Spatial Resolution Ch 28 - 29 PowerPoint

Spatial Resolution

the ability to differentiate two objects (attributed to sharpness/penumbra)

spatial resolution determines

sharpness of the structural lines in a digital image

High Spatial Resolution

• Edges of anatomical structures will appear sharp and easily visualized

• no motion present

Digital factors & spatial resolution

• matrix size

• pixel size

For high spatial resolution, generally factors should be

small

Increase pixel size results in

decreased spatial resolution

increase matrix results in

increase spatial resolution

unit of resolution

lp/mm (2 pixels per mm)

When assessing recorded detail, clinical evaluation includes

• trabecular pattern of bone

• cortex of bone

• bronchopulmonary markings of lungs

Matrix size

determined from the number of the columns and the number of rows of the image matrix

Results in increased spatial resolution

large image matrix

common plate matrix sizes

• 512 × 512

• 1024 × 1024

• 2048 × 2048

relationship between matrix size and pixel size

inverse

relationship between field of view (FOV) and pixel size

direct

matrix size is not

tech controlled

Sampling frequency is performed by

laser in CR equipment

frequency

number of times data is recorded on a digital plate when being read

With imaging plates: the ______ the sampling frequency = the higher spatial resolution

faster

spatial frequency

the number of details that can be fit into a given space

Modulator Transfer Function (MTF)

the ability of a system to record available spatial frequencies

best measurement of spatial resolution (for computer)

MTF

_____ have the highest MTF

Digital systems (DR)

An MTF of 1 would be

(typically, in 70% - 80% range)

a perfect imaging system

Dynamic range

the range of exposures that may be captured by a detector

dynamic range describes

number of gray shades that an imaging system can reproduce

Digital imaging has a _____ dynamic range

wide

Contrast resolution

ability to differentiate shades of gray

Exposure latitude

the range of exposures which produces quality images

Exposure latitude is the extent

which radiographs can be overexposed or underexposed and still achieve an acceptable result

Disadvantage of the wide exposure latitude that digital systems offer?

dose creep with overexposure to the patient

Quantization

takes electrical signals and turns them into digital bits of information. Occurs during ADC

ADC

analog to digital converter

Quantization is displayed as

specific brightness levels on the image

Bit Depth

total number of possible brightness levels that can be assigned to any individual pixel in the image

8 bits

256 shades of gray

computers manipulate data based on what is called

binary number system

1 byte

8 bits

Object to Image Distance (OID)

distance between patient and IR

Source to Image Distance (SID)

distance between tube and IR

Increase of

• SID

Would increase spatial resolution

true

Increase of

• OID

• FSS

• Motion

• Angulation

Would increase spatial resolution

false

Distortion

the misrepresentation of the true size or shape of an object on a radiographic image

Geometric factors of distortion

• Size Distortion

• Shape distortion (how we make image long or short)

the two ways shape distortion affect the image

• elongation

• foreshortening

Angulation of these factors result in shape distortion

• Image Receptor (IR)

• X-ray tube

• Anatomical part

Magnification is a form of size distortion primarily controlled by

OID

Increasing OID would also increase (related to distortion)

Magnification

For every 1” increase in OID, SID should increase

7-8”

Chest x-ray

72” SID (180 cm)

• decreases the magnification

• increases spatial resolution

PA chest is preferred over _____ because of the location of the heart within the thorax

AP chest

Performing an exam PA instead of AP can

reduce size of the anatomical part

Magnification Factor (MF)

As the x-ray beam diverges the image produced on the radiograph is always slightly larger than the original image

MF formula

SID/SOD (= image size/object size)

SOD

Source to Object Distance

SOD = SID - OID

Actual Size formula

Image Size / MF

Order of solving a MF and actual size problem

1. Find SOD

2. Calculate MF

3. Calculate AS

Image Size formula

Actual size x MF

How is elongation resulted?

angling the tube & image receptor

Foreshortening (f/art)

• angling the part

• AP Partial Flexion Elbow

• Trauma or Fracture scenario

How would you minimize size distortion?

• Decrease OID

• Increase SID

Which of the following would produce the largest image? And why

A. 40” SID, 3” OID

B. 40” SID, 5” OID

C. 72” SID, 3” OID

D. 72” SID, 5” OID

B.

Higher OID = larger image

Lower SID = more beam divergence (think larger shadow)

A short OID would (Decreased, small / Part closer to IR = Good)

bring the object close to the image receptor and improve spatial resolution

When a large OID is used (Increased / Part further from IR = Bad)

• photons will diverge and cause image blurring. Also cause magnification of the part decreasing visualization of small details

• Increased Penumbra (increased blurring)

Long, Increased SID (tube farther from IR = Good)

improves spatial resolution with less divergence of beam

Short, Decreased SID (tube closer to IR = bad)

photons will diverge more and cause image blurring

Focal spot size selection directly related to

size of the filament used

Smaller focal size produces

smaller effective focal spot and a more aligned beam (Higher SR)

Larger focal spot produces

larger effective focal spot and a less aligned beam (less SR)

Penumbra

Un sharpness or blurring at the edges of an object

Penumbra (unsharpness) can be controlled by

• SID

• OID

• SOD

• FSS

A larger patient will increase OID, thus

result in magnification