Nuclear Magnetic Resonance

1
Nuclear Magnetic Resonance

• Phillip W Patton, Ph.D.

2
Take Away 5 Things You Should Be Able to Explain
After the NMR Lectures

• The magnetic characteristics of the nucleus and
  the magnetic properties of matter
• How the NMR signal is generated and detected

• T1 and T2 relaxation how they arise and how they
  are measured
• Pulse sequence methods used and pulse sequence
  timing (e.g., TR and TE) and inherent NMR
  parameters (e.g., T1 and T2) give rise to tissue
  contrast
• How a 1D gradient can be used to provide an echo
  and allow for quick imaging with shallow flip
  angle sequences

3
2003 Nobel Prize for Medicine- MRI

• Laterbur and Mansfield (2003, medicine)
  discoveries concerning magnetic resonance imaging
  (MRI)
• Rabi (1944, physics) nuclear magnetic resonance
  (NMR) methodology
• Bloch and Purcell (1952, physics) NMR precision
  measurements
• Ernst (1991, chemistry) high-resolution NMR
  spectroscopy

4
Nuclear Magnetic Resonance

• NMR the study of the magnetic properties of the
  nucleus
• Magnetic field associated with nuclear spin/chg.
  distr.
• Not an imaging technique provides spectroscopic
  data
• High contrast sensitivity to soft tissue
  differences
• Does not use ionizing radiation (radio waves)

• Magnetic Resonance Imaging magnetic gradients
  and mathematical reconstruction algorithms
  produce the N-dimensional image from NMR
  free-induction decay data

• Important to understand the underlying principles
  of NMR in order to transfer this knowledge to MRI

5
Image Contrast What does it depend on?

• Radiation needs to interact with the bodys
  tissues in some differential manner to provide
  contrast
• X-ray/CT differences in e- density (e-/cm3 r
  e-/g)
• Ultrasound differences in acoustic impedance (Z
  rc)
• Nuclear Medicine differences in tracer
  concentration (r)
• MRI many intrinsic and extrinsic factors affect
  contrast
• intrinsic rH,T1, T2, flow, perfusion, diffusion,
  ...
• extrinsic TR, TE, TI, flip angle, ...

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 257.
6
Magnetism and the Magnetic Properties of Matter

• Mag. field generated by moving charges (e- or
  quarks)
• Most materials do not exhibit overt magnetic
  properties
• Exception permanent magnet
• Magnetic susceptibility extent to which a
  material becomes magnetized when placed in a
  magnetic field
• Three categories of magnetic susceptibility
• Diamagnetic opposing applied field
• Ca, H2O, most organic materials (C and H)
• Paramagnetic enhancing field, no self-magnetism
  
• O2, deoxyhemoglobin and Gd-based contrast agents
• Ferromagnetic superparamagnetic, greatly
  enhancing field
• Exhibits self-magnetism Fe, Co and Ni

7
Magnetism and the Magnetic Properties of Matter

• Magnetic fields arise from magnetic dipoles (N/S)
  
• N side the origin of magnetic field lines
  (arbitrary)
• Attraction (N-S) and repulsion (N-N S-S)
• Magnetic field strength (flux density) B
• Measured in tesla (T) and gauss (G) 1 T 10,000
  G
• Earth magnetic field 1/20,000 T or 0.5 G
• Magnetic fields arise from
• Permanent magnets
• Current through a wire or solenoid (current
  amplitude sets B magnitude)

8
Magnetism and the Magnetic Properties of Matter
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 374 and 377.
9
Magnetic Characteristics of the Nucleus

• Magnetic properties of nuclei are determined by
  the spin and charge distribution (quarks) of the
  nucleons (p and n)
• The magnetic moment (m) describes the nuclear B
  field magnitude
• Pairing of p-p or n-n causes m to cancel out
• So if the number of protons and neutrons is even
  ? no/little m
• If N even and P odd or P even and N odd ?
  resultant m (NMR eff.)

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 375.
10
Nuclear Magnetic Characteristics of the Elements

• Biologically relevant elements that are
  candidates for NMR/MRI
• Magnitude of m
• Physiologic concentration
• Isotopic abundance
• Relative sensitivity
• 1H (p) provides 104-106 times the signal as 23Na
  or 31P

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 376.
11
Nuclear Magnetic Characteristics of the Elements

• Spinning p considered classically as a bar
  magnet
• Thermal energy randomizes direction of m ? no net
  magnetization
• Application of an external magnetic field (B0) ?
  two energy states
• Lower energy m parallel w/ B0 and higher energy m
  anti-parallel w/ B0
• Number of excess m _at_ 1.0T and 310 K ? 3 ppm
  (very small effect)
• For typical voxel in MRI 1021 p ? 3x1015 more m
  in lower state

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 377.
c.f. http//www.hull.ac.uk/mri /lectures/gpl_page
.html
12
Larmor Frequency

• Classically a torque on m by B0 causes
  precession
• Precession occurs at an angular frequency
  (rotations/sec or radians/sec)
• Larmor equation w0(radians/sec) gB0
  f0(rotations/sec or Hz) (g/2p)B0
• g/2p gyromagnetic ratio (MHz/T) unique to each
  element
• Choice of freq. ? the resonance phen. to be
  tuned to a specific element
• For 1H _at_ 1.5T 64 MHz (Channel 3)

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 379.
c.f. Hendee, et al. Medical Imaging Physics, 4th
ed., p. 357.
13
Larmor Frequency US VHF Broadcast Spectrum
c.f. http//www.rentcom.com/wpapers/ telex/telex3.
html
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.18.
14
Nuclear Magnetic Characteristics of the Elements

• At equilibrium, no B field ? B0 (all along
  z-axis) Random distribution of m in x-y plane
  averages out Bxy 0 Small mz add up to
  measurable M0 (equilibrium magnetization)
  Absorbed radiofrequency EM radiation ? low-E to
  high-E High-E nuclei lose energy to environment
  return to equilibrium state and Mz (longitudinal
  magnetization) ? M0

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 378.
c.f. http//www.hull.ac.uk/mri /lectures/gpl_page
.html
15
Geometric Orientation

• Two frames of reference used
• Laboratory frame stationary reference from
  observers POV
• Rotating frame angular frequency equal to the
  Larmor precessional frequency
• Both frames are useful in explaining various
  interactions
• Mxy transverse magnetization, ? B0 (at
  equilibrium 0)
• When RF applied, Mz tipped into the x-y
  (transverse) plane

Rotating Frame
Lab Frame
Rotating Frame
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 380-381.
16
Resonance and Excitation

• Return to equilibrium results in RF emission from
  m with
• Amplitude proportional the number of excited
  nuclei (spin r)
• Rate depends on the characteristics of the sample
  (T1 and T2)
• Excitation, detection analysis the basics for
  NMR/MRI
• Resonance occurs when applied RF magnetic field
  (B1) is precisely matched in frequency to that of
  the nuclei
• Absorption of RF energy promotes low-E m ? high-E
  m
• Amplitude and duration of RF pulse determines the
  number of nuclei that undergo the energy
  transition (q)
• Continued RF application induces a return to
  equilibrium

17
Resonance and Excitation
RF Pulse Angle Tip 0 90 180
Higher energy state
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 382.
18
Resonance and Excitation

• B1 field component rotating at Larmor f0
  (off-freq. ? little effect)
• Rotating reference frame B1 stationary in x-y
  plane
• B1 applied torque to Mz ? rotation q g B1 t
  
• Common angles 90 (p/2 radians p/2 pulse) and
  180 (p radians)

• Flip angle (q) describes the rotation through
  which the longitudinal magnetization (Mz) is
  displaced to generate transverse magnetization
  (Mxy)

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 384.
19
Resonance and Excitation

• Time required 10-100 msec
• 90 pulse ? largest Mxy (signal) generated
• For flip angle (q) lt 90
• smaller Mxy component generated and less signal
• less time necessary to displace Mz
• greater amount of Mxy (signal) per excitation
  time
• Low flip angle (q) very important in rapid MRI
  scanning

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 384.
20
Free Induction Decay T2 and T2 Relaxation

• 90 pulse produces phase coherence of nuclei

• As Mxy rotates at f0 of the receiver coil (lab
  frame) through magnetic induction (dB/dt)
  produces a damped sinusoidal electronic signal
  free induction decay (FID)

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 385.
21
Free Induction Decay T2 and T2 Relaxation

• Decay of the FID envelope due to loss of phase
  coherence of the individual spins due to
  intrinsic micro magnetic field variations in the
  sample spin-spin interaction ? T2 decay constant
  
• Mxy(t) M0e-(t/T2) decay of Mxy after 90 pulse
  
• T2 time required for Mxy to ? to 37 (1/e) peak
  level
• T2 relaxation relatively unaffected by B0

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 385.
22
Free Induction Decay T2 and T2 Relaxation

• T2 decay mechanisms det. by the molecular
  structure of the sample
• Large, stationary structures have short T2

• B0 inhomogeneities and susceptibility agents
  (e.g., contrast materials) cause more rapid
  dephasing ? T2 decay

• Mobile molecules (e.g., CSF) exhibit a long T2 as
  rapid molecular motion reduces intrinsic B
  inhomogeneities

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 386.
c.f. http//www.hull.ac.uk/mri /lectures/gpl_page
.html
23
Return to Equilibrium T1 Relaxation

• Loss of Mxy phase coherence (T2 T2 decay)
  occurs relatively quickly
• Return of Mz to M0 (equilibrium) takes longer
• Excited spins release energy to local environment
  (lattice) spin-lattice relaxation ? T1 decay
  constant
• Mz(t) M01-e-(t/T1) recovery of Mz after 90
  pulse
• T1 time required for Mz to ? to 63 (1-e-1) M0
  
• After t 5 T1 ? Mz(t) ? M0

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 387.
c.f. http//www.hull.ac.uk/mri /lectures/gpl_page
.html
24
Return to Equilibrium T1 Relaxation

• Method to determine T1 use various Dt between
  90 pulses and estimate by curve fitting
  Dissipation of absorbed energy into the lattice
  (T1) varies substantially for various tissue
  structures and pathologies (prev. Damadian table)
  Energy transfer most efficient when the
  precessional frequency of the excited nuclei
  overlaps with the vibrational frequencies of the
  lattice

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 388.
25
Return to Equilibrium T1 Relaxation

• Large slow-moving molecules ? low vibrational
  freq. (very small overlap with f0 longest T1)
• Moderately sized molecules (e.g., lipids,
  proteins and fat) and viscous fluids ? low
  intermed. freq. (great overlap short T1)
• Small molecules ? low, intermediate and high
  freq. (small overlap with f0 long T1)
• T1 Soft tissue 0.1,1 and aqueous substances
  1,4
• T1 relaxation ? as B0 ?
• Contrast agents spin-lattice sink

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 389.
26
Comparison of T1 T2

• T1 gt T2 gt T2 (T2 4-10X shorter than T1) Small
  molecules long T1 and long T2 (e.g., water, CSF)
  Intermediate molecules short T1 and short T2
  (most tissues) Large/bound molecules long T1 and
  short T2 The differences in T1 and T2, as well as
  spin density (r) provide much to MRI contrast and
  exploited for the diagnosis of pathologic
  conditions

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 390-391.
27
T1 T2 vs. B Field Strength
1.5 T 64 MHz
3.0 T 128 MHz
c.f. Mansfield, et al. NMR Imaging in
Biomedicine, 1982, p. 23
28
Pulse Sequences

• Tailoring pulse sequence emphasizes the image
  contrast dependent on r, T1 and T2 ? contrast
  weighted images
• Timing, order, polarity, pulse shaping, and
  repetition frequency of RF pulses and gradient
  (later) application
• Three major pulse sequences
• Spin echo
• Inversion recovery
• Gradient recalled echo

c.f. http//www.indianembassy.org/dydemo/page3.htm
29
Spin Echo - Echo Time (TE)

• Initial 90 pulse (t 0) ? maximal Mxy and phase
  coherence
• FID exponentially decays via T2 relaxation
• At t TE/2 a 180 pulse is applied ? induces
  spin rephasing
• Spin inversion spins rotate in the opposite
  direction, undoing all the T2 dephasing through
  Dt TE/2 at t TE (Dt 2TE/2)
• An FID waveform echo (spin echo) produced at t
  TE

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 392.
30
Spin Echo - Echo Time (TE)

• Maximum echo amplitude depends on T2 and not T2
• FID envelope decay still dependent on T2
• SE formation separates RF excitation and signal
  acquisition events
• FID echo envelope centered at TE sampled and
  digitized with ADC
• Multiple echos generated by successive 180
  pulses allow determination of sample T2 -
  exponential curve fitting Mxy(t) ? e-t/T2

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 393.
31
SE - Repetition Time (TR) Partial Saturation

• Standard SE pulse sequences use a series of 90
  pulses separated by Dt TR (repetition time,
  msec) 300,3000
• This Dt allows recovery of Mz through T1
  relaxation processes
• Degree of partial saturation dependent on T1
  relaxation and TR

• After the 2nd 90 pulse, a steady-state Mz
  produces the same FID amplitude from subsequent
  90 pulses partial saturation

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 394.
32
Spin Echo Contrast Weighting

• How the NMR signal changes with different tissue
  types and pulse sequence parameters
• S ??? 1-e-(TR/T1) e-(TE/T2)
• r, T1 and T2 are tissue properties
• TR and TE are pulse sequence parameters
• Each of these values can alter voxel contrast

(x,y)
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 394.
33
Spin Echo T1-weighting

• Short TR to maximize differences in Mz during
  return to equilibrium
• Short TE to minimize differences in T2 dependency
  of the FID
• How T1 values modulate the FID
• When TR ranges 400-600 msec differences in Mz
  emphasized
• Short TE preserves the T1 FID differences with
  minimum T2 decay

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 395.
34
Spin Echo T1-weighting
(TR549, TE11)

• T1-weighted (TR500, TE8)
• Fat most intense signal
• White and gray matter with intermediate signal
• CSF with lowest signal
• Typical pulse sequence parameters
• TR 400-600 msec
• TE 5-30 msec

T1
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 395.
35
NMR T1 for Tumor and Normal Tissue
c.f. http//www.gg.caltech.edu/dhl/
c.f. Mansfield, et al. NMR Imaging in
Biomedicine, 1982, p. 22.
36
Spin Echo Spin (Proton) Density Weighting

• Image contrast due to differences in the nuclear
  spin density (r)
• Very hydrogenous tissues (e.g., lipids and fats)
  have high r compared with proteinaceous soft
  tissues
• Aqueous tissues (e.g., CSF) also have a
  relatively high r
• Long TR to minimize T1 differences (CSF gt fat gt
  GM gt WM)
• Short TE to minimize T2 decay

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 397.
37
Spin Echo Spin (Proton) Density Weighting
(TR2400, TE30)

• r-weighted (TR2,400, TE30)
• Fat and CSF relatively bright
• Slight contrast inversion between WM and GM
• Typical pulse sequence parameters
• TR 1,500-3,500 msec
• TE 5-30 msec
• Highest SNR for SE pulse sequences
• Image contrast relatively poor

r
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 397.
T1
38
Spin Echo T2-weighting

• Reduce T1 effects with long TR, accentuate T2
  effects with long TE
• T2-weighted signal usu. the second echo of a
  multi-echo sequence
• Compared with a T1-weighted image ? inversion of
  tissue contrast
• Short T1 tissues ? short T2, long T1 tissues ?
  long T2

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 398.
39
Spin Echo T2-weighting
(TR2400, TE90)

• T2-weighted (TR gt 2,000, TE gt 80)
• As TE increased, more T2 contrast is achieved at
  the expense of reduced Mxy
• Typical pulse sequence parameters
• TR 1,500-3,500 msec
• TE 60-150 msec

T2
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 398.
r
T1
40
Spin Echo Parameters
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 399.
41
Inversion Recovery (IR)

• Emphasizes T1 by expanding the amplitude of Mz by
  2X
• Initial 180 pulse inverts Mz ? - Mz
• After Dt TI (inversion time), a 90 pulse
  rotates Mz into Mxy
• At Dt TI TE/2, a second 180 pulse induces an
  FID echo at TE
• TR period between initial 180 pulses
• TR lt 5 T1 causes partial saturation

(x,y)
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 400.
42
Inversion Recovery (IR)

• Echo amplitude depends on TI, TE, TR and Mz
• S ??? 1-2e-(TI/T1)e-(TR/T1) e-(TE/T2)
• TI controls contrast between tissues
• Can produce negative Mz (out of phase) when short
  TI used
• FID amplitude phase (phase sensitive detection
  quadrature receiver coil) can be preserved or the
  magnitude taken

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 400.
43
IR - T2 Short Tau IR

• Short Tau Inversion Recovery (STIR)
• Uses very short TI and magnitude signal
  processing
• Materials w/ short T1 have lower sig. intensity
  (reverse of std. T1-weighting)
• All tissues pass through zero amplitude (Mz 0)
• Judicious TI selection ? suppress a given tissue
  signal (bounce point)

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 402.
44
IR - T2 Short Tau IR
(TR750, TE13)

• Null point TI ln(2) T1

(TI150, TR5520, TE29)

• Example fat suppression T1 260 msec (B01.5T)
  ? TI 180 msec
• Compared with a T1-weighted sequence, STIR fat
  suppression reduces distracting fat signal and
  eliminates chemical shift artifacts
• Typical STIR TI 140-180 msec TR 2,500 msec

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 402.
FLAIR
T2
(TI2400 TR10K, TE150)
(TR2400, TE90)
45
IR - Field Attenuated IR and Contrast Comparison

• Long TI increases the signal levels of CSF
  other long T1 tissues
• FLuid Attenuated IR (FLAIR) bounce point at CSF
  T1 (3,500 msec)
• Nulling CSF requires TI ln(2) T1 2,400
  msec
• TR 7,000 typically employed to allow reasonable
  Mz recovery
• Contrast comparison T1-, r-, and T2-weighted
  plus FLAIR

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 403.
46
Gradient Recalled Echo (GRE)

• Magnetic field gradient used to induce the
  formation of an echo
• Gradient changes local magnetic field (B0DB) f
  (g/2p)(B0DB)
• FID signal generated under a linear gradient
  dephases quickly
• Inverted gradient (opposite polarity) used to
  produce an FID echo
• Not a spin-echo technique does not cancel T2
  effects

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 404.
47
Gradient Recalled Echo (GRE)

• Echo time controlled through gradient magnitude
  or time offset
• Flip angle (q) a major variable determining
  contrast in GRE seq.
• Less time to excite the spins ? short TR ?
  smaller flip q
• For short TR (lt 200 msec) more Mz generated w/
  small flip q

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 404.
48
GRE Sequence with Long TR (gt 200 msec)

• For long TR (gt 200 msec) GRE and flip q gt 45
  contrast behavior similar to SE
• Major difference signal dependence on T2 rather
  than T2
• Mechanisms of T2 contrast different than T2,
  especially for contrast agents
• T1-weighting achieved with short TE
• For flip q lt 30 small Mxy reduces T1
  differences
• r differences the major contrast attributes for
  short TE
• Longer TE provides T2-weighting
• GRE not useful with long TR except for
  demonstrating magnetic susceptibility differences

49
GRE - Steady-state Precession withShort TR (lt 50
msec)

• Steady-state precession equilibrium of Mz and
  Mxy from pulse to pulse in a repitition sequence
• For very short TR (lt T2), persistent Mxy occurs
• During each pulse aMxy ? Mz and bMz ? Mxy (a, b
  lt1)
• Steady-state Mz and Mxy components co-exist in
  dynamic equilibrium
• GRASS Gradient Recalled Acq. in the Steady
  State
• FISP Fast Imaging with Steady-state Precession
• FAST Fourier Acquired STeady state
• Practical only with short and very short TR
• Flip q has the major impact on contrast

50
GRE - Steady-state Precession withShort TR (lt 50
msec) and Contrast Weighting

• Small flip q 5-30 r-weighted contrast
  Moderate flip q 30-60 T2/T1-weighted contrast
  (some T1) Large flip q 75-90 T2- and
  T1-weighted contrast Typical parameter values for
  contrast desired in GRE and steady-state
  acquisitions GRASS/FISP TR 35 msec, TE 3 msec
  and flip q 20
• Unremarkable contrast but flow

GRASS sequence (TR 24 msec, TE 4.7 msec, flip
q50) volume acquisition. Contrast unremarkable
for white/gray matter due to T2/T1-weighting
dependence. Blood appears bright MR angiography
reduce contrast of anatomy relative to
vasculature.
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., pp. 406-407.
51
Signal Flow

• The MR signal from moving fluids (vascular and
  CSF) is complicated by many factors
• Flow velocity
• Vessel orientation
• Laminar vs. turbulent flow patterns
• Pulse sequences
• Image acquisition modes
• Flow related mechanisms combine with image
  acquisition parameters to alter contrast
• Bright-blood to black-blood
• Can be a source of artifacts
• Exploited to produce MR angiography images

52
Signal Flow

• Low signal intensities high-velocity signal loss
  
• Nuclei move out of slice during echo reformation
  (nothing focused in Mxy plane ? no or little FID
  signal)
• Flow turbulence flow voids
• Dephasing of spins in blood (confused spin
  alignment)
• Black-blood double IR (TI 600 ms)
• IR sequence prefaced with non-selective, volume
  180 pulse
• Flow-related enhancement
• Even-echo rephasing (prominent in slow laminar
  flow veins)
• Gradient echo images (unsaturated blood) ?
  velocity, slice thinness and TR

53
Perfusion and Diffusion Contrast

• Tissues with ? H2O mobility have greater signal
  loss
• In vivo structural integrity of tissues measured
  ? apparent diffusion coefficient maps
• Sensitive indicator for early detection of
• Spine and spinal cord pathophysiology
• Ischemic injury
• Spin-echo and echoplanar pulse sequences with
  diffusion gradients
• Obstacles
• Sensitivity to head/brain motion
• Eddy currents

Diffusion-weighted image (DWI) with gray
scale-encoded diffusion coefficients.
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 410.
54
Magnetization Transfer Contrast

• Result of selective observation of the
  interaction between the p in free H2O molecules
  and p in macromolecular proteins due to coupling
  or chemical exchange
• Can be excited separately using narrow-band RF
• Magnetization transferred from macromolecular p
  to free H2O p
• Reduced signal from adjacent free H2O p

c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 412.
55
Magnetization Transfer Contrast

• This process affects only those p having
  chemical exchange with the macromolecules and
  improves image contrast
• Anatomic imaging of heart, eye, MS, knee
  cartilage and general MR angiography
• Tissue characterization possible as the
  magnetization transfer ratio (MTCon/MTCoff) is
  caused in part by tissue-specific surface
  chemistry

MR arthrograms of shoulder in 32-year-old man
with suspected gleno-humeral instability. Axial
3D gradient-echo MR image obtained using
parametric magnetization transfer pulses no
discernible magnetization transfer contrast in
injected fluid or in fatty marrow spaces, whereas
degree of magnetization transfer contrast varies
in skeletal muscle, cartilage, and capsular
supporting structures (color scale 0-100).
c.f. Yao L, Thomasson D. Magnetization transfer
contrast in rapid three-dimensional MR imaging
using segmented radiofrequency prepulses. AJR
2002 179 863-5 .