Recent developments in Magnetic Resonance Imaging

Acknowledgements

• Jeffrey Bezanson• Thanh Nguyen, PhD• Martin Prince, PhD, MD• Richard Watts, PhD• Yihong Yang, PhD

NIH R01HL60879, R01HL62994American Heart AssociationWhitaker Foundation

Medical imaging methods

Projection x-ray

CT

PET, etcUS MRI

Spatial encoding

xy-matrix detectors

-array detectors

-rotations

1-n detectors

line scan; y=2c/t

1-n detectors

k-space sampling

Contrast attenuation attenuation reflection,T1,T2,T2*,

flow,perfusion, etc

The richness of both encoding and contrast mechanisms in MRI nourishes the advances realized and the advances still in promise.

Magnetic Resonance

dm/dt = - (B0z + B1 + B) m spin – B field

- (mxx + myy) / T2 spin – spin

- (m0- mz)z / T1 spin – lattice

Bloch Equation Interactions

Spatial encoding – RF readout (frequency encoding)

m(x)

Spatial-varying resonance frequency during RF detection

S(t) = m(x)eikxxdx = S(kx), m(x) = FT{S(kx)}

S(t) ~ eit

S(t) ~ m(x)eiGxxtdx

kx = Gxtx

B = B0 + Gxx

RF volume excitation –slice selection

Spatial-varying resonance frequency during RF excitation

z

B1 freq band = 0 + Gzz

m = mx+imy ~ b1(t)e-iGzztdt = B1(Gzz)

Excited location

Slice profile

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) eikzzeikyyeiGxxtdxdydz

Major advances in MRI

• fMRI – pre-surgical mapping, neuroscience • MRA – replacing x-ray angiography for

diagnosis• Cardiac MRI – one-stop shop comprehensive

cardiac study, potentially revolutionizing cardiac medicine

By both clinical & scientific measures

fMRI

• BOLD mechanism – blood susceptibility (T2*) and brain activation

• Presurgical mapping

• Neuroscience?

T2* - intravoxel dephasing

time

900 RF

MR signal e-t/T2

e-t/T2*

Oxyhemoglobin and deoxyhemoglobin in veins

Oxyhemoglobin (diamagnetic) Deoxyhemoglobin (paramagnetic)

Rest Activation

Normal blood flow High blood flow

homogeneousless intravoxel dephasing

heterogeneousintravoxel dephasing

BOLD

Brain activity

Oxygen consumption Cerebral blood flow

OxyhemoglobinDeoxyhemoglobin

Magnetic susceptibility

T2*

MRI signal

Fundamental limitation of fMRI

2

impulse stimulus

Sig

nal c

hang

e (%

)

Time (sec)4 8 10 12

1

0

fMRI signal comes from hemodynamic response, a delayed convoluted effects associated with neuronal activities.

Presurgical mapping - AVM

displacement of visual function in left hemisphere with AVMBryan Mock

Presurgical mapping - tumor

Radiology. 1999;210:529-538

Displacement of motor cortex by a tumor

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

5 10 15 20 25

# of RFs (TR=10 ms)

.2

.4

.6

.8

1

sign

al T1=50ms

T1=1000ms

Gd enhanced blood

background

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

RF coil

station 2

Isocenter

station 1

station 3

20

25

30

35

40

Tim

e (se

c)

Arterial [Gd]

Bolus Chase MRA

Dynamic 2D MRDSA

FFT

time

mask

MRDSA of the Foot

Dyn

amic

2D

+ B

olus

Cha

se 3

D

over

com

e ve

nous

sig

nal

3D 2D

Real-time MRA

Accelerate Bolus Chase Acquisition

double sampling spacing kz

keep same kzm & excitation volumereduce scan time by half & no resolution loss

Reduced k-space sampling:

Bolus Chase with Reduced kz-sampling

Multiple channel parallel imaging

Coil 2

Coil 1

ideal actual

x

y

Coil sensitivity map

x

y

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-i2kycn(y)f(y) = dk'•Cn(k-k')F(k')

F = (CH-1C)-1CH-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

• vessel wall – plaque (~0.2mm)• vessel lumen – stenoses, flow reserve (~0.5-1mm)• myocardium – perfusion, mechanics, metabolism• valves and chambers – function

Noninvasive imaging of coronary artery disease is the holy grail of medical imaging.

Coronary MRA is recommended as the No.1 area of emphasis for NIH…...Radiology 1998; 208:573-576

Vessel wall imaging?

Fayad ZA; Fuster V et al. Circ. 2000;102;506-510

no fat sat fat sat

???

Coronary MRA

Radiology. 2000;217:270-277

Coronary MRA – image contrast

No Fat Sat Fat SatRCA

Coronary MRA – Gd-enhanced acquistion

Gd enhanced no Gd

Coronary MRA – motion problem

ReadoutECGMagnPrep

Cardiac motion – not available from ECG waveform

optimal delay?

rest period?

Respiratory motion: breath-holding? navigator gating?

Motion suppression techniques

• Motion detection – pencil beam navigator, volumetric navigator, …

• Motion suppression – gating, correction, view ordering, …

The navigator approach is to measure motion and modify data acquisition accordingly.

An intelligent real-time navigator system

algorithms navigator k-space

Pencil beam navigator – 2D selective excitation

Gx

Gy

Gz

RF

Bloch Equation Solution:

M(x) =iM0(x)kW(k)S(k)eix·kdk -300 -200 -100 0 100 200 300-300

-200

-100

0

100

200

300

k y

kx

0 64 128 192 2560

20

40

60

80

100

120pw_rfn = 5ms nav_size = 5

nav_size = 10 nav_size = 20

Magni

tude (a

rb.)

Position (FOV=24cm)

Real-time navigator gating

accept

time

lung

diaphragm

Navigator gated CMRA

no gating gating

Volumetric navigator

Optimal ECG trigger delay

500

550

600

650

700

750

800

700 800 900 1000 1100 1200

Cardiac cycle (ms)

Opt

imal

del

ay (

ms)

delay_image

delay_navigator

delay_weissler

delay_stuber

Optimal ECG trigger delay – minimize cardiac motion effects

450 msec 550 650

750 850 950

Coronary motion

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)

Coronary motion is dominantly (>91%) consisting of global motion (translation and rotation) and a LV-centered dilation. Invest Radiol 19:499-509, 1984

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

0

5

10

15

20

25

30

kr(1

/cm

)

Acquisition window

View ordering (VO)

std centric VO motion-matched VO

• motion caused PSF size ~ 0.5 mm

• voxel CNR ~ 4 (Rose Model)

• acquisition voxel size ~ 0.5 mm

Thesis to develop CMRA – 3 criteria

Respiratory motion (mm)

Resp: gateSI,correctCard: trigger

Resp: gate,correct,vorderCard: gate,correct,vorder

Resp: breathholdCard: trigger,vorder

0 0.5 1 1.5

0

0

.5

1

1.

5C

ardi

ac m

otio

n (m

m) ×

+

+×

Voxel size (mm3)0 1 2 3 4 5

01

23

4

CN

R+

+

+

*2T/TExscan eNTqVBW/1RcKCNR

Better contrast agentMore effective sampling

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?