Introduction to Medical Imaging Introduction to Medical Imaging
MRI Magnetic Resonance Imaging Guy Gilboa Course 046831 Slide 2 MRI
invention Several involved: Raymond Damadian 1971, idea still very
sketchy, no images produces. Paul Lauterbur 1973-4, mature
technique for 2D and 3D imaging. Produced first image of a living
mouse. Peter Mansfield - developed a mathematical technique where
scans take seconds rather than hours also producing clearer images.
Nobel prize 2003, Paul Lauterbur Sir Peter Mansfield (Damadian left
out, protests of him and colleagues). From top: Damadian,
Lauterbur, Mansfield. Slide 3 MRI scanner The lecture is based
mainly on: [1]
http://www.medphysics.wisc.edu/~block/bme_530_lectures.html [2] Ch.
5 of the book by N. B. Smith and A. Webb, Introduction to Medical
Imaging, Cambridge University Press, 2011. Slide 4 Typical brain
MRI Slide 5 MRI basic operation principle The MRI is comprised of 3
main components: A superconducting primary magnet 3 magnetic field
gradient coils RF transmitter and receiver Taken from
https://wiki.engr.illinois.edu/display/BIOE414/Team+4+
-+MRI+Radio+Frequency+Coils
https://wiki.engr.illinois.edu/display/BIOE414/Team+4+
-+MRI+Radio+Frequency+Coils Slide 6 Movie how MRI works (8.5 min.)
https://www.youtube.com/watch?v=Ok9ILIYzmaY Slide 7 Energy units
V=volt; s=second; m=meter; J=joule; A=ampere; Wb=webber; N=newton;
C=coulomb; g=gauss. Slide 8 Lorentz force Slide 9 Faraday induction
Slide 10 Some symbols Slide 11 Magnetic Fields used in MR: 1)
Static main field B o 2) Radio frequency (RF) field B 1 3) Gradient
fields G x, G y, G z Slide 12 Very strong magnets used in MRI Ohio
Akron Childrens Hospital: 3T MRI (Magnetom Skyra, Siemens). Weight
~7,500kg. Cost $3.5M (2011). Slide 13 Over 3T magnets very large
and expensive 7T (Stanford) 9.4T (Siemens)
http://news.stanford.edu/news/2005/may18/med-mri-051805.html
http://www.siemens.com/press/en/pressrelease/?press=/en/pressrelease/2008/imaging_it/juelich.htm
Slide 14 Gradient coils Create a weak magnetic field in any
direction in space. Magnetic field strength approximately 100 times
lower than the main field. Slide 15 Reference Frame z x y Slide 16
Magnetic Moments MR is exhibited in atoms with odd # of protons or
neutrons. Spin angular momentum creates a dipole magnetic moment
Intuitively current, but nuclear spin operator in quantum mechanics
Plancks constant / 2 Model proton as a ring of current. Which atoms
have this phenomenon? 1 H - abundant, largest signal 31 P 23 Na =
gyromagnetic ratio : the ratio of the dipole moment to angular
momentum Slide 17 Energy states Hydrogen has two quantized
currents, B o field creates 2 energy states for Hydrogen where
Energy of Magnetic Moment in energy separation resonance frequency
f o Slide 18 Nuclei spin states There are two populations of
nuclei: n + - called parallel n - - called anti parallel n+n+ n-n-
lower energy higher energy Which state will nuclei tend to go to?
For B= 1.0T Boltzman distribution: Slightly more will end up in the
lower energy state. We call the net difference aligned spins. Only
a net of 7 in 2*10 6 protons are aligned for H + at 1.0 Tesla.
(consider 1 million +3 in parallel and 1 million -3 anti-parallel.
But... Slide 19 There is a lot of a water! 18 g of water is
approximately 18 ml and has approximately 2 moles of hydrogen
protons Consider the protons in 1mm x 1 mm x 1 mm cube. 2*6.62*10
23 *1/1000*1/18 = 7.73 x10 19 protons/mm 3 If we have 7 excesses
protons per 2 million protons, we get.25 million billion protons
per cubic millimeter!!!! Slide 20 Torque mechanical analogy Spins
in a magnetic field are analogous to a spinning top in a
gravitational field. (gravity - similar to B o ) Top precesses
about Magnetic Torque Slide 21 Precession rotates (precesses) about
Precessional frequency: is known as the Larmor frequency. for 1 H 1
Tesla = 10 4 Gauss Usually, B o =.1 to 3 Tesla So, at 1 Tesla, f o
= 42.57 MHz for 1 H or Slide 22 Precession Movie (7 min.)
https://www.youtube.com/watch?v=7aRKAXD4dAg Slide 23 RF Magnetic
field The RF Magnetic Field, also known as the B 1 field To excite
nuclei, apply rotating field at o in x-y plane. (transverse plane)
B 1 radiofrequency field tuned to Larmor frequency and applied in
transverse ( xy ) plane induces nutation (at Larmor frequency) of
magnetization vector as it tips away from the z -axis. - lab frame
of reference Slide 24 RF general excitation (rotating frame) By
design, In the rotating frame, the frame rotates about z axis at o
radians/sec 1) B 1 applies torque on M 2) M rotates away from z.
(screwdriver analogy) 3) Strength and duration of B 1 determines
flip angle. This process is referred to as RF excitation. x y z
Slide 25 Coils diagram Simplified Drawing of Basic Instrumentation.
Body lies on table encompassed by coils for static field B o,
gradient fields (two of three shown), and radiofrequency field B 1.
Image, caption: copyright Nishimura, Fig. 3.15 Slide 26 Detection -
Switch RF coil to receive mode. Precession of M induces EMF in the
RF coil. (Faradays Law) EMF time signal - Lab frame t Voltage (free
induction decay) x y z M for 90 degree excitation Slide 27 T1 and
T2 relaxation times Application of RF pulse creates non-
equilibrium state (adding energy to the system). After the pulse is
switched off, the system is relaxed back to equilibrium. There are
2 relaxation times which govern the return to equilibrium: T1
(spin-lattice), equilibrium of z component. T2 (spin-spin), x and y
components. Slide 28 Tissue relaxation times for 1.5 Tesla Table:
5.1 from [2] TissueT 1 (ms)T 2 (ms) White matter79090 Gray
matter920100 Liver50050 Skeletal muscle87060 Lipid ( )290160
Cartilage ()106042 Slide 29 Bloch Equation Solution: Longitudinal
Magnetization Relaxation Component The greater the difference from
equilibrium, the faster the change Solution: Initial Mz Doesnt have
to be 0! Return to Equilibrium Slide 30 Transverse time constant T2
- spin-spin relaxation T 2 values: < 1 ms to 250 ms What is T 2
relaxation? - z component of field from neighboring dipoles affects
the resonant frequencies. - spread in resonant frequency
(dephasing) happens on the microscopic level. - low frequency
fluctuations create frequency broadening. Image Contrast: Longer
T2s are brighter in T2-weighted imaging, darker in T1-weighted
imaging Slide 31 MR: Relaxation: Some sample tissue time constants
- T 1 Image, caption: Nishimura, Fig. 4.2 fat liver kidney
Approximate T 1 values as a function of B o white matter gray
matter muscle Slide 32 Gradient Fields - key for imaging - Paul
Lauterbur Gradient coils are designed to create an additional B
field that varies linearly across the scanner as shown below when
current is driven into the coil. The slope of linear change is
known as the gradient field and is directly proportional to the
current driven into the coil. The value of B z varies in x
linearly. z BzBz BoBo slope = G z Whole Body Scanners: |G| = 1-4
G/cm (10-40 mT/m) Gz can be considered as the magnitude of the
gradient field, or as the current level being driven into the coil.
Slide 33 Basic Procedure 1)Selectively excite a slice ( z) -
time?.4 ms to 4 ms - thickness?2 mm to 1 cm 2) Record FID, control
G x and G y - time?1 ms to 50 ms 3) Wait for recovery - time?5 ms
to 3s 4)Repeat for next measurement. - measurements?128 to 512 - in
just 1 flip 5) Next: More on spatial encoding Slide 34 Phase and
frequency encoding It is not important which dimension encodes
frequency and which phase. We assume: X encodes frequency Y encodes
phase Slide 35 Frequency encoding Slide 36 Phase encoding Slide 37
K-space formalism (5.10) Slide 38 Image recovery Slide 39 Phase
Direction Frequency Direction One line of k-space acquired per TR
k-Space Acquisition Phase Encode DAQ Sampled Signal kxkx kyky Taken
from [1] Slide 40 Fast Fourier Transform FFT Slide 41 8 x 8 512 x
512 Slide 42 16 x 16 512 x 512 Slide 43 32 x 32 512 x 512 Slide 44
64 x 64 512 x 512 Slide 45 128 x 128 512 x 512 Slide 46 256 x
256512 x 512 Slide 47 Signal Intensity and SNR Slide 48 Multiple
slice imaging The TR time required between successive RF
excitations for each phase encoding step is much longer than TE. In
this time other adjacent slices are usually acquired (maximum of
TR/TE) Usually this is done in an interlacing fashion od numbered
slices are followed by even-numbered slices. Slide 49 Spin-echo
imaging sequence SS Slice selection, PE Phase encoding, FE
Frequency encoding, TR Time of repetition, TE Time of echo. Slide
50 T1, T2, PD A long TR and short TE sequence is usually called
Proton Density (PD) weighted. A short TR and short TE sequence is
usually called T1- weighted A long TR and long TE sequence is
usually called T2-weighted Taken from
http://www.imaios.com/en/e-Courses/e-MRI/MRI-signal-contrast/Signal-weighting
Slide 51 MR angiography Increase signal difference between flowing
blood and tissue Based on TOF (time-of-flight) technique, shorter
effective T1 due to flow if the slice is oriented perpendicular to
the direction of flow. Slide 52 Functional MRI Determines which
areas of the brain are involved in cognitive tasks and brain
functions such as speech and sensory motion. Based on the fact that
MRI signal intensity changes depending upon the level of
oxygenation of the blood in the brain (indicating increased
neuronal activity). Uses fast scans which can cover the brain in a
few seconds. Slide 53 Example of fMRI Brain activity changes of
teenagers playing violent video games. Taken from
http://www2.rsna.org/timssnet/media/pressreleases/pr_target.cfm?ID=304
http://www2.rsna.org/timssnet/media/pressreleases/pr_target.cfm?ID=304
Slide 54 MR contrast agents Positive Paramagnetic contrast agents,
shorten the T1 of tissue in which they accumulate. Based on
gadolinium ion (Gd). Used to detect tumors, lesions in the central
nervous system (brain and spine). Negative Superprparamgnetic (iron
oxides), reduce T2 relaxation time. Used in detection of liver
lesions. Slide 55 Image characteristics (5.20) Slide 56
Characteristics (cont) Contrast to noise Contrast is based on T1,
T2, PD scans. Can be manipulated by choices of TR, TE. For small
lesions, the contrast is increased by having higher spatial
resolution to minimize partial volume artifacts. Slide 57 Examples
brain Comparison of PD, T1, T2 and angiography. Slide 58 Cardiology
4 chamber view MR angiography of the chest (18 sec scan time) Slide
59 Some clinical applications of MRI Used widely to scan almost
every organ in the body, popular uses are: Neurological
applications Can diagnose both acute and chronic neurological
deseases. Method of choice for brain tumor detection. Most
protocols involve administration of Gd. Many pathological
conditions in the brain result in increased water content, which
gives high signal intensity on T2-weighted sequences. Slide 60
Clinical apps (cont) Liver and Muscoloskeletal Can diagnose well
lesions in fatty liver. Also iron overload, liver cysts, several
lesions. Muscle-skeleton system. Knee scans to diagnose arthritis
(joint inflammation). Cardiology To reduce motion artifacts - scans
are gated according to the cardiac cycle, based on
electrocardiograms (ECG). Detects myocardial infarcts, can measure
left ventricular volume and ejection fraction. Good contrast
between blood and myocardial wall. Diagnose coronary artery
stenosis using angiography. Slide 61 MRI summary Slide 62 MRI vs CT
Brain image Better contrast in MRI for soft tissues, easy to
distinguish between gray and white matter. Slide 63 Comparison
between MRI and CT CTMRI Ionizing radiationYesNo CostlowerHigher
(x3?) Speed10-30 s (full scan 5- 10 min). Several minutes (full
scan 30-60min) Data modesFewMany 3D imagesYes Resolution~7 lp/cm~3
lp/cm Work with metal in the body YesNo SNR increases asRadiation
increases, or body is smaller. Primary magnet is stronger (also
acquisition time) Slide 64
