Chapter 2: Digital Imaging Characteristics

The measurement of radiation energy absorbed in a unit of air. (kinetic energy released per unit mass of air)

Air Kerma (J or Gy)

Standardized radiation exposure (KSTD)

Standard exposure typical of an imaging receptor system that is made with additional filtration to stimulate patient tissue

Target equivalent air kerma value (KTGT)

A set of established values that represent optimal exposures for each specific body part and view

Indicated equivalent air kerma (KIND)

Measurement of radiation incident on the image receptor derived from pixel values produced by the exposure to an image receptor

Analog

Refers to a device or system that captures or measures a continuously changing signal

Digital

Images recorded as multiple numeric values and are divided into an array of small elements that can be processed in many different ways

Brightness

Refers to the appearance of an image on the display monitor of the computer and is a function of the monitor's ability to emit light through the surface of the display. Luminance

Contrast resolution

The ability of the image receptor to distinguish between shades of gray

Detective quantum efficiency (DQE)

a measurement of the efficiency of an image receptor in converting the x-ray exposure it receives to a quality radiographic image

Deviation index (DI)

The difference between actual exposure (KIND) and the target exposure (KTGT), except that it is expressed in a logarithmic fashion.

DI logarithmic equation

10 x log10(KIND/KTGT)

Dynamic range

the ability of an imaging system to respond to varying levels of exposure

Field of view (FOV)

The amount of body part or patient included in the image

Latitude

The range of exposure diagnostic image values that the image detector can produce

Matrix

A square arrangement of numbers/pixel values in columns and rows. In digital imaging, the numbers correspond to discrete pixel values

Modulation transfer function (MTF)

The ability of a system to record available spatial frequencies

Noise

anything that interferes with, distorts, or slows down the transmission of information

Noise power spectrum (NPS)

Describes the spatial frequency content of the noise as well as the spatial characteristics

Pixel

Short for "picture element," it is the fundamental unit of a digital image, typically a tiny square or dot which contains a single point of color of a larger image.

Pixel bit depth

The number of bits within a pixel

Pixel size

FOV/matrix size

Signal-to-noise ratio (SNR)

Measure of signal strength relative to background noise.

Analog signal wave is recorded and used in its ______ form.

Original

A digital image begins as an ______ signal.

Analog

The critical characteristics of a digital image are:

-Spatial contrast

-Contrast resolution

-Noise

-Dose efficiency of the receptor

An image formed by recording a continuous changing signal is known as:

Analog

Maximum contrast resolution

The process of associating the pixels with discrete (whole numbers only) values.

Pixel size ______ affects image spatial resolution

directly

True or false. The smaller the pixel, the lower the spatial resolution.

False

Pixel size may change:

-When matrix size changes

-The FOV changes

The worse the matrix size for the same FOV, the ______ the spatial resolution.

worse

Equation for the number of gray tones a pixel can produce.

2^bit depth

A pixel with a bit depth of 8 will have _____ shades of gray.

256

Is the gray level a factor in determining the image contrast resolution?

Yes

Each box within the matrix corresponds to:

-A specific location in the image

-A specific area of the patient's tissue

Each box of an image matrix will display a numerical value that can be transformed into a visual _______ or ______ level.

brightness, density

Matrices are made up of:

Pixels and voxels

Voxels

Pixels that represent a 3D volume of tissue. 3D pixels

Overall dimension of the image matrix or size of the detector

FOV

True or false. Spatial resolution is dependent on matrix size

True

True or false. The size of the matrix does not determine the size of the pixels.

False

If you have a 10 x 12 and a 14 x 17 PSP cassette, and both have a 512 x 512 matrix, then the ______ one will have smaller pixels.

10 x 12

The larger the matrix, the ____ the image file size.

Larger

For a given FOV, the larger the matrix size, the _____ (more or less) small individual pixels.

More

Increasing the number of _____ (smaller or larger) pixels will improve the quality of the image.

Smaller

The term FOV is synonymous with:

X-ray field

Changes in the FOV _____ (do or do not) affect the size of the matrix.

Do not

Changes in matrix _____ (do or do not) pixel size

Do

As the matrix increases and the FOV remains the same, the pixel size must _______ to fit into the matrix.

Decrease

Matrix size _____ (can or cannot) be changed without affecting the FOV.

Can

The FOV ______ (can or cannot) be changed without affecting the matrix size.

Can

A change in either the matrix size or the FOV _____ (will or will not) change the size of the pixels.

Will

The amount of tissue included in the image is referred to as:

FOV

Exposure index

Refers to the amount of exposure received by the image receptor, not the patient

In 2008, International Electrotechnical Commission (IEC) published a report titled "Medical electrical equipment -- Exposure index of digital x-ray imaging systems -- Part 1: Definitions and requirements for general radiography."

In 2009, the American Association of Physicists in Medicine (AAPM) released a report #116 titled "An Exposure Indicator for Digital Radiography."

This report expressed a need for what?

Standardized exposure indicators

Standardized Radiation exposure is abbreviated as:

KSTD

KSTD is a standard exposure typical of that imaging receptor system. The exposure is made with additional filtration that ______ the beam to simulate patient tissue. These standard conditions for the exposure are used to ensure that...?

Hardens, the equipment is functioning appropriately.

Indicated equivalent air kerma is abbreviated as:

KIND

KIND is the measurement of:

The radiation that was incident on the IR for that particular exposure.

The KIND measurement is derived from reading the _____ _____ produced by the exposure on an IR. Known as _____

Pixel values, for-processing pixel values (Q)

To derive the KIND, the median pixel value produced by the exposure is found after a data correction and the median value is compared to the ______.

KSTD

______ is the amound of exposure on the IR.

KIND

Target equivalent air kerma value is abbreviated as:

KTGT

KTGT is a set of specific values that represent an _______ exposure for each body part and view. These values are set by the manufacturer or system user.

optimal

The optimal exposure values of KTGT are listed in a _____ in the system by body part (b) and view (v), KTGT (b, v)

table

The difference between the actual exposure (KIND) and the target exposure (KTGT) is:

Deviation index (DI)

What is the perfect image DI value?

0

If the DI is negative, the image is _____

underexposed

To raise the DI +1, increase the technique by ____%

20%

To decrease the DI -1, decrease the technique by ____%

25%

Changing the DI by 1 changes the KIND by +___% or -___%

+25% or -20%

A DI increase of 1 is approximately ____% of the intended exposure

125%

a DI decrease of 1 is approximately ___% of the intended exposure.

80%

Can anything cause the reading of the pixel values (Q) or the DI number to be off? If so, list a few.

Yes

-A prosthesis

-Gonadal shielding

-Failure of the system to recognize the collimated border

-An unexpected body part in the image

Image quality characteristics

-Brightness

-Contrast resolution

-Spatial resolution

True or false. No technical adjustment during image acquisition will change the brightness of the image; it is strictly a display quality.

True

Contrast resolution in digital imaging is directly related to the _____ of the pixels in the image.

Bit depth

In digital, _____kVp and ____mAs can be used, lowering patient dose ______ affecting image contrast.

Higher, lower, without

Why is scatter control more important in digital radiography?

Because digital IRs are more sensitive and have greater latitude.

Spatial resolution

The ability of the imaging system to demonstrate small details of an object

True or false. Dynamic range of the image equates to higher spatial resolution.

False

Smaller pixels = ____ spatial resolution

higher

An AP knee digital image shows more soft tissue than F/S, why?

The wider dynamic recording range.

True or false. Wider dynamic range means more detail.

False

Modulation transfer function is abbreviated as:

MTF

_____ quanitifies the contribution of each system component to the overall efficiency of the entire system.

MTF

MTF is a ratio of the image to the object so taht a perfect system would have an MTF or ____ or ____%

1, 100%

In digital detectors, when an x-ray photon excites a phosphor/scintillation layer, the phosphor produces ____.

Light

In digital detectors, when an x-ray photon excites a phosphor/scintillation layer, spreading out of the light will _____ (always, never) occur.

Always

In digital detectors, when an x-ray photon excites a phosphor/scintillation layer, light spread ____ system efficiency.

Reduces

The more light spread, the _____ the MTF, the ____ the image looks like an object.

lower, less

Radiographic noise occurs during:

The acquisition of the image

Equipment noise is caused by:

Detector elements or nonuniform detector responses

Anatomic noise is caused by:

Superimposition of body parts

Noise power spectrum is abbreviated as:

NPS

The higher the NPS, the _____ the noise for a specific detector.

Higher

NPS describes the ______ ______ content of the noise as well as spatial characteristics.

Spatial frequency

As SNR increases, noise ______, but exposure _____ to the patient.

Decreases, increases

True or false. Latitude depends on the image detector.

True

The higher the dynamic range of the detector, the ____ values can be detected.

More