The Basics of MRI

Current MRI technology displays images as multiple sets of Current MRI technology displays images as multiple sets of gray tone images. Visualization and interpretation of the multiparameter gray tone images. Visualization and interpretation of the multiparameter images may be optimized by assigned color tissue segmentation. images may be optimized by assigned color tissue segmentation.

Researcher H. Keith Brown, Ph.D. has developed technology Researcher H. Keith Brown, Ph.D. has developed technology that creates color composite images that indicate the unique physical and that creates color composite images that indicate the unique physical and chemical properties of the human tissues represented by those images.chemical properties of the human tissues represented by those images.

NMR imaging is a powerful NMR imaging is a powerful technique to obtain high resolution technique to obtain high resolution images of comparable quality with images of comparable quality with CT scans without the disadvantages CT scans without the disadvantages of possible radiation damage.of possible radiation damage.

The important aspects which determine the The important aspects which determine the resolution and contrast in the resolution and contrast in the final imagefinal image are the are the bandwidth of the rf-signalbandwidth of the rf-signal which causes. The which causes. The resonance absorption (excitation of spin-up resonance absorption (excitation of spin-up protons to spin-down protons), the protons to spin-down protons), the relaxation relaxation timetime scale for establishing the equilibrium value, scale for establishing the equilibrium value, and the and the field gradient field gradient GG in the external magnetic in the external magnetic field field BBjj..

This loss process has an oscillatory exponential behavior: This loss process has an oscillatory exponential behavior:

The actual The actual TT22 relaxation is reduced relaxation is reduced to a true relaxation time to a true relaxation time TT22

** by field by field inhomogeneities and field gradients.inhomogeneities and field gradients.

Typically Typically TT11 > T > T2 2 but the relaxation times typically but the relaxation times typically

depend of the particular kind of body tissue (influence of depend of the particular kind of body tissue (influence of difference in proton density due to differences in molecular difference in proton density due to differences in molecular structure of body tissues)structure of body tissues)

MRI techniques use the differences in MRI techniques use the differences in relaxation time to highlight different relaxation time to highlight different tissue materials and to obtain optimum tissue materials and to obtain optimum contrast and resolution!contrast and resolution!

Different techniques are used for pulse sequences for the rf-signals.Different techniques are used for pulse sequences for the rf-signals.

The rf-signal typically has a certain bandwidth around the Larmor The rf-signal typically has a certain bandwidth around the Larmor frequency for the material to be observed. For medical MRI this is frequency for the material to be observed. For medical MRI this is typically hydrogen. typically hydrogen.

Two pulse sequence techniques are typically used; the saturation Two pulse sequence techniques are typically used; the saturation recovery sequence, recovery sequence, SRS SRS and the and the spin-echo sequencespin-echo sequence, , SES.SES.

The The SRSSRS is comprised of a series of 90° pulses separated by a period is comprised of a series of 90° pulses separated by a period of time (time of repetition of time (time of repetition TD) TD) Each applied 90° rf-pulse rotates the Each applied 90° rf-pulse rotates the magnetization from z-direction into the xy-plane, an antenna is used to magnetization from z-direction into the xy-plane, an antenna is used to pick up the signal (FID) induced by the change of magnetization. The pick up the signal (FID) induced by the change of magnetization. The oscillating (oscillating (00)) FID signal decays following the time constant FID signal decays following the time constant ti ti before before

the next pulse occurs.the next pulse occurs.

Before complete relaxation has occurred (relaxation time Before complete relaxation has occurred (relaxation time TT11 a seconda second

90° pulse follows.90° pulse follows.

Saturation recovery sequenceSaturation recovery sequence

The free induction signal The free induction signal (which contains many frequencies) (which contains many frequencies) is converted into absorption mode is converted into absorption mode signal by Fourier transformation, it signal by Fourier transformation, it has a Lorentzian form:has a Lorentzian form:

In the In the SESSES the 90° pulse is followed by an the 90° pulse is followed by an additional 180° pulse at a time Tadditional 180° pulse at a time TEE/2 /2 (T(TEE echo echo

time) to refocus the time) to refocus the MMxyxy magnetization before the magnetization before the

next 90° pulse occurs. This causes an additional next 90° pulse occurs. This causes an additional echo signal. echo signal.

The signal for the The signal for the SES SES image is image is described as a function of pulse described as a function of pulse repetition time repetition time TD TD and echo time and echo time TE:TE:

This equation allows to make the choice of scanning parameters This equation allows to make the choice of scanning parameters TDTD and and TE TE to emphasize the differences for to emphasize the differences for TT11 and and TT22 in different tissue in different tissue

materials. materials. TD TD emphasizes the weighting of emphasizes the weighting of TT11 and and TE TE emphasizes the emphasizes the

weighting of weighting of TT22..

The right choice of pulse repetition The right choice of pulse repetition TD TD and echo-time and echo-time TE TE allows to allows to emphasize the emphasize the TT11 relaxation time characteristics for different body relaxation time characteristics for different body

tissues. Short tissues. Short TD TD emphasizes tissues with short relaxation times emphasizes tissues with short relaxation times TT11

like fat and blood, short like fat and blood, short TE TE minimizes minimizes TT2 2 decay effects. Long decay effects. Long

repetition times repetition times TD TD emphasize tissues with long relaxation times emphasize tissues with long relaxation times TT11

like cerebral tissues.like cerebral tissues.

The The contrastcontrast is determined by the difference in the is determined by the difference in the relative signal intensity:relative signal intensity:

The relative intensity in the NMR signal for different body tissues The relative intensity in the NMR signal for different body tissues can be calculated as a function of the relaxation times can be calculated as a function of the relaxation times T1 and T2T1 and T2

for different choices of retardation time for different choices of retardation time TDTD and echo-time and echo-time TETE. . The relative intensity of a signal for body tissue The relative intensity of a signal for body tissue i i is:is:

EXAMPLE: EXAMPLE: Compare the intensity of the MRI signal Compare the intensity of the MRI signal for a magnetic field strength of Bfor a magnetic field strength of B00=1.5 T for cerebrospinal =1.5 T for cerebrospinal

fluid (CSF) and gray matter (gm) and calculate the contrast fluid (CSF) and gray matter (gm) and calculate the contrast in the MRI image.in the MRI image.

For a magnetic field strength of B=1.5 T the relaxation For a magnetic field strength of B=1.5 T the relaxation times for cerebrospinal fluid and for gray matter are:times for cerebrospinal fluid and for gray matter are:

T1(CSF) = 2400 ms, T2(CSF) = 160 msT1(CSF) = 2400 ms, T2(CSF) = 160 ms

T1(gray matter) = 900 ms, T2(fat) = 100 msT1(gray matter) = 900 ms, T2(fat) = 100 ms

TDTD TETE IICSFCSF IIgmgm II

500ms500ms 10ms10ms 0.3860.386 0.1990.199 0.2090.209

Nuclear Magnetic Resonance Image AcquisitionNuclear Magnetic Resonance Image Acquisition

Most of the MRI imaging methods are based on the fact thatMost of the MRI imaging methods are based on the fact thatthe resonant frequency is proportional to the field strength.the resonant frequency is proportional to the field strength.

Thus a small field gradient Thus a small field gradient G = G = B/B/zz is added along the axis of field is added along the axis of fieldBB00, , z z which causes the resonance frequency which causes the resonance frequency 00 to change with position to change with position z:z:

If the resulting If the resulting FID FID signal is Fourier transformed to obtain the frequencysignal is Fourier transformed to obtain the frequencydistribution, the frequency axis would be equivalent to the distribution, the frequency axis would be equivalent to the zz-displacement.-displacement.

A field gradient A field gradient GGssss (slice-selection gradient) can be used to(slice-selection gradient) can be used to

localize the MR excitation to a region within the body.localize the MR excitation to a region within the body.

If the rf-pulse has only a small bandwidth (If the rf-pulse has only a small bandwidth (ss ss 1-2 kHz), 1-2 kHz),

only spins in a thin slice resonating at frequencies within that only spins in a thin slice resonating at frequencies within that bandwidth would be excited (selective excitation).bandwidth would be excited (selective excitation).

Each position zEach position zii corresponds corresponds

to a resonance frequency to a resonance frequency 00

The choice of field gradient The choice of field gradient and band width of the rf-pulse and band width of the rf-pulse determines the slice thickness:determines the slice thickness:

A fixed gradient A fixed gradient GGssss allows to modify the slice thickness by allows to modify the slice thickness by

changing the band width of the rf-signal. Typically, however, the band changing the band width of the rf-signal. Typically, however, the band width is fixed and the gradient varies. width is fixed and the gradient varies.

If an NMR facility has a field of B=2 Tesla and a gradient of G=0.01xB (T/cm)If the field is directed along the length axis of the head

We can calculate the NMR Larmor frequency as a function of the position along that axis…

0= .B + .z.G where = 42.58 MHz/T

So the equation becomes…

0= 85.16 + (0.85 x z)

For z = 1.0 cm 0= 86.0 MHz

For z = 5.0 cm 0= 89.4 MHz

For z = 10.0 cm 0= 93.7 MHz

4.7 Tesla/33cm SISCO IMAGING SYSTEM4.7 Tesla/33cm SISCO IMAGING SYSTEM

This Scanner has a magnetic field of 4.7T and 200 MHz resonance This Scanner has a magnetic field of 4.7T and 200 MHz resonance frequency for protons.  The shielded gradient coils and Oxford gradient frequency for protons.  The shielded gradient coils and Oxford gradient power supply are able to produce a gradient field of 6.5 G/cm.  The diameter power supply are able to produce a gradient field of 6.5 G/cm.  The diameter of the space in the magnet bore available to users is 22 cm.  The usual size of the space in the magnet bore available to users is 22 cm.  The usual size of the objects for MR imaging is 14 x 14 x 14 cm.of the objects for MR imaging is 14 x 14 x 14 cm.

Siemens 3 Tesla Magnetom Allega MR HeadscannerSiemens 3 Tesla Magnetom Allega MR Headscanner

The Siemens 3T Allegra is a state-of-the-art system designed The Siemens 3T Allegra is a state-of-the-art system designed especially for neurological and cognitive fMRI studies. especially for neurological and cognitive fMRI studies.

The Allegra system provides a gradient strength of 40 mT/m and a The Allegra system provides a gradient strength of 40 mT/m and a slew rate 400 T/m/sec. It has excellent linearity across a 22 cm FOV. It will slew rate 400 T/m/sec. It has excellent linearity across a 22 cm FOV. It will facilitate  simultaneous optical and MR or eye-tracking and MR recording. facilitate  simultaneous optical and MR or eye-tracking and MR recording.

Varian 600 MHz Wide-Bore SpectrometerVarian 600 MHz Wide-Bore Spectrometer

The Varian 600 MHz wide-bore The Varian 600 MHz wide-bore system will be equipped for both high system will be equipped for both high resolution NMR spectroscopy and resolution NMR spectroscopy and microimaging. With gradients in place the microimaging. With gradients in place the clear bore of the magnet is 3.5 cm. Proton clear bore of the magnet is 3.5 cm. Proton 5 mm and 10 mm probes are available for 5 mm and 10 mm probes are available for imaging studies. In addition, there are four imaging studies. In addition, there are four receiver channels for the implementation of receiver channels for the implementation of multiple coils, phased arrays and parallel multiple coils, phased arrays and parallel acquisition schemes. High resolution and acquisition schemes. High resolution and imaging software is available, with the imaging software is available, with the identical operating system to the 4. 7 Tesla identical operating system to the 4. 7 Tesla SISCO system. SISCO system.