Spatial Resolution (RAD 1102)

Photographic Properties

visibility of detail is brightness and contrast

Geometric Properties

sharpness of detail is spatial resolution and distortion

spatial resolution

ability of an imaging system to accurately display objects in two dimensions

What does spatial resolution refer to that can be detected in an image?

smallest object

The ability of a radiographic image to demonstrate ______ ______ determines the quality of the spatial resolution, and there will always be some loss of resolution

sharp lines

Spatial resolution is determined by:

- matrix size

- number of pixels

- gray scale bit depth

- Detector Elements (DEL)

- sampling frequency (CR)

- pixel size

- pixel pitch

Spatial resolution is determined by...

matrix size

matrix

series of square boxes giving form to an image

How is the matrix laid out?

by rows and columns

What is each box inside the matrix called?

a pixel

Each pixel is a single numeric which is represented as a single brightness level on the monitor

Each pixel corresponds to a shade of gray representing an area in the patient. This is called a what?

voxel or volume element

A voxel (volume element) is...

3D

A voxel is converted into a __________ value contained in each pixel

numerical

What limits spatial resolution?

pixel size

Is there a digital system that can resolve an object that is smaller than the pixel size? If so, what is it called?

no, there is no digital system that is able to make out objects that are smaller than the pixel size

Spatial resolution is determined by...

the number of pixels

pixel density

number of pixels/mm in an image

A matrix size of 1024 x 1024 contains 1,048,576 pixels while a 2048 x 2048 matrix contains 4,194,304 pixels. The amount of pixels is known as the....

pixel density

The matrix size and pixel density have a(n) ______ relationship

direct

If the matrix size is increased, the number of pixels will __________ which will __________ spatial resolution

increase; increase

↑ matrix size = ↑ # of pixels (pixel density) = ↑ spatial resolution

What determines pixel density?

pixel pitch

An increase in pixel density results in a(n) _________ in number of pixels which ____________ resolution

increase; increases

Which image has the most pixel density?

C

Which image has the least pixel density?

A

pixel pitch

space from the center of a pixel to the center of the adjacent pixel

pixel size

physical size of the pixel square

Spatial resolution is determined by...

bit depth

Each pixel in an image is represented by what?

a series of bits

How many bits are in 1 byte in each pixel of an image?

8 bits

Bit depth is determined by...

ADC (analog to digital converter)

pixel bit depth

the number of bits that have been made available in the digital system to represent each pixel in the image

Pixel bit depth = ______ of bits/pixel

number

Bit depth determines...

the amount of shades of gray available to display

The formula for bit depth is...

2^n

If the pixel depth is increased to 8 bits (2^8), what happens to the pixel and spatial resolution?

the pixel can now have 256 different gray values available to it which increases the gray values resulting in better (more) spatial resolution

Bit depth and spatial resolution have a(n) ________ relationship

direct

↑ bit depth = ↑ spatial resolution

If there is an increase number of bits, the signals from the detector are...

more accurately digitized

The numerical size of an image is the product of what?

number of pixels (pixel length x width of image)

and bit depth (bits per pixel = (m)(n) (bit depth))

Effects of Pixel Size and Bit-Depth

Spatial resolution is determined by...

DEL or Detector Element

DEL (or Detector Element)

flat panel system that is the pixels on the image in which the number of DEL's corresponds to the number of pixels

The number of DEL's and pixels as well as spatial resolution have a(n) _________ relationship

direct

↑ DELs = ↑ # of pixels = ↑ spatial resolution

Spatial resolution is measured by:

- point spread function

- spatial frequency

- modulation transfer function (MTF)

point spread function (or line spread function)

measurement of spatial resolution that expresses the boundaries of an image and measures the amount of blur (penumbra)

spatial (sampling) frequency

measurement of spatial resolution that measures the distance between pairs of lines that can be imaged as distinct from one another

↑ spatial (sampling) frequency = ↑ recorded details/spatial resolution

The more samples available equals better spatial resolution

modulation transfer function

measurement of spatial resolution that measures the accuracy of an image compared to the original object on a scale of 0-1

↑ modulation transfer function = ↑ recorded details/spatial resolution

The more efficient the overall system is equals better spatial resolution

If the modulation transfer function is measured at 1, that means the image is ____________ ___________. Is a measurement of 1 realistic? Why or why not?

completely perfect but is impossible to do as noise and such is also converted into the image

system noise

background information that the image receptor receives

Noise and spatial resolution have a(n) __________ relationship

indirect

If there is an increase in noise, the spatial resolution of an image is __________

decreased

↑ noise = ↓ recorded details/spatial resolution

Digital imaging systems sample the incoming signal at discrete points. This requires that the spatial resolution frequency signal be sampled twice from each cycle.

Nyquist Criterion

the processing algorithm that averages the incoming analog by using the distance between the imaging detector elements

Motion Sharpness

Categories of Recorded Detail

1. Motion (i.e. involuntary & voluntary)

2. Geometric (i.e. OID & SID)

3. Intrinsic

The biggest detriment to recorded detail is....

motion

What are types of motion?

involuntary and voluntary

How do you control involuntary motion?

- knowledge of system

- vary exposure time (decrease exposure time = alleviate motion of patient)

How do you control voluntary motion?

- patient cooperation

- immobilization of part

- suspension of respiration

- short exposure time (to do so, use high mA causing short time, increase KV 15%, or decrease SID)

Example Photo of Motion in A Chest X-ray

When trying to obtain a short exposure time in order to control voluntary motion, you can use a _______ mA technique cause of mA & time's inverse relationship

high

When trying to obtain a short exposure time in order to control voluntary motion, a 15% __________ of KV can be used as this technique uses half the amount of mAS used for an exposure

increase

When trying to obtain a short exposure time in order to control voluntary motion, you can ________ the SID to do so

decrease

Motion causes what of the part in an image? How does this affect recorded detail?

blurring of the part which causes a decrease of recorded detail

↓ exposure time (shortest) = ↑ mA or KV & ↓ SID = ↓ motion = ↑ recorded detail

Geometric Sharpness

Geometric Unsharpness

- penumbra

- umbra

penumbra

blur at edge of a structure

umbra

image proper

Penumbra and recorded detail have a(n) __________ relationship

inverse

If there is an increase in penumbra, there is a(n) _________ in recorded detail

decrease

penumbra = blur = unsharpness

↑ penumbra = ↓ recorded detail (causing unsharpness of image)

Geometric unsharpness occurs when the part to be radiographed is _________ or ___________

magnified or distorted

Geometric sharpness is affected by:

1. OID (object image distance)

2. FSS (focal spot size)

3. SID (source image distance)

OID (object image distance) affects _________ _________

geometric sharpness

OID and penumbra have a(n) __________ relationship

direct

When OID is increased, there is a(n) ___________ in penumbra

increase

OID and recorded detail have a(n) __________ relationship

inverse

When the OID increases so does penumbra, this will result in a(n) __________ in recorded detail

decrease

↑ OID = ↑ penumbra = ↓ recorded detail (causing unsharpness of image)

OID also includes _________ _______

patient size

When a patient is larger, there is a(n) ___________ in OID. Why is this?

increase because the part being examined is farther from the IR

In order to reduce OID or magnification of an examine part, you want to place the part as _________ as possible to the IR

close

↑ patient size = ↑ OID = ↑ penumbra = ↓ recorded detail (causing unsharpness of image)

Focal spot size (FSS) affects _______ ________

geometric sharpness

focal spot size

source of x-ray beam is at the target (anode) in the tube in which it is an area on the target rather than a single point at which the x-rays are formed

Focal spot size and penumbra have a(n) __________ relationship

direct

When the focal spot size is increased, the penumbra will ____________

increase

Focal spot size and recorded detail have a(n) __________ relationship

inverse

When focal spot size increases so does the penumbra, there will be a(n) ________ in recorded detail

decrease

When a small focal spot is used, there is _______ recorded detail in an image in comparison to a large

better

Ideally, would you want to use a small focal spot or large focal spot for an image? Why?

small focal spot as it has better recorded detail in an image

↓ focal spot size = ↓ penumbra = ↑ recorded detail (sharper image)