My Mri

Basic principles of MRI

Dr. Lysyy Oleg Assaf Harofeh Medical Center Imaging department

What is MRI? Magnetic resonance imaging (MRI) is an imaging technique used to produce images of the inside of the human body and based on the principles of nuclear magnetic resonance (NMR) – absorption and emission of energy in the radio frequency

History 1946- Felix Bloch (Stanford) & Edward Purcell (Harvard) discovered nuclear magnetic resonanceNobel Prize in 1952

History 

1971- Raymond Damadian shows tumors appear different than normal tissue (in vitro)

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1975 - Ernst suggests frequency & phase gradients and Fourier transform

History

1973-Lauterburgimaging of a mouse (in vivo)

History July 3,1977 Dr.Raymond Damadian – first human imaging

5 hours

History

History

How does it work?  

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Put subject in big magnetic field Transmit radio waves into the subject Turn off radio wave transmitter Receive radio waves retransmitted by the subject Convert measured RF data to image

1. Put subject in big magnetic field

Magnet The imaging magnet is the most expencive part of the system.  Most magnets are of the superconduction type 

Magnetic Intensity 

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The magnets in use today in MRI are in the 0.5-tesla to 3.0-tesla range( 5,000 to 30,000 gauss). Magnetic fields greater than 3 tesla have not been approved for use in medical imaging. More powerful magnets - up to 60 tesla are used in research. Earth's magnetic field is 0.5-gauss

Spin Physics 

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Spin is a fundamental property of nature like electrical charge or mass. A moving electric charge,be it positive or negative,produces a magnetic field The faster it move or the larger the charge,the larger the magnetic field it produces

Hydrogen 

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In nuclear magnetic resonance, it is unpaired nuclear spins that are of importance – 1H, 13C, 19F, 23Na Biological abundance of 1H is 63% Produces small but noticeable magnetic field The biggest sources of protons in the body are water and fat

Spin Physics In the absence of an external magnetic field, nuclear spins are randomized

Spin Physics When an external magnetic field is applied, the nuclear magnets line up parallel (low energy) or antiparallel (high

Spin Physics 

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Anti-parallel & parallel protons can cancel each other There are more parallel protons on the low energy level When we put the patient in the external magnetic field he has his own magnetic field

Precession Protons do not just lay in the external magnetic field but precess around the magnetic field lines

Larmor equation

ν= γ B0 ν

is precession frequency

B

is the strength of the ext. mag. Field 0

γ

is gyromagnetic ratio. For 1H, γ =42.58 Mhz/T

2. Transmit radio waves into the subject A brief RF signal ,whose frequency equals the frequency of wobble of certain protons ν ,knocks those protons out from low to high energy level Lower Higher

What happens? 

Some protons pick up energy and thus decrease the amount of longitudinal magnetization and produces a new transversal magnetization

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The protons get in synch, start precess in phase

What happens?

3. Turn off radio wave transmitter When RF signal ceases,photons snap back to low energy level ,emitting a RF signal of their own, that announces the presence of a specific tissue

What happens?

4. Receive radio waves from the subject Need a receive coil tuned to the same RF frequency as the exciter coil

5. Convert measured RF data to image Fourier transform is operation which converts function from time to frequency domains

T1 relaxation – flipped nuclei realign with the magnetic field

T2 - Transverse relaxation due to spin dephasing

T1 and T2 are constant and specific for every tissue Tissue Grey matter White matter Muscle CSF Fat Blood

T1(ms) 950 600 900 4500 250 1200

T2(ms) 100 80 50 2500 60 100-200

These relaxation time differences use to generate image contrast

T1 Weighting Image

Contrast is predominantly dependent

T2 Weighting Image

Contrast is predominantly dependent

Imaging Sequences  

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Generate tissue contrast Minimize artifacts Gradient Echo Spin Echo Fast Spin Echo Inversion Recovery

An example

MR signal intensities T2WI Solid mass

bright

PD/FLAI R bright

Cyst

bright

dark

dark

bright

bright

dark

gray

bright

bright

Subacute bright blood Acute & dark chronic blood Fat dark

T1WI dark

MRI of the Brain Sagittal

T1 Contrast TE = 14 ms TR = 400 ms

T2 Contrast TE = 100 ms TR = 1500 ms

Proton Density TE = 14 ms TR = 1500 ms

Contrast Agents Contrast agents are chemical substances introduced to the anatomical or functional region being imaged, to increase the differences between different tissues or between normal and abnormal tissue, by

Contrast Agents Gadolinium cause a reduction in the T1 relaxation time-increased signal time intensity on T1WI (appearing bright)

Brain Tumor T1

T2

Gd T1

Indications CNS : - demyelination disease -CVA -primary brain tumors  Spine: -demyelination disease -discopathy -cord tumors 

Indications Orthopedic: -ligaments & meniscuses evaluation -soft tissue & bone tumors 

Body MRI: -confirmation of uncertain diagnoses -extension of tumors 

Absolute Contraindications  

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Cardiac pacemakers Ferromagnetic or electronically operated stapedial implants Hemostatic clips (CNS) Metallic splinters in the orbit

Relative contraindications        

Insulin pumps and nerve stimulators Lead wires or similar wires Non-ferromagnetic stapedial implants Cochlear implants Prosthetic heart valves (in high fields, if dehiscence is suspected) Makeup and tattoos Claustrophobia Pregnancy

Possible hazards  

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Static magnetic fields Varying magnetic fields (gradient fields) Radiofrequency fields Noise

Pregnancy MR scanning should be avoided in the first three months of pregnancy.  MR imaging is indicated for use in pregnant women if other nonionizing forms of diagnostic imaging are inadequate, or if the examination provides important information which would otherwise require exposure 

Diffusion Weighted MRI 

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The process by which molecules or other particles intermingle and migrate due to their random thermal motion Most sensitive procedure to detect cerebral ischemia (cytotoxic edema) The diffusion coefficient of brain decreases within minutes of onset

Diffusion Weighted MRI Ischemic diffusional changes occur much earlier- within minutes Vs standard MR sequences within 3 hours

Perfusion-weighted imaging 

Perfusion-weighted imaging (PWI) is used to identify regions that are receiving enough blood supply to remain structurally intact, but not enough to function normally.

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The measurement of blood flow via MRI after injection of gadolinium contrast

Perfusion-weighted imaging The infarct area will show little or no perfusion at the core or central zone, and the penumbra will show decreased perfusion

Functional MR Imaging 

Detection of changes in cerebral blood oxygenation using gradient echo MR images, an effect termed “blood oxygen level dependent” (BOLD)

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These changes are secondary to neuronal activation and are linked to a local increase in cerebral blood flow

Functional MR Imaging Current benefit to clinical practice in the presurgical identification of the primary motor cortex and language cortex

MR Spectroscopy 

Metabolites in extremely small concentrations (parts per million) are measured after suppressing water and fat signals from a small tissue volume

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Important metabolites studied are NAcetylaspartate (NAA), creatine (Cr) and phosphocreatine, choline (Cho).

MR Spectroscopy 

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Diagnosis of tumors: Choline levels are increased and NAA levels are decreased in tumors Diagnosis of stroke: The measurement of N-acetylaspartate (NAA) signals directly correlate with neuronal loss secondary to ischemia

MR Spectroscopy

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Normal Tumor Necrotic tumor

MRA

MRA      

Detailed images of blood vessels and blood flow No risk of damaging an artery The procedure itself and the time needed to recover are shorter MRA is less costly than catheter angiography No exposure to X-rays Useful for patients prone to allergic reactions

MRI Advantages / Limitations 

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No exposure to ionizing radiation Three dimensional viewing High contrast imaging No known side effects

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More expensive than CT Long examination time Sensitivity to motion Bones are not visualized Very confined space: claustrophobia Not be used by

Thank you