Radiographic Technique for Digital Imaging

In film, technique...

mattered significantly for final image production

in film, kVp =

contrast

less kVp typically indicated higher contrast

in film, mAs =

density

darker meant more mAs

lighter meant less mAs

In digital; brightness, contrast, noise, sharpness, and magnification have all been

tampered with by the time the technologist sees the final image

What is the new king in digital when it comes to technique?

IR exposure

What is techniques primary role?

To ensure that adequate signal reaches the detector system such that computer algorithms can be successful in making corrections and refinements to the image

Tecniques primary role necessitates

high signal to noise ratio

does kVp still control image contrast?

yes but to a degree

kVp still controls he

subject contrast of the remnant beam signal that reaches the detector

subject contrast

The degree of differential absorption resulting from the differing absorption characteristics of the tissues in the body

the percentage penetration of the X-ray beam is

a critical part of ensuring an adequate signal

final digital image qualities are still primarily the

result of the computer operations on the acquired data

no amount of mAs can compensate for

insufficient kVp

what modifies the contrast of the incoming data set?

rescaling

What makes the greatest impact on the contrast displayed in the final image?

LUT applied during gradation processing

The biggest manipulator of image contrast

LUT applied during gradation processing

As long as we have sufficient kVp

Contrast may be further altered after LUT application by

windowing after the initial image is displayed

Primary role of kVp in DR

to provide sufficient penetration to ensure adequate signal reaches detector

Primary controlling factor for grayscale/contrast in DR

LUTs

Primary controlling factor for grayscale/contrast in film-screen

kVp

Primary controlling factor for brightness/density in DR

Rescaling

Primary controlling factor for brightness/density in film-screen

mAs

Primary controller of sharpness for DR

Pixel size

Primary controller of sharpness for film-screen

Focal spot size

Primary controller of magnification for DR

Matrix size

Primary controller of magnification for film-screen

Distancess (SID/SOD) ratio

Primary controller of shape distortion for both DR and film-screen

Alignment

Beam penetration

percentage or ratio of remaining/remnant x-ray photon quantity compared to starting/original X-ray photon quantity

If you had 100 photons to start, and 10 remaining, what is the percentage of penetration

10%

If you have 50 photons of a higher kVp, and 10 remaining, what is the percentage of penetration?

20%

The total exposure at the detector is based on

mAs and kVp

Detector only cares about getting sufficient

quantity to create the image

BUT THIS COMBO OF BOTH STARTING QUANTITY AND PENETRATION

It is extremely important to visualize

subject contrast

What is the percentage of the subject contrast difference between tissues in film?

10% or greater difference in contrast was discernable

What is the percentage of the subject contrast difference between tissues in digital?

It can be as low as 1%

What is the percentage of the subject contrast difference between tissues in CT?

Objects with 0.5% contrast variation

Effects of higher kVp values on production of scatter were previously

over-emphasized

Primary causes of scatter

Patient Size

Collimation

The effects of kVp when compared to tissue volume on scatter are

secondary

Digital equipment is remarkably resilient to the

effects of scatter radiation caused during an exposure

Consider latitude in choosing

exposure factors, including high kVp levels

Research shows that increasing kVp levels by 15% across the board for technique charts has not caused

substantial generation of scatter radiation

and allows for all mAs values to be cut in half

While the impact of increasing kVp by a single 15% step does marginally impact scatter, the impact on contrast of the final displayed image is so

slight as to be generally negligible, and often discernible

Recent research indicates that many radiologists immediately

window images they examine

increasing/decreasing contrast according to the anatomy or particular condition being ruled out

A 15% kVp increase that marginally decreases contrast is

practically irrelevant when the radiologists immediately window contrast to personal preference

How much percentage of magnification should you use for evaluation of mottle?

200%

Mottle is best evaluated by examining a

relatively homogenous tissue, or "smooth" tissue density area

What are some good examples of where you can evaluate mottle the best?

Joint spaces of AP knee

Soft tissue area between transverse process of L5 and scaral ala on AP abdomen

When decreasing mAs and increasing kVp, reviewing images for mottle is not a case of is there mottle, but rather

is there an increase in mottle between images

Recommendation for reducing patient exposure

apply the 15% rule to film techniques for conversion to digital uses

which this is practically already done in DR

By applying the 15% rule to film technique for the conversion to digital uses, the

-surface exposure can be reduced by 1/3

-absorbed dose to the patient can be reduced even more (due to increased penetration)

Exposure latitude (under and over exposure) is defined as

the margin of error in setting technique that a system will allow

Film-screen had a latitude from

-30% to +50% (of ideal exposure)

CR and DR have a latitude margin from

-50% to more than +400%

Underexposure limits are

easy to discern since mottle becomes quickly apparent

Saturation requires how much to be visible

8-10x the normal exposure

a practical upper limit eludes us

Due to the hair-trigger nature for underexposure and the excessive latitude with overexposure limits, what has become the new norm?

Dose creep

Repeat rates have dropped from 5% to

3%

Although repeat rates have dropped, an increase of radiation delivered per exam has been documented at approx

40-50%

What are the 4 factors related to an increase in patient exposure?

1. Popular selection of the 200 speed class on CR/DR systems rather than 300 or 400 that are readily available

2. The irrational resistance to using higher kVps across the board within a department

3. The legitimate fear of image mottle resulting from underexposure

4. The amazing inherent ability of CR and DR to "correct" for overexposure

Myths of CR and DR for kVp

Never use less than 70 kVp

Never use more than 80 kVp

Myths of CR and DR for mAs

This system is mAs driven, adjust mAs only

This system is kVp driven, adjust kVp only

True or false: you cant collimate with CR

false

unless less than 30% of the field is in it

True or false: you cant use grids with CR or DR

false

There is no longer a what in DR for which techniques need to be adjusted

extremity cassette

All procedures in DR are processed at the same speed of

200 or more

Similar thicknesses and tissue densities can share

similar technique

What is an example of anatomy that are similar to one another, that could use similar technique?

Lateral skull

AP C spine

AP shoulder

All 1/2 of a PA skull technique

Skills learned from manual techniques are

essential

For ever 4cm for part thickness

mAs should be doubled

15% rule still

applies

Distance rule of thumb are still

valuable to memorize

When CR was first installed to a location with film, what recalibration was required?

AEC calibration

If AEC is calibrated correctly for CR/DR system, it will work with

reasonable consistency

If mottle is still apparent, the main culprit is likely

continued use of low kVp

high kVp is necessary to assure adequate penetration of the X-ray beam to the AEC detectors

What should you avoid adjusting?

The density setting to +2 or +3

Why should we avoid adjusting the density to +2 or +3?

It will increase mAs unnecessarily

If image mottle is a problem, kVp should be

increased until sufficient signal is reaching the detectors AEC cells

What may be no longer needed as much?

Grids

Why are grids not necessarily needed anymore?

Digital processing can compensate for the effects of scatter, supplanting the traditional purpose of grids

Digital processing is taking over

grids

Virtual grid software literally eliminates need for grid use except for in

the most extreme circumstances

What is the revision of grids in the digital age?

By reducing scatter radiation, grids comprise one method of reducing the overall amount of noise the computer must deal with

In digital, grids reduce

overall amount of noise the computer must deal with

What is moire artifact?

An interference pattern; false presentation of artifactual lines

Aliasing may be also known as

Moire effect

Grid errors are seen more with

CR

Moire: grid error

where grid lines are parallel or near parallel to the scan direction

Short dimension grids

Have grid lines running crosswise rather than lengthwise

What are short dimension grids beneficial for?

Portables (chest)

The grid lines often run in the same direction as the

scan direction

Aliasing is most significant when

lines are near parallel

Grid/scan direction best orientation

90 degrees

Extraordinary exposure latitude of DR allows for more flexibility in choosing

not to use a grid for anatomic regions somewhat thicker than traditional 10 cm (4'') film-screen threshold

Best practice for grid use with DR

1. Any pediatric procedure for anatomy 13cm (5.1'') or less should be performed non grid

2. Aerated thorax images (chest, t-spine, sternum, ribs, etc.) contain considerable amounts of gaseous air that does not produce significant amounts of scatter; therefore, for routine patient habituses, non-grid techniques may be beneficial (dose savings without degradation of image quality)

3. Use lower grid ratios whenever possible

4. For all mobile procedures, 6:1 ratio grid use is generally recommended

What is the part thickness in which a pediatric procedure should be performed non-grid

13 cm (5.1'') or less

Skull/torso procedures in film-screen needed 10:1-12:1 grids, what grids would you use in DR?

6:1-8:1

For all mobile procedures, what grid ratio is recommended?

6:1

Virtual grid software may

overtake the grid practice in coming years

Which is more acceptable, mottle or scatter?

Scatter