mri-for-beginners

M -AGNETICR -ESONANCE I -MAGING

CONTENT

• PHYSICS

• SAFETY

• APPLICATIONS

• QUESTIONS

PHYSICS

TYPES OF ATOMIC MOTION

1. The electron orbits the nucleus

2. The electron spins on its own axis

3. ***The nucleus spins on its own axis***

MRI USES THE HYDROGEN ATOM

•1 electron orbits the nucleus•The nucleus contains no neutrons but contains 1 proton

THE HYDROGEN NUCLEUS HAS A NET POSITIVE CHARGE

•Hydrogen nucleus is a spinning, positively charged particle

LAW OF ELECTROMAGNETISM

•A charged particle in motion will create a magnetic field•The postitively charged, spinning hydrogen nucleus generates a magnetic field

WHY HYDROGEN?

•Very abundant in the human body-H20•Has a large magnetic moment

MAGNETIC MOMENT

The tendency of an MR active nuclei to align its axis of rotation to an applied magnetic field

MR ACTIVE NUCLEIodd # protons

orodd # neutrons

or BOTH

e.g. Hydrogen1, Carbon13, Nitrogen15, Oxygen17, Fluorine19, Sodium23, Phosphorus31

STABLE ATOMS # protons = # electrons

IONS# protons # electrons

When a body is placed into the bore of the scanner, the strong magnetic field will cause the individual hydrogen nuclei to either:A) ALIGN ANTI-PARALLEL TO THE MAIN MAGNETIC FIELD

(B0)OR

B) ALIGN PARALLEL TO THE MAIN MAGNETIC FIELD (B0)

B0NMV

Anti-parallelhigh energy

Parallellowenergy

NET MAGNETIZATION VECTOR

• An excess of hydrogen nuclei will line up parallel to B0 and create the NMV of the patient

N

S

N

S

direction

size

The magnetic vector

THE NUCLEI WILL ALSO PRECESS…

PRECESSION• Due to the influence of

B0, the hydrogen nucleus “wobbles” or precesses (like a spinning top as it comes to rest)

• The axis of the nucleus forms a path around B0 known as the “precessional path”

Hydrogennucleus

B0

Precessional path

PRECESSION• The speed at which hydrogen precesses depends

on the strength of B0 and is termed the “precessional frequency”

• The precessional frequency of hydrogen in a 1.5 Tesla magnetic field is 63.86 MHz

• The precessional paths of the individual hydrogen nucleus’ is random, or “out of phase”

WE NEED THEM TO BE “IN-PHASE” OR TO RESONATE…

RESONANCEOccurs when an object is exposed to an oscillating perturbation that has a frequency close to its own

natural frequency of oscillation

•Ella Fitzgerald•Tacoma Narrows bridge failure

RESONANCE con’t

• Frequency of the hydrogen proton in a 1.5T magnetic field can be found in the RF band of energy in the electromagnetic spectrum

RADIOFREQUENCYENERGY

• Follows the Law of Electromagnetism (charged particles in motion will generate a magnetic field)

• Magnetic field known in MR as B1

• Applied as a “pulse” during MR sequences

• The RF pulse is applied so that B1 is 90 to B0

DURING RESONANCE…1) The hydrogen atoms begin to precess “in phase”

1)

2) The hydrogen atoms align with the RF’s magnetic field (B1) and they flip!!

B0 B0

B1 B1

NMV NMV flips!RF

PULSE

AS THE NUCLEI PRECESS IN-PHASE IN THE B1 PLANE, A CHANGING MAGNETIC FIELD IS

CREATED

IF YOU PLACE A RECEIVER COIL (ANTENNA) IN THE PATH OF THE CHANGING MAGNETIC

FIELD, A CURRENT WILL BE INDUCED

THIS IS FARADAY’S LAW OF INDUCTION

FARADAY’S LAW OF INDUCTION

A changing magnetic field will induce an electrical current in any conducting medium

COILSUsed to:

•transmit pulses of radiofrequency energy•receive induced voltage - MR SIGNAL•increase image quality by tuning in to one body part at a time

RELAXATION

When the RF pulse is turned “off”, the NMV “relaxes” back to B0 (away from B1)

B0

B1

NMV

•RF pulses are applied very quickly in succession - RF PULSE SEQUENCE•3 minute sequence (20 slices, axial brain) - 60 RF pulses may be applied

MR SIGNAL

• Collected by a coil

• Encoded through a series of complex techniques and calculations (magic?)

• Stored as data

• Mapped onto an image matrix

TR - REPETITION TIME

Time from the application of one RF pulse to another RF pulse

TE - ECHO TIME

Time from the application of the RF pulse to the peak of the signal induced in the coil

T1 WEIGHTING•A short TR and short TE will result in a T1 weighted image•Excellent for demonstrating anatomy

T2 WEIGHTING•A long TR and long TE will result in a T2 weighted image•Excellent for demonstrating pathology

MANY OTHER DIFFERENT TYPES OF IMAGES THAT COMBINE ABOVE AND

INCLUDE OTHER PARAMETERS

T1 WEIGHTED IMAGE

T2 WEIGHTED IMAGE

SAFETY

THE MAGNET IS ON ALL THE

TIME!!!

OHM’S LAW OF RESISTANCE

V = IRV = voltage I = current R = resistanceR depends on the material, the length, the

cross-sectional area, and the temperature of the loops of wire through which the current

flows**Decreasing the temperature of the wire

will decrease resistance to the flow of electricity

SUPERCONDUCTING MAGNET

• No resistance to flow of electricity

• Coils of wire surrounded by cryogen bath (Helium) at -273 C

• No external source of energy required

• Magnetic field present ALL THE TIME!!!

Gauss - measure of magnetic field strength

refrigerator magnet - 150-250 G

10,000 Gauss = 1T

MRI - 0.2T - 1.5T 100x stronger that fridge magnet

THE STRONG MAGNETIC FIELD OF THE MAGNET CAN TURN THE

FOLLOWING INTO DANGEROUS PROJECTILES:

• coins• scissors• trauma boards• sandbags• safety pins

• wheelchairs• oxygen tanks• I.V. poles• I.D. tags• keys

•Monitoring equipment•Infusion pumps•Credit cards•Cellular telephones•Any electronic device

THE CHANGING MAGNETIC FIELDS CAN DO DAMAGE TO:

•Gold•Silver•Digital watches•Eyeglass frames•Snaps/zippers fastened to clothing•Dental work

THE FOLLOWING ARE (USUALLY*) OKAY:

APPLICATIONS

ADVANTAGES

• Superior soft tissue contrast resolution - excellent pathological discrimination

• No ionizing radiation

• Direct multi-planar imaging (transverse, coronal, sagittal, any oblique)

• Non-invasive - vascular studies can be performed without contrast

KNEE

ANGIOGRAPHIC TECHNIQUES

• Circle of Willis angiograms without any contrast

ANGIOGRAPHIC TECHNIQUES

•Studies using contrast can also be performed

RENAL MRA

GADOLINIUM

USEFUL FOR DETECTION OF:

• Tumours pre- and post-operative

• Infection

• Inflammation

• Post-traumatic lesions

• Post-operative changes

• MRA’s

DISADVANTAGES OF MRI

• Expensive

• Long scan times

• Audible noise (65-115dB)

• Isolation of patient (claustrophobia, monitoring of ill patients)

• Exclusion of patients with pacemakers and certain implants

BRAIN• Hemorrhage (stages of)• Demyelinating disorders (M.S.)• Infectious processes (encephalitis, meningitis)• Abscesses• Neoplasms• Neurofibromatosis• Trauma• Vascular disorders (AVM’s, infarcts, aneurysms)

BRAIN (cont’d)

• Metastasis• Internal auditory canal pathology• Pituitary pathology• Hydrocephalus• Child abuse• Cranial nerve pathology• Congenital anomalies (for anatomical review)• Epilepsy (seizures in general)

AXIAL T2 BRAIN

SPINE

• Radiculopathy• Tumours• Trauma/contusion• Syringomyelia• Metastasis• Vascular disorders• Cord edema• M.S. plaques

SPINE (cont’d)

• Cauda equina syndrome

• Tethered cord

• Arachnoiditis

• Marrow-replacing processes

• Degenerative disc disease

• Discitis

• Congenital anomalies

SPINE

MUSCULOSKELTAL(shoulder, knee, ankle, wrist, elbow, TMJ)

• Meniscal pathology• Ligament/tendon injury• Muscle/nerve impingement• Avascular necrosis• Labral tears (shoulder, hip)• Chondromalacia• Inflammation (osteomyelitis)• Primary bone tumours• Soft tissue tumours

SHOULDER

ABDOMINAL/PELVIC

• Liver pathology

• Kidney pathology

• Renal artery MRA

• Fetal abnormalities

ABDOMINAL IMAGING

• Breath-hold scans to overcome motion artifact problem

• MRCP’s - images of the biliary and pancreatic ductal systems performed non-invasively (no contrast or endoscope!) within seconds

• Fetal imaging very diagnostic

MRCP

FETAL BREATH-HOLD IMAGE

FETAL ENCEPHALOCELE

CARDIAC

• Co-arctation

• RV dysplasia

• Cinematic studies

• Measure cardiac output, stroke volume, ejection fraction

MR Spectroscopy (MRS)• Information obtained is in the form of a spectrum

which provides the biochemical information contained within a selected voxel of tissue

• Used to detect the absence or presence of a certain compound

• Assists in differential diagnosis when standard clinical radiological tests fail or are too invasive

Spectrum

MRS Current Applications

• Multiple Sclerosis• Leigh’s• Huntington’s• Parkinson’s• Alzheimer’s• Epilepsy• other dementias• metabolic disorders

• Stroke• Asphyxiation or

ischemic injury• Tumours and

intracranial lesions• Prostate cancer• Encephalopathies• Leukodystrophies

Functional MRI (fMRI)• research topic

• Detects changes in blood flow or metabolism associated with specific motor or sensory functions or stimuli

• Performed by scanning specific areas of the brain/spine while: a) the subject performs a certain motor task or b) exposing the subject to certain external/internal stimuli

fMRI cont’d

• Subjects are scanned at rest and then during exercise or exposure to various stimuli

• The two conditions are subtracted to reveal areas of brain activation

• Areas of activation will have increased levels of blood flow and are therefore detectable

fMRI cont’d

• Mapping of the brain’s motor and sensory areas

• Delineating primary cortical areas prior to surgery on patients with tumours (to avoid paralysis when operating on tumours in dangerous locations)

• Assessment of brain function following injury

MANY OTHER WORKS IN PROGRESS…

QUESTIONS?????