Model Based Iterative Reconstruction, ASIR, and FBP in the Same Patient:

Impact on Image Noise and Image Quality

William P. Shuman MD, FACRUniversity of Washington

SCBTMR Annual Course Washington DC, Oct. 23-26, 2011

Conflict of Interest Statement

• I administer a grant from GE Healthcare which supports clinical investigations in body CT

• No support for equipment or salaries

Generations of CT Reconstruction

Filtered back Projection (FBP)– Assumes “perfect” model

1st Generation

Image Filters & Kernels– Image space approach– Filter noise, Application specific

2nd Generation

Statistical Iterative Reconstruction– Raw Date Recon Approach– Models noise (Photon & Electronic)

3rd Generation

4th GenerationFull Iterative Recon - Model Based– Models CT system geometry – Reconstruct more accurate imagesIQ

Imp

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& D

ose

Re

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510(k) pending at FDA. Not available for sale in the U.S.

“Idealized” Optics to Real Optics

Filtered Back-Projection

pencil beam

point source

Assumes perfect noise sample

point detector

real source

Real measurementnoise

real detector

Model-Based Iterative Reconstruction

point voxel real 3D voxel

510(k) pending at FDA. Not available for sale in the U.S.

fan beam

Optimize algorithm to run faster on multi-core

processorsMaximize performance

density: more computing powerBlade Center

High-performanceProcessor Technology

Enterprise class blade center with efficient architecture

Proprietary optical modeling algorithm

MBIR Reconstruction

510(k) pending at FDA. Not available for sale in the U.S.

Raw data

SYSTEM NOISE STATISTICS

REAL 3D SYSTEM OPTICS

noise vs. detail low dose, detail lower dose and better detail

SYSTEM NOISE

STATISTICS

Statistical Model-based

Rapid Rapid 30+ mins

510(k) pending at FDA. Not available for sale in the U.S.

Evolution of Image Reconstruction

FBPDirect Reconstruction

FBP ASIR MBIR

120 kVp, variable mAs (NI=36), 1.375 pitch. 0.625/0.8 mm slice: Width = 400, Level = 40 HU, BMI=33Kinahan et al

Soft Tissue ConspicuityIncreased noise Increased conspicuity

Three-phase liver

FBP

MBIR

Arterial Venous 5 min delay

120 kVp, variable mAs (NI=36), 1.375 pitch. 0.8 mm slice: Width = 400, Level = 40 HU, BMI=25

Kinahan et al

Liver: FBP vs ASIR vs MBIR

• 36 cirrhosis patients with known liver lesions• Scanned with low-dose ATCM (NI = 36)

• Clinically optimized for 40% ASIR• kVp 120, BMI up to 35

• Raw data from late-arterial phase reconstructed with FBP, 40% ASIR, MBIR

• Images shown to 2 independent readers• Identify lesions• Characterize as hyperdense, hypodense, mixed• Score lesion conspicuity

Liver: FBP vs ASIR vs MBIR

• Lesion, liver HU density measured

• Noise measured in liver, air, fat, aorta

• Three separate “Truth Readers”• Unblinded – looked at all reconstructions, EMR,

comparison studies• Established lesion number, character, location

MBIR ASIR FBP

Arterial

Delayed

MBIR ASIR FBP

Arterial

Delayed

Liver: FBP vs ASIR vs MBIR

• Preliminary Results:

• Apparent equal lesion detection among the reconstruction types (51 lesions)

• Apparent equal lesion characterization among the reconstruction types

• Subjective lesion conspicuity appears slightly better on MBIR

Liver: FBP vs ASIR vs MBIR

• Preliminary Results:

• Background noise in air for MBIR was one quarter that of ASIR and one fifth of FBP

• Contrast-to-noise ration for MBIR was five times greater than ASIR and six times greater than FBP

Conclusion :

• When CT scanning factors are optimized for 40% ASIR:• Recon of the same raw data set with MBIR,

ASIR and FBP results in subjectively comparable liver lesion detectability, lesion characterization, and lesion conspicuity

• Measured image noise is dramatically lower with MBIR and measured lesion CNR is much greater

Conclusion :

• Suggests that when CT scanning factors are optimized for MBIR, patient radiation dose can be much lower (~ 80 to 90% lower)

MBIR FBPASIR

100 kVp, 25-38 mA, 0.4 sec

Images courtesy of Pr Maher, Cork University Hospital, Ireland 510(k) pending at FDA Not available for sale in the U.S.

Low Dose abdomen + pelvis - 0.6 mSv

39 cm