Transcript Iterative Reconstruction Technique for Reducing

Iterative Reconstruction Technique
for Reducing Brain CT Radiation
Dose: A Study in Pediatric Patients
Following Brain Trauma
ASNR 2016
eP-182
J Becker1, A Janicek1, U Udayasankar1, E
Krupinski2, W Erly1
1University of Arizona, Tucson, AZ,
2Emory University, Atlanta, GA
• No disclosures.
Purpose
1. To evaluate the reduction in CT radiation
dose to pediatric brains following trauma as a
result of iterative reconstruction compared to
filtered back projection using standard
manufacturer imaging protocols and
reconstruction algorithms.
2. To evaluate the quality of the images in
quantitative terms, assessing for SNR and
CNR.
Background
• CT of the brain is playing an increasingly important role in the triage
of pediatric patients who have suffered cranial trauma, as
evidenced by the steady nationwide increase in CT during the past
decade. The percentage of Emergency Room (ER) visits for cranial
trauma resulting in CT brain examinations rose steadily from 12% in
1997 to 34% in 2008 1.
• Increased awareness of the potential long-term hazards of
radiation, particularly in children, has engaged physicians and
regulators in attempts to decrease CT radiation dosage without
compromising image quality2- 7.
• It is estimated that 170 additional fatal cancers will develop due to
head CT examinations performed in children younger than 15 years
of age in the US in a single year, with an excess of 1 case of
leukemia and 1 brain tumor per 10, 000 CT brain scans. Cumulative
doses from 2-3 CT brain scans are estimated to almost double the
risk of brain tumors, with 5-10 CT brain scans tripling the risk of
leukemia 5.
Iterative Reconstruction (IR)
• Successful dose reduction has been achieved with the use
of iterative reconstruction technique in the abdomen and
adult brain, with preservation of image quality 7-9.
• In terms of quantitative imaging parameters, a linear
regression equation suggests that standard quality may be
preserved using IR until approximately 20% dose
reduction7.
• In the adult brain dose reduction of 31% has been achieved
in the adult brain, without significant loss of contrast to
noise (CNR) or diagnostic image acceptability 8.
• In the abdomen IR showed a reduction of CTDI and DLP by
an average of between 25% and 50% and up to 65% in
some patients, with up to 3% loss of image sharpness 8-9.
Advantages of IR Compared With
Filtered Back Projection (FBP)
• Although new to CT, IR is widely used in PET imaging and
was initially used when CT was introduced 6.
• A limitation of iterative reconstruction is the long
computing time.
• A modified and computationally faster iterative
reconstruction technique ASIR has been developed to
overcome this using a single technique and has been shown
to overcome the major limitation of FBP namely the
reduction in SNR and CNR with dose reduction.
IR Calculation
• IR is a reconstruction algorithm whereby image data are corrected
with an assortment of models.
• Adaptive statistical iterative reconstruction (ASIR), uses only one
corrective model to address image noise.
• Mathematically, the ASIR model is based on matrix algebra whereby
the measured value of each pixel (y) is transformed to a new
estimate of the pixel (y'). This pixel value is compared with the ideal
answer predicted with the noise model (A).
• If needed, another iteration ensues.
• x = argmin{L(Ax, y) + αG(x)} x ˆ
• A final pixel value for the adaptive statistical iterative reconstruction
image (xˆ) results when repeated y' values ultimately converge.
• G is the regularization term that enforces smoothing and edge
preservation of the data
Methods
• Retrospective review of 101 consecutive pediatric trauma
patients with normal CT brain.
• Aged 21 days to 17 years 300 days.
• All 101 had CT brain performed and reconstructed using
standard manufacturer protocols
• 4 different scanners
• Scanner 1: Siemens FLASH (Siemens, Erlangen,
Germany)using ASIR filtered back projection
• Scanner 2: Siemens Definition AS (Siemens, Erlangen,
Germany) using ASIR filtered back projection
• Scanners 3 & 4 : 64-slice Toshiba Aquilion (Toshiba,
Tustin, CA) using filtered back projection.
Sample Image 1:
Siemens FLASH
kVp 120 variable mAs max 134,
DLP 645 mGycm CTDI 31.97
Sample Image 2:
Siemens FLASH
kVp 100, mAs variable max 168
DLP 369 mGycm, CTDI 22.38
Sample Case 3:
Siemens Definition AS
kVp 120, variable mAs max 113
DLP 397 mGycm, CTDI 24.74
Sample Case 4:
Toshiba 3
kVp 120, variable mAs max 100
DLP 1311.3 mGycm CTDI 61.9
Sample Case 5:
Toshiba 4
kVp 120, variable mAs max 198
DLP 3471.3 mGycm CTDI 200.1
A significant reduction in
radiation dose was achieved using
the Siemens FLASH and Definition
AS.
DLP of Siemens FLASH and
Definition AS were 37% and 18%
of Toshiba 3 and 41% and 20% of
Toshiba 4 respectively.
CTDI of Siemens FLASH and
Definition AS were 44% and 22%
of Toshiba 3 and 35% and 17% of
Toshiba 4 respectively.
The various physical measures were
compared for significant differences as a
function of machine using an Analysis of
Variance (ANOVA) with post hoc Fisher’s
Protected Least Squares Difference
(PLSD) tests.
SIGNIFICANTLY HIGHER SIGNAL
TO NOISE RATIOS (SNR) WERE
SEEN WITH THE SIEMENS
DEFINITION AS in frontal grey
matter, frontal horn of the lateral
ventricle, caudate head, dentate
nucleus and cerebellar white
matter compared to the other
scanners.
No significant difference was
found in these regions between
the other scanners except for
significantly higher SIEMENS
FLASH SNR in cerebellar white
matter compared with Toshiba 3.
CONTRAST TO NOISE (CNR)
CNR
1. SIEMENS
FLASH
2. SIEMENS
DEFINITION
AS
3. TOSHIBA
4. TOSHIBA
1.5
(0.47)
2.27
(0.56)
0.24
(5.85)
0.76
(0.51)
No significant difference in
CNR between the different
scanners
F = 1.05
P = 3.75
Conclusion
• We have shown significant reduction in CT brain doses
using ASIR compared with FBP using manufactures
standard sequences and reconstruction algorithms
without significant reduction in quantitative measures
of image quality:
– DLP of Siemens FLASH and Definition AS were 63% and
82% lower than Toshiba 3 and 59% and 80% lower than
that of of Toshiba 4 respectively.
– CTDI of Siemens FLASH and Definition AS were 66% and
88% of Toshiba 3 and 65% and 83% of Toshiba 4
respectively.
– No significant difference in CNR .
References
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