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Introduction toMagnetic Resonance Imaging

MRI of the brain,ca. 1978.

ca. 1993 ca. 2006

2014

Modality Characteristics andComparison

• Radiography

• CT scanning

• Nuclear medicine

• MRI

• Ultrasound

use electromagnetic energy

a small portion of thespectrum is useful for medicalimaging

uses sound waves1-20MHz

“reflection modality”

use ionizingradiation

“transmissionmodalities”

“emissionmodality”

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Is MRI Dangerous ? No.

Some Kidney Patients Suffer MRI PoisoningA growing number of people are becoming afflicted with an incurable, man-made disease that is related to a common medical procedure performedevery single day in this country, a KCRA 3 investigation has found.MSNBC

Note that this was NOT MRI poisoning – this was gadolinium poisoning, anoccasionally-used MR contrast agent

How urban legends begin….. :

Is MRI Dangerous ? MAYBE A LITTLE…

http://www.youtube.com/watch?v=5z33ZcDgavY

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courtesy Joe Hornak, RIT

Very briefly – how it works

courtesy Joe Hornak, RIT

Very briefly – how it works

The very strong magnet……with gradient coils inside

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courtesy Joe Hornak, RIT

Very briefly – how it works

Introduction to MRI:• Spin

– Microscopic view– Macroscopic view

• RF excitation – how do we turn “spin” intodetectable signal?

• Relaxation – what makes the detectablesignal decay?

• Spin echoes – how can we overcome thisdecay?

• Contrast – how can we use relaxation to tellus about the properties of our sample?

• Gradients & signal localization• Frequency encoding

• Slice selection• Readout

• Image formation

Nuclear Magnetic ResonanceNMR

Magnetic Resonance ImagingMRI

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• Protons have a small electric(positive) charge

• They spin => produce smallmagnetic field (“spin”)

• Under normal conditions, thespins are randomly oriented

• Water is the largest source ofprotons in the body (hydrogennuclei), followed by fat

• Nuclei that have an odd numberof protons have “net spin”, or amagnetic moment, and arecandidates for NMR

• Fortunately, hydrogen has oneproton and thus a net spin thatwe can influence

Spin – microscopic view:

• Just like a compass aligning withthe earth’s field, a spinningproton placed within a largeexternal magnetic field, B0, willalign with or against the field.

• A slight excess will align with thefield

• The net result is an alignmentwith the external field

• Notes that will affect our“macroscopic” view later:

– a photon at the “correct” frequency for themagnetic field can cause the individual nucleito change their direction of alignment (to flip)

– Reality of “alignment”:

Spin – microscopic view:

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• The protons aligned with the field are in a lower energy state thanthose aligned against the field

• The larger the external B0 field, the greater the difference in energylevels

• The larger the external B0 field, the larger the excess number ofprotons aligned with the field.

Spin

Distribution of2 million protonsat different field strengths

Consider: how many excess protons are there in a single voxel of water at 1.5T ?

• Assume a voxel is 2 x 2 x 5mm = 0.02ml• Avogadro’s Number says 6.02 x 1023 molecules/mole• Facts: 1 mole of water weighs 18 grams (16g of O and 2g of H), has 2

moles of H, and fills 18ml• So 1 voxel of water has

• From previous slide, “excess” protons in the low-energy state at 1.5Tare 9/2million=>

or 6 million billion

• Point: we can ignore the microscopic (quantum) view and focus onthe macroscopic (classical) view

Spin – macroscopic view

protonstotal10338.1ml/mole

ml/voxel

18

02.0

Hmole

Hmolecules1002.6

watermole

Hmoles2 2123

1002.69102

10338.1 15

6

21

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• The total number of excess spins is calledM0, the net magnetization, and it isaligned along the main field, B0

Spin & Larmor frequency – macroscopic view

• M0 in a magnetic field, B0, acts like adreidle in the presence of gravity andprecesses about the field at a frequencyproportional to B0

f0=γB0 , the Larmor Frequency

where γ , gamma, is the gyromagnetic ratio =

42.56 MHz/Tesla for protons in hydrogen

• When you input a magnetic field, B1, at the Larmor frequency(an RF field generated by an RF coil ), you can “tip” themagnetization vector into the x-y plane, or transverse plane

• The magnetization continues to precess at the Larmorfrequency in the transverse plane and this moving magneticfield can be detected by a pickup coil.– Transverse magnetization can be detected

• D Ădž�ƐŝŐŶĂů�ǁ ŝƚŚ�ŵĂdž�ƚƌĂŶƐǀ ĞƌƐĞ�ŵĂŐŶĞƟnjĂƟŽŶ�ї �α=90o

– Longitudinal magnetization is not detectedwith the RF coil

RF excitation – getting a detectable signal

A note on the Rotating FrameA frame of reference that isrotating at the Larmorfrequency. i.e. x’ and y’ axesare rotating at f0 and z’=z.

Rotating frame

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Relaxation– the decay of the detectable signal

• T1 relaxation, or longitudinal relaxation:The z component recovers, decreasing thenet transverse (detectable) component:

• T2 relaxation, or transverse relaxation, orspin-spin relaxation: The magnetizationvector is, in reality, composed of manymany spins that are not all exposed to theexact same magnetic field due to purelyrandom spin-spin interactions => they allprecess at slightly different frequencies.This is called dephasing.

Relaxation– the decay of the detectable signal

Note: usually T2 is so much shorter than T1 that we consider the signaldecay to be due primarily to T2 effects. We consider T1 to be “good”since that is how we grow back our magnetization to use again.

T1 - Growth of longitudinal

magnetization

When the transmitter isturned off, the protonsimmediately begin to re-radiate the absorbedenergy from the RFpulse that tipped them.

FID – FreeInduction Decay:The signalunaffected by anygradient. Containsno positionalinformation

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• Reality:

T2* relaxation: Not only do the spins dephase because ofspin-spin interactions, but also because of (repeatable, non-random, fixed) imperfections in the magnetic field

Relaxation– the decay of the detectable signal

• The effects of the non-random, repeatable,parts of T2* decay can be overcome

Spin Echoes – overcoming (some of) the decay

• A note on gradient echoes: instead of using the 180o RF pulse to achieve rephasing (an echo), a “gradient echo”is achieved by forcing dephasing of the spins by deliberately placing a change in the magnetic field (a“gradient”) across them for a time, and then reversing the polarity of that gradient to force rephasing. This is afast way to create an echo, and works well in homogenous main fields (where the “repeatable, non-random”parts of T2* decay we discussed are small). You will be using spin echoes.

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• So far, our signal (FID or echo) isfrom the entire sample

• Recall: the frequency of the signal we receive is a functionof the strength of the magnetic field

Therefore: if we can control the strength of the magneticfield in space, we will receive back a signal whose frequencycontent is spatially dependent. (then we just need an IFT)

We need gradients in our main magnetic field

• The gradient coils do not change the direction of the main magnetic field.They add or subtract from the magnitude of the main field. The amountthey add or subtract is a function of spatial position.

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0 ˆ( )x y zB B G x G y G z z

X -gradient

Y -gradient

z -gradient

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• Place a gradient in the magnetic field thatyields a frequency distribution across thesample that is a function of position in z:

• If a pure sinusoidal RF excitation at a specificfrequency were applied, only an infinitesimallythin slice of the body would undergo forcedprecession, or tipping, and yield signal. This isnot feasible (or desirable) in practice. Actually,create a waveform that has a range offrequencies (frequency content or spectrum orbandwidth) and excites a “slice”.

NMR → MRI : frequency encoding - slice selection

Slice(along z)

B0

Bandwidth of RF pulse

• Turn on a readout gradient during the acquisition (or readout/digitization)of the signal, and the signal will have frequency content that is a functionof x. i.e. the signal is spatially encoded in the x direction.

NMR → MRI : frequency encoding - readout

object:x

y

Gradient strength FID FT – spectrum – “projection”

0

weak

strong

f

f

f

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• Now consider doing the same thing with agradient along y:

NMR → MRI : image formation

object:x

y

FID

f

FT – spectrum – “projection”

• For you to form an image, you could acquiremany of these projections and then reconstruct

• Typically, to create an MR image, you need to fill 2D“k -space” with “raw data” – the repeated echoesyou collect

NMR → MRI : image formation

2D-FFT

kX

ky

Nf samples

Np

Ph

ase

Enco

de

Rep

etit

ion

s k-space domain Image domain

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• How long does it take to fill k-space, or to create araw data set?– It depends on how long we wait between pulses of RF. This is our

TR, or repetition time.

Longer TR →more time for the longitudinal magnetization to grow backaccording to T1→more magnetization to tip into the transverse plane→more signal

• By controlling our TR and TE (echo time – time wewait before digitizing our echo), we can control ourimage contrast.

NMR → MRI : image formation

T1W TSE with CLEAR0.5 x 0.6 x 4.0 mm13 slices in 5:04 min

T2W TSE with CLEARSENSE Spine coil0.5 x 0.6 x 4.0 mm13 slices in 5:22 minCourtesy: KyungheeUniversity Hospital,Korea

STIR TSEwith CLEAR0.5 x 0.9 x 4.0mm13 slices in 9:58 min

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Ultra high resolution PDW TSE Wrist using a-TSE and SENSE to increase resolutionand reduce scantime - 0.15 x 0.15 x 2.0 mm, 16 slices in 6:14 min

Ultra high resolution PDW TSE Foot using a-TSE and SENSE to increase resolution andreduce scantime ---- 0.2 x 0.2 x 2.5mm, 18 slices in 7:16 mins

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Hi-Res T2W TSE Whole Body Imagingusing the integrated body coil0.5 x 0.5 x 6.0 mm12 slices, 2:27 min / station

Whole BodyAngiography in 4stations using theintegrated body coilOptimal resolution perstation using MobiFlex,scan time 1:10 minCourtesy: CatharinaHospitalEindhoven, TheNetherlands

FiberTrak of relevant tracts in a patient with large pathology15 directions DTI using SENSE2.0 x 2.0 x 2.0 mm60 slices in 5:13 minCourtesy: University of Michigan, Ann Arbor, MI, USA

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High resolution Time of Flight of the Circle of Willis0.2 x 0.2 x 0.5 mm, 148 slices

High resolutionVenous BOLD usingSENSE0.5 x 0.5 x 0.5 mm,200 slices

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2D CSI (TE 144 ms) using SENSE to reduce scantime, 4:32 minCho / Cr / NAA / Lip ratios differ in lesions compared to normal tissueCourtesy: University of Michigan, Ann Arbor, MI, USA

IViewBold processing package for functional studiesMotor task experiment on a patient with a large meningiomaCourtesy: Erasme University Hospital, Brussels, Belgium

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Quantitative Flow in a breath hold with color overlay2.0 x 1.3 x 8.0 mm, 40 phases, 16 sec

Where is MRI going? MR is diversifying-Dedicated systems for head, cardiac, extremities

Open systems for MR guided surgery already existHIGH FIELD – METABOLIC MEDICINE

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Cost-Reduction Through Technology

Low-cost, dedicated MRI unitsare now on the market, and arebeing developed for manyapplications

Other technologies in futurecould lead to further reductions

Prepolarized MRIHigh-Tc superconductors

4.7T40cm

MRI at A&M

4.7T33cm

geeks

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Evolution of MRI

clinical/anatomical/hospital MRI

Lab-on-a-chipBench-to-bedside

MRI analysis`

Bloch

Purcell

Nuclear Magnetic Resonance

1946

Magnetic Resonance Imaging

Clinical MRI at1.5T => SNR

Proton DensityT1 weightingT2 weighting

ChemicalSeparation“FATSAT”

FlowMRA

FastGradients

BOLD fMRI

DCE MRI

High Fields &Parallel MRI

Ernst2D localization

DamadianRelaxation times

2000

“Real Time” MRI

CSI

quantitativediagnosticclinical MRI

AnalyticalNMR

LauterburSpatial

localization

MansfieldHuman MRI

1992

1978

1984 1988 2004

1973 1982

MRI at A&M

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In vivoExperimentSetting

Warmed Knockdownchamber

Isofluraneevaporator

Water pumpand heater

Heatedsurgical bed

NoseconeLizard Lamp

Scavenging filter

FanControl heater

Maincontrolmodule

Respiratorymodule

Simulator

ECG/tempmodule

Nonmagnetic oxygen tank

Nonmagneticisofluraneevaporator

Scavenging filter

Loadingcoil/animalholder

MRI at A&M

Fig 2: MR images of the head region of the rat obtained with a 4.7Tesla/33 cmdiameter scanner interfaced to a Varian Inova console. Shown here are axialscans of 4 slices (1.5mm) taken after injection of Gd-DTPA contrast agent. Theimaging parameters were: TR = 500 msec, TE = 8 msec, 2 averages, field ofview: 50mm x 50 mm, resolution: 256x256, imaging time: 4 min and 22secs. Animals were anesthetized with injectable anesthesia and contrast dyewas injected via an in-dwelling tail vein catheter.

A

DC

B

MRI at A&M

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ISBI & ISMRM 2013 abstracts:U Mich (3T) and UTSW (7T) collaborations

Figure 3: Eight gradient echo images acquired on aGE 3T clinical scanner with loop array transmit coil.Only one channel was used to transmit for eachimages Inter-element coupling is well suppressed.

Courses

• ECEN 411 – Introduction to MRI – Lab (3)

• ECEN 648 – Principles of MR Imaging (3)

• ECEN/BMEN 427/627 – MR Engineering Lab (3)

• ECEN 617 – Advanced Signal Processing for MRI

• BMEN 489/689 – Biomedical Electromagnetics