Download Recent development in MRI

Recent developments in
Magnetic Resonance Imaging
Acknowledgements
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Jeffrey Bezanson
Thanh Nguyen, PhD
Martin Prince, PhD, MD
Richard Watts, PhD
Yihong Yang, PhD
NIH R01HL60879, R01HL62994
American Heart Association
Whitaker Foundation
Medical imaging methods
Projection
x-ray
Spatial
xy-matrix
encoding detectors
Contrast
CT
PET, etc
US
MRI
r-array detectors
q-rotations
1-n detectors
line scan; y=2c/t
1-n detectors
k-space sampling
reflection
r,T1,T2,T2*,
flow,perfusion, etc
attenuation attenuation
The richness of both encoding and contrast mechanisms in
MRI nourishes the advances realized and the advances
still in promise.
Magnetic Resonance
Bloch Equation
Interactions
dm/dt = - g(B0z + B1 + DB)  m spin – B field
- (mxx + myy) / T2
spin – spin
- (m0- mz)z / T1
spin – lattice
Spatial encoding –
RF readout (frequency encoding)
Spatial-varying resonance frequency during RF detection
B = B 0 + G xx
S(t) ~ eigBt
S(t) ~ m(x)eigGxxtdx
m(x)
x
S(t) = m(x)eikxxdx = S(kx),
kx = gGxt
m(x) = FT{S(kx)}
RF volume excitation –
slice selection
Spatial-varying resonance frequency during RF excitation
w
w = w0 + gGzz
B1 freq band
z
Excited location
Slice profile
m = mx+imy ~ g b1(t)e-igGzztdt =
B1(gGzz)
Phase encoding
After volume excitation & before readout (kx), apply
y-gradient pulse that makes spin phase varying
linearly in y (=kyy), apply z-gradient pulse that
makes spin phase varying linearly in z (=kzz).
Repeat RF excitation and detection with different
gradient areas (ky, kz).
S(kz, ky, t) =  m(x,y,z) eikzzeikyyeigGxxtdxdydz
Major advances in MRI
By both clinical & scientific measures
• fMRI – pre-surgical mapping, neuroscience
• MRA – replacing x-ray angiography for
diagnosis
• Cardiac MRI – one-stop shop comprehensive
cardiac study, potentially revolutionizing
cardiac medicine
fMRI
• BOLD mechanism – blood susceptibility
(T2*) and brain activation
• Presurgical mapping
• Neuroscience?
T2* - intravoxel dephasing
MR signal
e-t/T2
e-t/T2*
time
900 RF
Oxyhemoglobin and deoxyhemoglobin in veins
Oxyhemoglobin (diamagnetic)
Deoxyhemoglobin (paramagnetic)
Rest
Activation
Normal blood flow
High blood flow
heterogeneous
intravoxel dephasing
homogeneous
less intravoxel dephasing
BOLD
Brain activity
Oxygen consumption
Cerebral blood flow
Oxyhemoglobin
Deoxyhemoglobin
Magnetic susceptibility
T2*
MRI signal
Fundamental limitation of fMRI
Signal change (%)
fMRI signal comes from hemodynamic response, a
delayed convoluted effects associated with neuronal
activities.
1
0
2
4
impulse stimulus
8
10
12
Time (sec)
Presurgical mapping - AVM
displacement of visual function in left hemisphere with AVM
Bryan Mock
Presurgical mapping - tumor
Displacement of
motor cortex by
a tumor
Radiology.
1999;210:529-538
MRA
• Contrast enhanced MRA, overcoming
intravoxel dephasing problem associated
with time-of-flight or phase-contrast MRA.
• Clinical impact: achieving an accuracy
close to x-ray angiography (XRA),
replacing XRA in many diagnosis.
Contrast enhanced MRA
1
signal
.8
T1=50ms
.6
Gd enhanced blood
.4
.2
T1=1000ms
5
10
15
20
background
25
# of RFs (TR=10 ms)
Contrast enhanced 3D MRA
Carotid angiography
TOF MRA
CE MRA
X-ray Angio
Radiology 1999 211: 265-273
CEMRA vs X-ray angiogrpahy
Radiology. 1999;210:683-688
Bolus Chase Acquisition
20
Isocenter
25
station 1
30
station 2
35
Time (sec)
RF coil
40
station 3
Arterial [Gd]
Bolus Chase MRA
Dynamic 2D MRDSA
mask
FFT
MRDSA of the Foot
3D
2D
Dynamic 2D + Bolus Chase 3D
overcome venous signal
Real-time MRA
Accelerate Bolus Chase Acquisition
Reduced k-space sampling:
double sampling spacing Dkz
keep same kzm & excitation volume
reduce scan time by half & no resolution loss
Bolus Chase with Reduced kz-sampling
Multiple channel parallel imaging
Coil sensitivity map
y
y
Coil 1
Coil 2
x
x
ideal
actual
Parallel imaging (SENSE/SMASH)
s1,y = c1,yfy + c1,y+FOV/2fy+FOV/2;
s2,y = c2,yfy + c2,y+FOV/2fy+FOV/2. (wrapping around artifacts)
fy = (c2,y+FOV/2 s1,y – c1,y+FOV/2 s2,y) / (c2,y+FOV/2 c1,y – c1,y+FOV/2 c2,y);
fy+FOV/2 = (c1,y s2,y - c2,y s1,y) / (c2,y+FOV/2 c1,y – c1,y+FOV/2 c2,y).
(recovered image)
Sn(k) =  dy•e-i2kycn(y)f(y) =  dk'•Cn(k-k')F(k')
F = (CHY-1C)-1CHY-1S, (least squares fitting).
Example
Individual coil
image (S1)
reconstructed from
individual coil ( f )
acquired with full
k-space data
Pruessman, et al MRM 42:952-962, 1999
Parallel imaging – increase speed
Sodickson, et al, Radiology. 217(1):284-9, 2000
Cardiac MRI
Noninvasive imaging of coronary artery
disease is the holy grail of medical imaging.
•
•
•
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vessel wall – plaque (~0.2mm)
vessel lumen – stenoses, flow reserve (~0.5-1mm)
myocardium – perfusion, mechanics, metabolism
valves and chambers – function
Coronary MRA is recommended as the No.1 area of
emphasis for NIH…...Radiology 1998; 208:573-576
Vessel wall imaging?
???
no fat sat
fat sat
Fayad ZA; Fuster V et al. Circ. 2000;102;506-510
Coronary MRA
Radiology. 2000;217:270-277
Coronary MRA – image contrast
RCA
No Fat Sat
Fat Sat
Coronary MRA – Gd-enhanced acquistion
Gd enhanced
no
Gd
Coronary MRA – motion problem
Cardiac motion – not available from ECG waveform
ECG
Readout
Magn
Prep
optimal delay?
rest period?
Respiratory motion:
breath-holding? navigator gating?
Motion suppression techniques
The navigator approach is to measure motion
and modify data acquisition accordingly.
• Motion detection – pencil beam navigator,
volumetric navigator, …
• Motion suppression – gating, correction, view
ordering, …
An intelligent real-time navigator system
algorithms
navigator
k-space
Pencil beam navigator – 2D selective excitation
300
200
ky
100
Bloch Equation Solution:
-100
-200
-300
-300 -200 -100
120
0
kx
100
200
300
pw_rfn = 5ms
nav_size = 5
nav_size = 10
nav_size = 20
100
80
Magnitude (arb.)
M(x) =igM0(x)k
W(k)S(k)eix·kdk
0
60
40
20
0
0
64
128
Position (FOV=24cm)
RF
Gx
Gy
Gz
192
256
Real-time navigator gating
lung
accept
diaphragm
time
Navigator gated CMRA
no gating
gating
Volumetric navigator
Optimal ECG trigger delay
Optimal delay (ms)
800
delay_image
delay_navigator
delay_weissler
delay_stuber
750
700
650
600
550
500
700
800
900
1000
Cardiac cycle (ms)
1100
1200
Optimal ECG trigger delay –
minimize cardiac motion effects
450 msec
550
650
750
850
950
Coronary motion
Coronary motion is dominantly (>91%) consisting of
global motion (translation and rotation) and a LVcentered dilation.
Invest Radiol 19:499-509, 1984
Linear translation:
FT{f(x-t)} = eikt F(k)
Rotation:
FT{f(Rx)} = F(Rk)
Dilation:
FT{af(a(z-z0)+z0)} = eikz0(1-1/a) F(k/a)
Adaptive motion correction (AMC)
gating at 8 mm
gating at 8 mm
+ AMC (0.4)
gating at 3 mm
Minimizing cardiac motion effects motion matched view ordering
30
kr(1/cm)
25
20
15
10
5
0
Acquisition window
View ordering (VO)
std centric VO
motion-matched VO
Thesis to develop CMRA – 3 criteria
• motion caused PSF size ~ 0.5 mm
• voxel CNR ~ 4 (Rose Model)
• acquisition voxel size ~ 0.5 mm
1.5
0
Cardiac motion (mm)
1
0.5
+
×
+
×
0
0.5
1
1.5
Respiratory motion (mm)
3
+
Better contrast agent
More effective sampling
2
CNR
4
+
0
1
+
0
1
2
3
4
5
Voxel size (mm3)
CNR  K c  R1 / BW  DV  q  Tscan N x  e TE / T 2*
Future coronary MRA
Integrated navigator contrast-enhanced 3D:
• Reduce cardiac and respiratory motion effects
using intelligent navigator echoes.
• Boost SNR/resolution using intravascular
contrast agents.
Summary
• fMRI – oxyhemoglobin , T2*, signal
• MRA – Gd enhanced, T1 ; bolus chase;
MRDSA; accelerated sampling; parallel
imaging; real-time imaging
• Cardiac MRI – navigator methods to reduce
cardiac and respiratory motion
Future of MRI
• Higher resolution anatomy
• Functional information
• Molecular imaging?