: Dim-light vision using rods; night vision.

Luminous Flux

Luminous flux represents the total amount of visible light emitted by a monitor. It's a measure of how much light the monitor produces.

Luminance Intensity

Luminance intensity refers to the total intensity of light from a source.

Luminance

Luminance is the intensity of light emitted from a surface. Similar to luminance intensity, but it is another measure of the brightness of a source, such as a digital display device (LED, LCD).

Cosine Law

The cosine law relates to the reduced projected surface area.

When a monitor is viewed straight on, the luminance intensity is at its maximum.
When viewed from an angle, the contrast and luminous intensity are reduced.
Luminous intensity falls off rapidly as one views a digital display device at larger angles from a perpendicular view.

Soft Copy Viewing

Soft copy viewing is performed on a liquid crystal display (LCD) or light-emitting diode display (LED). This means images are displayed (viewed) on the monitor.

Liquid Crystal Display (LCD)

A liquid crystal material exists in a state between a liquid and a solid. It has a molecular structure (crystal) and properties of viscosity (liquid).

A pixel is a basic picture element on a display.
LCDs are constructed pixel by pixel and emit light when electrically stimulated.
They have an intense white backlight that illuminates each pixel.
Each pixel contains light-polarizing filters to control the intensity and color of the light transmitted through each pixel.
Advantage: LCDs are designed to reduce the influence of ambient light on image contrast.
Disadvantage: They may have less image contrast due to the increasing angle view of the monitor.

LCDs typically divide each pixel into vertical subpixels, with each pixel having three filters: red, green, and blue.

LCDs are not limited by veil glare, which allows for good contrast resolution and low image noise.

Image Luminance in LCDs

Approximately 10% of the backlight is transmitted through the monitor.

Part of the inefficiency is due to light absorption in the filters and by the pixel that is blocked by the TFT (thin-film transistor).
The "aperture ratio" measures the image luminance of LCDs. It refers to the part of the "pixel face" that is available to transmit light.

Light Emitting Diode (LED) Display

Phosphor: Any material that emits light in response to an outside stimulus, resulting in visible light called luminescence.
Luminescence: The emission of light. When the material is stimulated, the outer shell electrons are raised to excited energy levels.

How Luminescence works:

Electrons are raised to excited energy levels, which creates a hole in the outer electron shell (unstable).
The hole is filled when the excited electron returns to its normal state.
This transition is accompanied by the emission of visible light photons.

Types of Luminescence

Fluorescence: Emission of visible light only during stimulation.
Phosphorescence: Emission of visible light during and after stimulation.
Electroluminescence: The phenomenon of a material emitting light when electric current or an electric field is passed through it.

LED Characteristics

LEDs have electroluminescence characteristics.

A diode is an electronic device that allows electric current to flow in only one direction.
LEDs emit light when electronically stimulated.
LEDs do not replace LCD digital display monitors; they provide the backlight.

Difference Between LCD and LED

The main difference is in the backlight:

LCD: Uses fluorescent backlights.
LED: Uses light-emitting diodes.

Advantages of LED Backlight in Digital Display Devices

Thinner and have a larger active area for the screen.
Used in the newest generation of curved video screens.
Monitors have a longer life.
Lower power consumption.
Produce a great deal less heat (which increases radiology reading room comfort).

Preprocessing

Preprocessing is designed to produce artifact-free digital images and is done automatically to correct:

Signal interpolation
Lag
Noise

It provides electronic calibration to reduce pixel-to-pixel, row-to-row, and column-to-column response differences.

Signal Interpolation

Defective pixels are corrected automatically using preprocessing.

The defective pixels are averaged, and that value is assigned to the defective pixel to make the correction on the image.

Latent Image/Image Lag

Each type of digital receptor generates an electronic latent image; however, the image is not always made visible. There is often some degree of image lag.

Image lag occurs as a variation in voltage.

Example: Switching from DSA (Digital Subtraction Angiography) to Fluoroscopy (or high dose to low dose techniques).
Image lag is corrected by offset voltage.
It is a buildup of information resulting from leftover signal or exposures in rapid sequence due to the difference in voltage between the outputs of two operational amplifiers.

Line Noise

Line noise is a power problem caused by a fluctuation in electrical current.

It causes linear artifacts to appear on the final image.
Solution: Apply the correct voltage from a row or column of pixels in the unirradiated areas of the IR (imaging receptor).

Preprocessing Calibration

This type of calibration is known as flatfielding, which reduces noise and improves contrast.

Offset images and gain images are automatic calibration images designed to make the response of the image receptor uniform.
Offset images are generated many times daily.
Gain images are generated every few months.

Post Processing

Post-processing includes anything that can be done to an image after it is acquired by the imaging system.

Allows visualization of all shades of gray.
Requires manipulation from the technologist and radiologist.

Annotation

Annotation is the process of adding text to an image.

Dynamic Range

Digital images have ranges up to 16-bit (2^{16} = 65,536 gray levels).

The human eye is only able to visualize approximately 30 shades of gray.
When using window and level adjustment, the tech can make all 65,536 shades of gray visible which is one of the most important features of digital imaging.

Matrix Size

A large matrix size results in better spatial resolution due to smaller pixels.

Large Matrix : Small Pixel Size
Small Matrix: Large Pixel Size

Image Flip

Image flip is the process of flipping images horizontally or vertically for correct image annotation.

Image Inversion

Image inversion is when an image is "inverted" from white to black.

Also known as image or digital subtraction.
The purpose is to enhance image contrast.

Pixel Shift

Pixel shift is used to assist with better resolution of an image.

This is a post-processing technique used in digital subtraction angiography.

Region of Interest (ROI)

A region of interest is an area of an anatomical structure on a reconstructed digital image, as defined by the operator using a cursor.

Can identify items such as lung nodule, stone identification, etc.

Edge Enhancement

Edge enhancement is the application of image processing that alters pixel values in an image to make the edges of structures appear more prominent.

Intended for use on images with high spatial resolution and low signal-to-noise ratios.
Effective for fractures and small high contrast tissue.

Highlighting, Pan, Scroll, Zoom

Highlighting is used for diffuse, non-focal disease.

Allows for careful, precise regions of an image modification.

Collimation

Collimation is a radiation protection method to reduce the field size in order to reduce the amount of scatter radiation impacting the patient. Also can improve image quality. Collimation leads to lower exposure and a better image.