Pediatric bone X-ray imaging is characterized by. â low contrast due ... Today, exposure parameters for digital radiography are mostly based on conventional .... in_digital_radiography_of_pediatric_extremities White_Paper.pdf. [5] J Gronau, U ...
Optimizing Dose Efficiency for Digital Radiography of Pediatric Extremities Robert Hess*, Ulrich Neitzel, Dirk Manke Philips Healthcare Diagnostic X-Ray Hamburg, Germany
* now at Hamburg University of Applied Sciences, Hamburg, Germany
Disclosure Statement
All authors were employees of Philips Healthcare at the time of this study.
Background • Pediatric bone X-ray imaging is characterized by – low contrast due to immature bone development – higher sensitivity of children to radiation
• Challenge: To optimize the dose efficiency, i.e.,
– to minimize the dose for the required image quality – to maximize the image quality for a given dose
or,
Background • Today, exposure parameters for digital radiography are mostly based on conventional film radiography:
Typical tube voltages for pediatric extremity exams: 46 – 60 kVp Conventional dose reduction paradigm: use harder beam quality (increase kVp, add pre-filter)
Harder beam quality leads to contrast loss Exposure index (corresponding to film density in conventional radiography) is kept fixed independent of kVp
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Hypothesis
For digital radiography the optimum dose efficiency for pediatric extremity exams is achieved at a different beam quality than commonly used.
Why?
Digital detectors have wide dynamic range
No need for fixed exposure index (or film density)
Retrospective contrast and brightness correction possible
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Study objective
Find the exposure parameters that give the best relation of image quality and applied dose for digital radiography of pediatric extremities.
Variables studied:
Tube voltage
Added pre-filter (0.1 mm Cu / 1.0 mm Al vs. no filtration)
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Methods: Phantom study set-up • Phantom: 10 mm PMMA (soft tissue) 1 mm Al (bone contrast)
X-ray tube Pre-filter
• Digital flat detector X-ray system: Philips DigitalDiagnost 2.1 CsI-based flat detector Al
• Parameters varied: Tube voltage 40 … 66 kVp Pre-filter yes / no mAs
PMMA
Detector
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Methods: Image quality measure X-ray Tube
• Contrast-to-noise ratio (CNR): Pre-filter
CNR
signal1 signal2 1 2
(noise1 noise 2 ) 2
2
noisex: standard deviation of signalx Al PMMA +1mm Al
Detector
ROI1
10mm PMMA
ROI2
noise1
signal1
Pixel value signal2
noise2
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Methods: Dose measure X-ray tube
• Mean absorbed dose (MAD):
Pre-filter
Mean value of absorbed energy per mass:
MAD
Eabs m
Eabs absorbed energy m irradiated mass
Entrance dose Al PMMA
• MAD determined by Monte-Carlo simulation
Detector
Mean absorbed dose (MAD)
Detector dose
• MAD ~ Effective dose ~ Risk of stochastic radiation effects 9
Results: MAD @ fixed CNR (CNR =26)
• the MAD is lower at lower kVp • the MAD is lower without added pre-filter • the MAD can be reduced by 56% by removing the pre-filter and lowering the tube voltage from 57 kVp to 40 kVp
60 µGy
Mean absorbed dose, MAD
For fixed image quality (CNR)
with pre-filter
50 µGy
40 µGy 30 µGy
56%
without pre-filter
20 µGy 10 µGy
0 µGy 40 kVp
50 kVp
60 kVp
70 kVp
Tube voltage
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Results: CNR @ fixed dose (MAD = 25 µGy) 40
• the CNR increases for lower kVp • the CNR is higher without added pre-filter • the CNR can be increased by 42% by removing the pre-filter and lowering the tube voltage from 57 kVp to 40 kVp
Contrast to noise ratio, CNR
For fixed dose (MAD)
+42%
without pre-filter
30
20
with pre-filter
10
0 40 kVp
50 kVp
60 kVp
70 kVp
Tube voltage
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Clinical example without pre-filter
50 kVp
40 kVp (same patient, same MAD)
Doesn't 40 kVp cause higher dose? Comparison of dose profiles in conventional radiography 20
• Lowering kVp while keeping the film density (or exposure index, EI) constant – increases the MAD – increases the entrance dose (ED)
Absorbed dose (µGy)
• For each situation the mean absorbed dose (MAD) is the average level of the corresponding curve
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10
5 40 kVp, same EI 55 kVp (reference)
• At soft beam quality a higher detector dose is required due to the lower screen/film sensitivity
0 0
5 Depth (mm)
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Doesn't 40 kVp cause higher dose? Comparison of dose profiles for fixed MAD or ED 20
• Lowering kVp while keeping the mean absorbed dose (MAD) constant
– slightly increases entrance dose (ED)
• Lowering kVp while keeping the ED constant – decreases the exposure index
Absorbed dose (µGy)
– decreases the exposure index
15
10
5
– slightly decreases the MAD
55 kVp (reference) 40 kVp, same MAD 40 kVp, same ED
0 0
5 Depth (mm)
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Summary • Highest dose efficiency for a situation simulating a pediatric extremity exam is achieved – at 40 kVp (lowest voltage attainable on a RAD generator) – without additional pre-filter • Compared to standard (higher) tube voltages the mean absorbed dose (MAD) can be reduced without image quality degradation • Exposure value (mAs) needs to be adapted such that the MAD remains the same • Detector signal will be lower at 40 kVp; this is covered by the dynamic range of the detector and compensated by image processing
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Outlook
This study suggests that dose optimization in digital radiography may be reached by lowering kVp instead of increasing it as traditionally done in conventional radiography. A similar trend was observed in previous studies for other application areas [1,2].
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Literature [1] MJ Tapiovaara, M Sandborg, DR Dance. A search for improved technique factors in paediatric fluoroscopy. Phys. Med. Biol. 44 (1999) 537–559 [2] AJ Gislason, AG Davies, AR Cowen. Dose optimization in pediatric cardiac x-ray imaging. Med. Phys. 37 (2010) 5258-5269 [3] R Hess, U Neitzel. Optimizing image quality and dose for digital radiography of distal pediatric extremities using the contrast-to-noise ratio. Fortschr. Röntgenstr. 184, 643-649 (2012) [4] R Hess, U Neitzel. Optimizing image quality and dose in digital radiography of pediatric extremities, White Paper, Philips Healthcare. http://www.healthcare.philips.com/us_en/about/events/rsna/pdfs/Optimizing_image_quality_and_dose_ in_digital_radiography_of_pediatric_extremities White_Paper.pdf [5] J Gronau, U Neitzel, R Hess, R Wolf. Improvement of image quality in paediatric patients by lowering the tube voltage to 40 kV at fixed effective dose in digital radiography. Simulating radiography of extremities. European Congress of Radiology 2012, Vienna, B-0867. Insights Imaging (2012) 3(Suppl 1) S331
