Spatial Resolution (RADS 342)

Spatial Resolution

The sharpness of structural edges or lines as recorded in the radiographic image

-how well we can see it/how crisp

Spatial Resolution determines...

accuracy of the anatomical structural lines

Resolution

the ability of the imaging system to produce separate images of closely spaced objects

Spatial Resolution is determined/calculated by...

the line pair test tool

Line pair

a line and a space

What is the greates enemy to spatial resolution?

Motion as it detroys it and causes blurring

Two types of motion

voluntary and involuntary

Voluntary motion

motion that can be consciously controlled by the patient's will (could happen if pt. doesn't listen to instructions)

Involuntary Motion

attributed to the physiological action of the systems of the body (peristalsis, heart beat - issue for CT)

-something we cannot control

Ways in which motion is controlled:

-Proper immobilization of the part of patient

-proper explanation to the patient

-reducing exposure time (high mA, low time)

-suspension of respiration

Any motion will cause...

unsharpness causing a decrease un spatial resolution because the part becomes blurred

Object Shape and Spatial Resolution

the shape of the structure can lead to unsharpness

-we cannot control this as most anatomical structures have rounded borders

Focal Spot Size only affects...

Spatial Resolution

Focal Spot

the precise point at which x-rays are produced

Line Focus Principle design gives...

a larger heating area while maintaining a smaller effective focal spot

Line Focus Principle

accomplished by angling the face of the anode 5-20 degrees from vertical, most common is 11 degrees

Effective Focal Spot Size

refers to the focal spot size as measured directly under the anode target, has less heat and will always be smaller

-area seen by pt. or the IR

Actual Focal Spot Size

the size of the area on the anode target that is exposed to electrons from the tube current, will always be larger and uses more heat

-measurable area

Most x-ray tubes are what kind of tubes?

Dual Focus Tubes w/ and small and large focal spot

Small Focal Spot

better spatial resolution/ sharper the image but more heatload on tube

Large Focal Spot

less spatial resolution/ more unsharpness but more heat dissapation to not crack anode

Umbra

term used to decribe the image proper

Penumbra

term used to describe the outer edge (hazy edge), the amount of unsharpness

-area after lines intersect

The penumbra is more pronounced at... (SID)

shorter SIDs, than long, telling us that longer SID's will give better spatial resolution and a sharper image

The penumbra is more pronounced at... (OID)

larger OIDs, than a smaller one, telling us that the smaller OID the better the spatial resolution anf a sharper image

Formula to calculate the geometric unsharpness (blur or penumbra)

P= FSS x OID/ SOD

SOD is calculated by...

SID - OID

Monitor Resolution

digital equipment has less capability for spatial resolution that the older film technology because analog film is physically capable of drawing a sharper line than a digital sensor can

SFS

0.5-0.6 mm

LFS

1.0-1.2 mm

Typical LCD monitor

0.15-0.2 mm pixels

When it comes to spatial resolution, techs should always use:

small focal spot size, maximum SID, and minimum OID

Matrix

a series of colums and rows

-digital image recorded as this

Pixels

picture elements

The larger the matrix size,

the smaller the pixel size which means the btter the spatial resolution

The smaller the pixel size,

the greater the spatial resolution

Pixel Pitch

the distance from the center of one pixel to the center of another adjacent pixel

Pixel pitch and pixels relationship

direct

- when pixels are smaller, pixel pitch is smaller

Pixel Pitch and Spatial Resolution relationship

indirect

the smaller the pixel pitch, means smaller pixel size and increases spatial resolution

Two types of images

Analog & Digital

Analog Image

captures a continuously changing signal, what the human eye needs to see the image

Digital Image

recorded as multiple values, divided into an array of smaller pieces (pixels)

-binary data, each pixed represented by a numerical value that correspons to a brightness level

Sampling Frequency

how often the analog signal is reproduced in its digitized form

- how frequently we take a sample

Increasing sampling frequency...

increases spatial resolution by decreasing pixel size and pixel pitch

Modulation

the process of varying a property of waveform

-for x-ray we dont want this

Modulation Transfer Function

the ability of the imaging system to render objects of different sizes onto an image or to display small objects

A perfect system would have an MTF of...

1 or 100%

High MTF results in...

very little change in wave form which is what we want, and a good SR

Low MTS results in...

the wave form changing a lot which is what we dont want, and bad SR

Which objects are harder to image?

objects with higher spatial frequency so smaller objects

With increasing spatial frequency, line pairs become more...

blurred because they are smaller

Signal to noise ration (SNR)

noise that is related to less photon (quantum), which limits the ability to see an objects edge

-want signal high and noise to be 1

Signal to noise ratio and spatial resolution relationship

the higher the SNR, the better the image in terms on spatial resolution

contast to noise ratio (CNR)

the overall grayscale quality of an image and how an image is percieved by the viewer

contrast to noise ratio and spatial resolution relationship

the more noise the less contrast visualized

Resolution Test Tools

plastic-encased lead bar patterns that are imaged to contruct a modulation transfer function