Diffusion Physics
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Transcript of Diffusion Physics
Diffusion Physics
H2O in the body is always in random motion due to thermal agitation
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
- Diffusion Coefficient is dependent on Temperature and Viscosity of Tissue
Diffusion Coefficient (rate of motion)
Temperature
Size of MoleculeViscosity
Diffusion Physics
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
-The “rate” of water motion is determine by a diffusion coefficient, “D”.
-Mean displacement of water molecules is related to “D” by Einstein’s equation:
TimeDiffusionCoefficient
MeanDisplacement
Diffusion Physics
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Detecting Diffusion with MRI - Intravoxel Incoherent Motion
Ellingson et al., Concepts in MR, 2008
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010 From: Ellingson, Concepts in MR, 2008
Detecting Diffusion with MRI - Intravoxel Incoherent Motion
Detected DWI Signal
MRI Signal w/o Diffusion Sensitivity
Variability inPhase of “Tagged” H2O Level of Diffusion Weighting
Diffusion Coefficient
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Intravoxel Incoherent vs Coherent Motion
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Diffusion Effects (Incoherent)
Flow Effects (Coherent) -- Phase Contrast (PC)-MRI
d = /3 radians Velocity
Proton on H2O
Image Voxel = t1
= t2
= t3
MR
I Sig
nal
Diffusion Time (or level of diffusion weighting)
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Factors that affect diffusion coefficient, D
Diffusion Time, t -Physical time between gradients used to “tag” H2O
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Factors that affect diffusion coefficient, D
Size of Compartment(s)- If we set a limit for r, then we observe an apparent diffusion coefficient, ADC
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Physical Compartment Size
Expected Compartment Size
Factors that affect diffusion coefficient, D
Tortuosity of the Compartments- More tortuous paths look like slow diffusion
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Tortuosity
Actual Path
Expected Path
Factors that affect diffusion coefficient, D
Viscosity and Temperature
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Diffusion Coefficient (rate of motion) Temperature
Viscosity
Pure H2O CSF Infection (WBCs) Lymphoma
Factors that affect diffusion coefficient, D
-Diffusion Time, t -Physical time between gradients used to “tag” H2O
-Size of Compartment(s)- If we set a limit for r, then we observe an apparent diffusion coefficient, ADC
-Tortuosity of the Compartments- More tortuous paths look like slow diffusion
-Temperature
-Viscosity
*** We can only measure “ADC” because of all the factors that change “D”! ***
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Steps in Performing DWI
• DWI (isotropic):– Collect a DWI (b = 1000 or 500 s/mm2) dataset by applying motion probing gradients
in the x, y, and z-direction.• Make sure TE is low and TR is long to increase SNR• For higher resolution scans, use a lower b-value
– Collect a T2w dataset (b = 0 s/mm2)
– Collect a low b-value, flow nulled dataset (b = 50 s/mm2)
– Average DWIs from 3 directions
– Calculate ADC
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
For b-values < 1000
DWI vs. ADC
• DWI– Images collected during application of a “diffusion sensitizing gradient”
– Contains T1, T2, and ADC effects
– “Restricted diffusion”, long T2, and short T1 all influence DWIs
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
b = 1000
From: Taouli, Radiology, 2010
b = 750
DWI vs. ADC
• DWI– Influence of T2 in DWIs is known as “T2 shine through”
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
From: Taouli, Radiology, 2010
b = 500 ADC Map
DWI vs. ADC
• ADC– Quantitative
– Calculated from DWI and T2w (b = 0 or low b-value)
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
From: Taouli, Radiology, 2010
DWI vs. ADC
• ADC– Reflects diffusion magnitude
– Eliminates long T2 and short T1 effects
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
From: Taouli, Radiology, 2010
DWI vs. ADC
• DWI– Influence of T2 in DWIs is known as “T2 shine through”
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
From: Taouli, Radiology, 2010
b = 500 ADC Map
Diffusion Tensor Imaging (DTI)
Isotropic Diffusion Anisotropic Diffusion
From: Ellingson, Concepts in MR, 2008
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Diffusion Tensor Imaging (DTI)
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Directional Encoding
6 directions (min) 15 directions
25 directions 41 directions
Diffusion Tensor Imaging (DTI)
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
The Diffusion Tensor:
Diffusion Tensor Imaging (DTI)
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Isotropic Diffusion
1 = 2 = 3
Anisotropic Diffusion
1 > 2, 3
From Ellingson, Concepts in MR, 2008
Fractional Anisotropy (FA)
Isotropic Anisotropic
FA = 0 FA = 1
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
DTI Tractography
• In the CNS and MSK, lADC is parallel to axon orientation
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
DTI Tractography
• In the CNS, lADC is parallel to axon orientation
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
DTI Tractography
• In the MSK, lADC is parallel to muscle fiber orientation
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From: University of Rochester
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
DTI Tractography
• In the heart, lADC is also parallel to muscle fiber orientation
From: University of Oxford
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
QuickTime™ and a decompressor
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QuickTime™ and a decompressor
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From: Eindhoven University of Technology
DTI Tractography
• Spinal Cord Injury
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
From Ellingson, Neurosurgery:Spine, 2010
Steps in Performing DTI
• DTI (6 directions):– Collect a DWI (b = 1000 or 500 s/mm2) along 6 non-collinear directions
– Collect a T2w dataset (b = 0 s/mm2)
– Calculate Diffusion Tensor:
• Calculate D in 6 different directions
• Set up the encoding matrix
• Define Tensors
• Solve Tensor Equations
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Applications of Diffusion MRIin the Abdomen
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
• Common pulse sequences– Single-shot spin-echo (SE) echoplanar with fat saturation
• With breathhold (20-30 seconds; sensitivity for lesion detection = 84.3%Parikh, 2008)– Need thicker slices (8-10 mm) for good SNR and good liver coverage
• Resp. gated (3-6 min; sensitivity for lesion detection = 93.7%Parikh, 2008)– Thinner slices can be used (5 mm)
– Better image quality, SNR and ADC quantificationTaouli, 2009
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
• Common pulse sequences– Single-shot spin-echo (SE) echoplanar with fat saturation
• With breathhold (20-30 seconds; sensitivity for lesion detection = 84.3%Parikh, 2008)– Need thicker slices (8-10 mm) for good SNR and good liver coverage
• Resp. gated (3-6 min; sensitivity for lesion detection = 93.7%Parikh, 2008)– Thinner slices can be used (5 mm)
– Better image quality, SNR and ADC quantificationTaouli, 2009
• Common b-values– b = 0 image (no diffusion weighting…essentially a “poor man’s” T2w image)
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
• Common pulse sequences– Single-shot spin-echo (SE) echoplanar with fat saturation
• With breathhold (20-30 seconds; sensitivity for lesion detection = 84.3%Parikh, 2008)– Need thicker slices (8-10 mm) for good SNR and good liver coverage
• Resp. gated (3-6 min; sensitivity for lesion detection = 93.7%Parikh, 2008)– Thinner slices can be used (5 mm)
– Better image quality, SNR and ADC quantificationTaouli, 2009
• Common b-values– b = 0 image (baseline with no diffusion weighting…essentially a “poor man’s” T2w
image
– Low b-value (b < 150 s/mm2) “Flow Nulled”• Nulls the intrahepatic vascular signal
• Allows for better detection of focal liver lesions(van den Bos, 2008; Parikh, 2008; Okada, 1998; Hussain, 2005)
~2% change in ADC
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
• Common pulse sequences– Single-shot spin-echo (SE) echoplanar with fat saturation
• With breathhold (20-30 seconds; sensitivity for lesion detection = 84.3%Parikh, 2008)– Need thicker slices (8-10 mm) for good SNR and good liver coverage
• Resp. gated (3-6 min; sensitivity for lesion detection = 93.7%Parikh, 2008)– Thinner slices can be used (5 mm)
– Better image quality, SNR and ADC quantificationTaouli, 2009
• Common b-values– b = 0 image (baseline with no diffusion weighting…essentially a “poor man’s” T2w
image
– Low b-value (b < 150 s/mm2) “Flow Nulled”• Nulls the intrahepatic vascular signal
• Allows for better detection of focal liver lesions(van den Bos, 2008; Parikh, 2008; Okada, 1998; Hussain, 2005)
– High b-value (500 < b < 1000 s/mm2)• Useful for focal liver lesion characterization (Taouli, 2003; Kim, 1999)
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
• Visual liver lesion characterization with DWI
From: Taouli, Radiology, 2010
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
• Visual liver lesion characterization with DWI
From: Taouli, Radiology, 2010
b = 500 ADC
From: Xu, J Comput Assist Tomogr, 2010
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
• Visual liver lesion characterization with DWI
From: Xu, J Comput Assist Tomogr, 2010
- DWI has higher specificity than CE
Diffusion MR Imagingof the Liver
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
From: Colagrande, Radiol Med, 2006
Cirrhotic Liver -- ADCLiver Cysts -- ADCAngioma -- ADCFNH -- ADCHepatocarcimoma -- ADC (mixed results)Metastasis -- ADC (mixed results)
Diffusion MR Imagingof the Kidneys
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Directionality -- DTI?
Diffusion MR Imagingof the Kidneys
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Directionality -- DTI?
From: Kido, Acta Radiol, 2010
Fractional Anisotropy (DTI)
Diffusion MR Imagingof the Kidneys
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
From: Colagrande, Radiol Med, 2006
b = 0
b = 500
ADC
From: Kido, Acta Radiol, 2010
From: Kilickesmez, J Comput Assist Tomogr, 2009
Diffusion MR Imagingof the Kidneys
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Renal Fibrosis
Animal Model -- From: Togao, Radiology, 2010
Diffusion MR Imagingof the Kidneys
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
Renal Cell Carcinoma
From: Kilickesmez, J Comput Assist Tomogr, 2009
Contrast-Enhanced T1 b = 1000 ADC
Diffusion MR Imagingof the Kidneys
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010
From: Kilickesmez, J Comput Assist Tomogr, 2009
Summary• Diffusion MRI is an MR technique that can quantify the magnitude of H20
diffusivity within tissues– Microstructural information
• ADC calculated from diffusion MR images is influenced by: – Cellularity– Tissue viscosity and temperature– Diffusion Time– Compartment Size (cell size and shape)– Tortuosity of environment
• DTI is useful for directionality of diffusion restrictions– CNS, MSK, Kidney
• DWI/ADC maps can be used to characterize many pathologies of the abdomen– Liver and Kidney pathologies are most common abdominal diffusion MR studies
B.M. Ellingson, Ph.D., Dept. of Radiological Sciences, David Geffen School of Medicine at UCLA, 2010