Imaging parameters and image quality in digital radiography · 1. The optimal imaging parameters...

Imaging parameters and image quality in digital radiography

Hanna Matikka

Medical Physicist, PhD

Department of Clinical Radiology

Kuopio University Hospital

Content

• Imaging parameters• mAs• kV• Added filtration

• Automatic exposure control• Basic principles• Benefits and pitfalls

• General pitfalls of image processing

with some practical tips for optimization

14.5.2019 Hanna Matikka

Motivation

• Radiography is the most common ionizing radiation examination of children

• 250 million pediatric radiological examinations worldwide per year(including dental)

• The most common examinations are chest, extremity, spinal and abdominal radiography

• The need for dose and image quality optimization is emphasized on children as they are more sensitive to adverse effects of radiation

• We as health care professionals have a key role in utilizing and promoting technigues which support optimization


How to build exposure protocols• Large variation in age, size and co-operation of pediatric patients

→ Should the exposure protocols be based on patients' age? Or weight? Or thickness?

• From physicist perspective the thickness of the target would be recommendable. However impractical and probably also unpleasant for the patient.

• A common way is to create exposure protocols based on patients' age

• The recommendable way to build body protocols is based on patient weight and for head protocols based on patient’s age

**(RP185, 2018)14.5.2019 Hanna Matikka

How to select the imaging parameters

• mAs

• kV

• Added filtration

• Automatic exposure control (AEC) or fixed values


mAs and image quality• mAs adjusts the number of photons

• Increasing mAs:• Increases the patient dose• Does not affect the image contrast• Reduces the image noise

• The statistical fluctuation in number of photons ↓

• The electrical noise of the detector ↓

→ Improves the detectability of smallobjects with low contrast

• Ideal images are not free of noise! Increase the mAs only if the noise levelmay hinder the diagnosis

• For a shorter SID, use a lower mAs

3 mAs 14 mAs22 mGycm2 76 mGycm2


Increasing mAs and dose

Seminars in Respirator and Critical Care Medicine, vol 35, No 1/2014

The contrast does not change while noise decreases.The detectability of small details improves.14.5.2019 Hanna Matikka

The kV affects tissues contrast and penetrability

• By increasing the voltage:• Photoelectric effect ↓

• Scattered radiation ↑

→ The tissue contrast decreases• Grids or image processing

• At the same time• The likelihood that x-rays interact with

atoms of tissue ↓

• The radiation becomes more penetrable

• The amount of produced photons/ mAs ↑

→ More photons reach the image receptor

→ Increase in mAs is not necessary The Essential Physics of Medical Imaging, 3rd ed. 2012

15% increase in kV has the same effect on amount of radiation as 50% increase in mAs


133 kV, 0.9 mAsDAP 15 mGycm2

65 kV, 0.5 mAsDAP 3 mGycm2

Image plate: 70 kV, 0.5 mAsDAP not available


In body protocols use the highest applicable kV and the lowest mAsthat produces the desired image quality

1. Choose a kV that provides enough penetration and tissue contrast

2. Choose mAs or AEC setting that provides a suitable noise level


Added filtration• All x-ray tubes have an inherent permanent filtration and most devices

also provide added filtration (e.g. 1mm Al and 0.1-0.2 mm Cu).• The added filtration

• Removes the lower energy photons from the x-ray beam• Decreases the dose of superficial organs and tissues. Lowers the effective dose.• Increases the mAs that is needed and may prolong the exposure time• Decreases the tissue contrast slightly

• In body protocols, at least 1mm Al ja 0.1 mm Cu should be used for children over 3 years. Highly recommendable also for smaller children ifthe exposure time and contrast do not suffer too much.

• On higher kVs a stronger added filtration (0.2 mm Cu) can be applied• Added filtration can also be used to decrease the minimum achievable

dose


The filtration has a major effect on patient dose:The comparison of kVs and mAs can be misleading!

3y PA (sitting) 117 kV, 0.9 mAs1 mm Al ja 0.1 mmCu2 mGycm2

3y AP (lying )86 kV, 0.3 mAsNo added filtration6 mGycm2


A case example

• Thorax and abdomen of a 600g premature baby

• 10 days between the images

• Same device, kV and mAsbut a drop in image quality

• An added filtration of 3.5 mm Al was forgotten to thelast image

• Detector doses about8.5 uGy and 3.5 uGy


Automatic exposure control, AEC

• The purpose of AEC is to stop the radiation when the desired receptordose has been reached and to ensure reproducible image quality

• In radiography it determines the mAs and does not adjust otherexposure parameters


Let’s vote:If you want a similar image quality for a 5-year old child as for an adult,

the AEC cut off dose should be:

A) The same as for an adult

B) Higher than for an adult

C) Lower than for an adult


• The AEC chamber records both scattered and nonscattered x-rays. Only thenonscattered x-rays bring relevant information (contrast) to the image.

• Children have less scattering soft tissues than adults

→ A lower cut off dose will produce the same image quality

• Moreover, noisier images are generally accepted

→ Use a lower dose for AEC than for adults

• On some devices the lowest selectable cut off dose is not low enough for smallchildren

→ Use fixed values (mAs)


AEC or fixed technique?

AEC

+ Reproducible image quality and dose

+ Easy to use: Takes into account thevariation in patient size and attenuation

- The chamber size may be large comparedto beam size

- Requires careful positioning of the objectand centering of the beam

- Use of radiation shielding more demanding

- Not available in mobile DR systems

Do not use adults’ backline of mAs on AEC

Fixed values (kV, mA, s)

+ The patient dose is always on the desiredlevel

+ Easier to use radiation shields

+ Free positioning of the target withrespect to detector

+ More accurate and reproducible for smallchildren (under 3y)

- Requires more experience

For a given mAs, use the shortest availableexposure time. Prefer small focal spot.


In practice

• The desired contrast and image quality depends on users’ conventionand varies between departments

• The optimal parameters vary also depending on the device and image receptors

• The optimization usually starts from the default (factory) protocols• Often by lowering the dose levels and increasing added filtration

• The image processing parameters (LUT curves, filtration) must also beoptimized as they have a strong effect on image appearence (contrast, sharpness)


Virtual grids

• Help in optimization as a physical grid is no longer needed even for thicker (> 20 cm) objects.

• A lower mAs can be applied

• Less grid-related artefacts and other challenges

• Removes scatter from the original image by means of image processing

• Most software have some limitations (patient weight, body area)

• Enhances image contrast and highlights edges like tubes and bones

• Adult settings may not be optimal for tissue contrast of children


Image processing example: Virtual grid


Collimation•The careful collimation of the radiation beam

•Lowers the patient dose (DAP)•Improves the image quality

•Decreases scattering and noise•Enables optimal use of greyscale on the ROI

•The digital collimation of the image•Optimizes greyscale values and image processing

•May save the image and a retake!

SUNY Upstate Medical University, 2010

Insights Imaging (2013) 4:723–727


Summary

1. The optimal imaging parameters vary between DR systems, cliniques and image receptors.

2. In body protocols use the highest applicable kV and the lowest mAsthat produces the desired image quality

3. When comparing mAs and kV, remember also added filtration!

4. AEC: Do not use adults’ dose level or mAs –backline

5. Optimization of image processing is important

6. Careful beam collimation minimizes scatter and patient dose and improves the quality of image processing


References

• Dorfman AL et al. Use of Medical Imaging Procedures With Ionizing Radiation in Children: A Population-Based Study. Arch Pediatr Adolesc Med. 2011, 165(5): 458–464.

• European Guidelines on Diagnostic Reference Levels for Paediatric Imaging. Publications Office of the European Union, 2018

• Knight SP et al. A paediatric X-ray exposure chart. Journal of Medical RadiationSciences 2014 Sep; 61(3): 191–201

• IAEA: Radiation Protection in Digital Radiology

• Ween B. et al. Pediatric digital chest radiography, comparison of grid versus non-grid techniques. European Journal of Radiography 2009, Vol. 1 (4): 201–206

• Optimizing Imaging Quality and Radiation Dose by the Age-Dependent Setting of Tube Voltage in Pediatric Chest Digital Radiography. Korean J Radiol. 2013 14(1): 126–131.


Imaging parameters and image quality in digital radiography · 1. The optimal imaging parameters...

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