Equip/Image - Resolution/Distortion (Unit 2-4)

What are the two geometric properties of image quality?

• Spatial Resolution

• Distortion

Spatial Resolution

Degree of geometric sharpness or accuracy of the structural lines actually recorded in the image.

• Definition, sharpness, recorded detail

Poor resolution = ___________ sharp definition of fine detail.

Lack of

Poor resolution is caused by unacceptable levels of _________ (edge unsharpness around periphery of the image)

Penumbra

Primary film-screen unit of resolution is _______________________.

Line pairs per millimeter (lp/mm)

Most human visual acuity is limited to the range of _________ (each line is ___ mm wide and seen by the human eye).

• 5 lp/mm

• 0.1

The higher the lp/mm, the __________ the resolution.

Better

Digital imaging spatial resolution is determined primarily by 3 factors:

• Matrix size

• Pixel size

• Grayscale bit depth

Digital imaging spatial resolution is commonly described as _______________.

Spatial Frequency

Higher frequency represents pairs of lines that can be visualized __________________ = _________________. It is good for _______ objects.

• Very close together

• High resolution

• Smaller

Lower frequency represents pairs of lines that are __________________ = _________________. It is good for ______ objects.

• Further apart

• Low resolution

• Larger

Pixel Pitch

Distance from the center of one pixel to the center of the next one in the display.

A _______ lp/mm value indicates better resolution.

Higher

What is the formula for calculating resolution based on pixel pitch?

Modulation Transfer Function (MTF)

Mathematically expresses spatial resolution on a scale of 0-1.

0 MTF

No signal, no image

1 MTF

Highest resolution possible.

In medical imaging, the goal is to produce an image with an MTF value as close as possible to ______.

1 or 100%

Sampling frequency is expressed as ____________.

Pixels/mm

Digital imaging requires that the spatial resolution frequency be sampled ______________.

Twice per cycle

_____________ sampling 2x/cycle produces an accurate, high-frequency digital signal.

Nyquist Frequency

A higher sampling frequency __________ image resolution (fidelity) because more information is detected and captured for accuracy.

Increases

Aliasing

Occurs when the Nyquist criterion is violated, and sampling occurs less than twice per cycle.

Aliasing produces an _________, fluctuating, ___-frequency digital signal.

• Inaccurate

• Low

Aliased data create a _________ artifact.

Moire pattern

Spatial resolution is often improved through factors that ________________.

Increase patient dose

Resolution problems should be approached in this order: (4)

1. Eliminate motion

2. Reduce OID

3. Reduce focal spot size

4. Increase SID

_________ of the beam is the most important factor for spatial resolution.

Geometry

Because the x-ray emanates from a small point (the focal spot), the photons ________ as they move further from their source.

Diverge

SID=___ + ___

SOD + OID

SOD=____-____

SID - OID

Resolution is improved when OID ___________ and degraded whenever it ____________.

• Decreases

• Increases

Affected side or part of interest is always positioned _____________ to decrease OID.

As close to IR as possible

Resolution is improved when SID ___________ and degraded when it ____________.

• Increases

• Decreases

Focal Spot

Area of the focal track that is impacted by the electron beam at one time.

Actual Focal Spot

The physical area of the focal track that is impacted; a for arrival.

Effective Focal Spot

The area of the focal spot that is projected out of the tube toward the object being radiographed; e for exit.

Line Focus Prinicple

Minimizes the effective focal spot while maximizing the actual focal spot to absorb the heat.

Focal spot size is a major controller of spatial resolution because it controls __________.

Penumbra

As the focal spot decreases in size, penumbra ____________, and resolution ___________.

• Decreases

• Increases

The width of penumbra can be calculated by:

Penumbra decreases when: (3)

• Focal spot decreases

• OID decreases

• SID increases

Penumbra happens because x-rays _______.

Diverge

Penumbra almost always happens in diagnostic x-ray because overall general shape of human anatomical structures are _____________.

Round or oval

_______________ can improve spatial resolution by reducing part thickness and decreasing OID and penumbra.

Part compression

________ affects spatial resolution because it fails to permit enough time for a well-defined image to form.

Motion

As a result of motion, no ________ can be perceived.

Fine detail

Voluntary Motion

Motion that is under the direct control of the patient.

Involuntary Motion

Motion not under the conscious control of the patient.

The first choice of who should hold a patient is _____________, followed by ____________ or __________.

• Non-pregnant relative

• Hospital employee

• Radiographer

Increase in SID causes _______ in spatial resolution.

Increase

Increase in OID causes ________ in spatial resolution.

Decrease

Increase in patient thickness causes _________ in spatial resolution.

Decrease

Increase in focal spot size causes _______ in spatial resolution.

Decrease

Increase in motion causes ________ in spatial resolution.

Decrease

As focal spot size decreases: (select all that apply)

• Penumbra decreases

• Sharpness increases

• Spatial resolution increases

• Penumbra decreases

• Sharpness increases

• Spatial resolution increases

Calculate the penumbra for an image taken with a 2.0 mm focal spot, at a 40” distance, and OID of 6 in.

0.35 mm

Distortion

Misrepresentation of the size or shape of the structures being examined.

Magnification is _____ distortion.

Size

Elongation and foreshortening are _______ distortion.

Shape

Magnification

Size distortion that occurs during exposure; controlled by SID, OID.

Reduced magnification size distortion ________ the spatial resolution.

Increases

______________ is the exception and requires a special machine with a tiny focal spot to magnify and elevate very subtle skeletal anomalies.

Magnification Radiography

Magnification size distortion is controlled by positioning the body part and tube to maximize ____ while minimizing ____.

• SID

• OID

The greater the SID, the __________ the mag.

Smaller

Magnification Factor Formula

If the SID is 40" and the OID is 2", what is the magnification factor?

1.05

Actual Size Formula

If object B is 2" in diameter on the image and is measured to be 3" from the image receptor, what is the actual size if the SID is 40"?

1.85”

% Mag Formula

Shape Distortion

Misrepresentation by unequal mag of the actual shape of the structure being examined.

Elongation

Projects object so it appears to be longer than it really is.

Foreshortening

Projects it so it appears shorter than it really is.

Elongation occurs when the _______________ is improperly aligned.

Tube or the IR

Foreshortening occurs _________________ is improperly aligned.

Only when the part

Changes in the tube angle cause __________, never ____________.

• Elongation

• Foreshortening

Increase in SID causes _________ in magnification.

Decrease (size)

Increase in OID causes _________ in magnification.

Increase (size)

Increase in improper alignment causes _______ in elongation or foreshortening.

Increase (shape)

If a projected image measures 5.5 in and the magnification factor is 1.25, what is the size of the acutal object?

4.4”

Which of the following occurs when the IR is misaligned with the body part?

Elongation

Image Critque Steps

1. Classify the image

2. Determine the cause of the problem

3. Determine appropriate corrective action

The first step in classifying the image is to:

Evaluate the entire image and determine if it is within the diagnostic acceptance limits (is it repeatable?).

Common Repeatable Issues

• C: Central Ray is centered or angled wrong

• R: Radiation exposure factors (technique) is wrong

• A: Anatomy is clipped

• P: Positioning is wrong

Slowly narrow the search by first classifying the problem into one of three major categories:

• Technical factor problem

• Procedural factor problem

• Equipment malfunction problem

Exposure Index (EI)

Actual Exposure

Target EI (EIt)

Ideal Exposure

Deviation Index (DI)

% deviation from target. DI value of 2 or more needs to be evaluated for repeat.

Geometric problems with detail include:

• Spatial resolution

• Distortion problems

Spatial resolution problems are the result of:

• Geometry of the beam (focal spot size, SID, or OID)

• IR (matrix, pixel size)

• Patient motion

Distortion problems are a result of:

• Size distortion (magnification)

• Shape distortion

Size distortion problems are the result of:

SID or OID

Shape distortion problems are the result of ___________________________ and result in ______________________.

• Improper tube, par, and/or IR alignment

• Elongation, foreshortening

A ________________ factor problem is one that involves ___________________ or _____________________ for the x-ray examination.

• Procedural

• Patient positioning

• Patient preparation

The “classify” step of image critique …

• Is step 2 of the image critique process

• Involves classifying the image as acceptable or repeatable.

• Involves classifying the cause of a repeatable image

• Involves classifying the correctable action needed

Match the causes of imaging problems with their corresponding letter.

• _____ Procedural Factor Problems

• _____ Technical Factor Problems

• _____ Geometric Problems

• _____ Equipment Malfunction

• _____ Photographic Problems

Quality Assurance

Activites that ensure that radiology service is consistently high-quality.

__________________ regular evaluation of equipment to determine its safe and accurate operation.

Quality Control (QC)

Linearity

Given mAs value, using different mA stations with appropriate exposure time adjustments to equal the same mAs, will provide consistent intensity mGya/mAs.

The max acceptable variation in linearity is _____ from one mA station to an adjacent mA station.

10%