dual energy ct: approaches and...
TRANSCRIPT
Dual Energy CT:Approaches and Applications
Dianna Cody, Ph.D.
Professor
Department of Imaging Physics
Objectives• How does it work?
– Approach by vendor
• What is it good for?– Established applications
– Emerging applications
Dual Energy CT• Recycled idea from 1980’s
• Bone QCT Densitometry
• Acquisition of attenuation data at two distinct energy levels
• Produce two sets of images
• Data processing for basis materials
Material DecompositionInterleaved High- and
Low-kVp ProjectionsData Acquisition
Low kVp Projections High kVp Projections
80 kVp 140 kVp
split
Filtered Back Projection
40keV –140keV
....)()()()()()()()()( 1
2
1
2
1111 iPiPiPiPiPiiPiiP highlowhighlowhighlow
....)()()()()()()()()( 2
2
2
2
2222 iPiPiPiPiPiiPiiP highlowhighlowhighlow
water
iodine
FBP
Attenuation-to-material density transformation
Monochromatic Generation
Iodine Density Water Density
70 keV
Iodine Projections Water Projections
Blended Images
How to acquire data at two energies? Varies by vendor
• GE – FAST kVp switching
• Philips – Two layer detector
• Siemens – Two x-ray tubes
• Toshiba – Two sequential rotations
Important Issues
• Temporal match of data acquired at separate kVs
• Adjust mA for kV? (Increase for low kVp, decrease for high kVp)
• Field of View limitations?
• Dose impact?
• Post-processing options
GE Approach
• Newest scanner model only (HD750)
• Fast kVp switching: 140 kV & 80 kV
• Full field of view
• 1/3 projections obtained at 140 kV
• 2/3 projections obtained at 80 kV
• Excellent temporal matching
• “Tissue Signature” function
GE Approach - Dose• Measured on HD750 June 2009
• 120kVp, 300mAs, 40mm beam– CTDIw 13.7 mGy
• 80 & 140 kVp, 600mAs, 40mm beam– CTDIw 31 mGy
• DE dose about 13% higher than 120kVp predicted by mAs change alone
Dose Penalty?• YES – difficult to obtain dual energy CT
images at same dose as single average 120kVp scan
• BUT – if standard practice is C-/C+ scans, a single pass dual energy CT scan may provide virtual non-contrast images
• Small dose increase may justify the additional information obtained
Philips Approach• Two-layer detector
• No extra x-ray tube technology
• Excellent temporal matching
• Full field of view
• Dose?
Siemens Approach
• Available on Definition
• Field of view limited (depends on model)
• Two x-ray tubes operated simultaneously at different kVps
• Adjust mA value independently
• Temporal matching somewhat limited (90° out of sync)
• Cross-scatter
• Dose?
Toshiba Approach• Dual rotations
• One at low kVp, next at high kVp
• Then move forward
• Limited to axial mode?
• Adjust mA for kVp?
• Temporal matching worst-case
• Full Field of view
• Dose?
Established Applications
Kidney Stone Characterization
Automated Bone Removal (Angiography)
Potential Applications
Liver & Kidney Lesion Characterization
75 keV - With Contrast
Images Courtesy of Dr. Amy Hara, Mayo Clinic, Scottsdale, AZ
Water Density(virtual non-contrast)
Iodine Density
Liver Fat Quantification
Virtual Non-Contrast Images
Calcified Plaque Processing
Pulmonary Embolism Detection
Metal Artifact & Beam Hardening Reduction
80 kVp 80 keV
Metal Artifact Reduction
140 kVp 70 keV w/MAR
80kVp
140kVp
80keV
More?• Improve detection of small pancreatic
lesions
• Discriminate between mucinous and nonmucinous pancreatic cystic lesions
• Characterize solid lesions (tumor vsfibrosis)
• Proton dose estimation in spine (fatty replacement of bone marrow)
Tissue Signature
70 keV (mono)
m1 (water) m2 (iodine)
Tissue Signature
• Optimize imaging parameters for new targeted tracers?
• Useful for discriminating solid lesions?
• Other applications yet to emerge?
Summary
• Vendors have diverged in approach to dual energy CT
• Advantages & Disadvatages
• Established applications
• MANY emerging applications…!
