Week 10 A Magnetic Resonance Imaging

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Week 10 A Magnetic Resonance Imaging

• Material from Clinical Imaging with Skeletal,
  Chest and Abdomen Pattern Differentials
• by Dennis M. Marchiori

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Magnetic Resonance Imaging

• Developed from Nuclear Magnetic Resonance used in
  laboratories to evaluate composition of
  laboratory samples.
• Raymond Damadien used the technology to produce a
  crude image of a rat tumor in 1974. He produced
  an image of a full body in 1977. His machine is
  now in the Smithsonian.

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MRI Benefits

• MRI has superior tissue contrast compared to
  computed tomography and radiography.
• Tissue contrast in CT is based upon attenuation
  properties where MRI uses properties of tissue
  nuclei, principally hydrogen. This information is
  more sensitive than x-ray attenuation.

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MRI Benefits

• MRI does not use ionizing radiation so it is not
  associated with ionizing radiation hazards.
• In general the parameters of MRI are without
  significant health risks but it has not been
  studies enough to assume that it is absolutely
  safe.

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MRI

• MRI used a strong magnetic fields and
  radiofrequencies to analyze the magnetic spin
  properties of hydrogen nuclei.
• Principle components of the MRI scanner include
• A large homogeneous magnetic field
• Gradient Magnetic coils
• Radiofrequency coils
• Computer system.

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MRI

• Looks like a CT scanner. The gantry is much
  longer than a CT unit. The machine is composed
  of
• A gantry containing the primary magnet
• A couch for the patient
• A computer system

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Types of Magnets

• There are three principle types of magnets used
  to generate the magnetic fields needed for MRI.
• Superconducting magnets
• Permanent magnets
• Resistive electromagnets

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Superconductive Magnets

• These magnets consist of a primary magnetic coils
  that are super-cooled by cryogens such as liquid
  helium or liquid nitrogen.
• Super-cooling dramatically reduces electrical
  resistance.

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Superconductive Magnets

• At infinite conductivity, the primary coils no
  longer requires a power supply.
• The magnetic field can be disrupted only by
  ramping down the magnet by excelling the cryogens.

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Superconductive Magnets

• The operating cost is high due to cost of
  cryogens.
• Able to make higher field strengths 1.5 T and
  higher for better signal to noise ration and
  greater detailed images.
• A problem is the large fringe field that can
  effect medical pacemakers and other devices.

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Permanent magnets

• Permanent magnets are constructed of bricks of
  ferromagnetic material.
• As a result, they can be constructed as open
  units.

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Permanent magnets

• They cannot obtain the high field strengths of
  super conductive units.
• Typical field strength is 0.2 tesla or less.
• Less problems with claustrophobia and smaller
  fringe field are the advantages.

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Resistive Electromagnets

• This design uses the classic electromagnet design
  where large amounts of power is conducted through
  coil loops of wire.
• Units use massive power consumption.
• Low to middle field strength 0.3 tesla or less.

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Field Strength

• Low field strength magnets are less than 0.2
  Tesla.
• Middle field strength is between 0.2 and 0.6
  Tesla
• High field strength is 1.0 Tesla and above.
• Field strength allows for higher resolution and
  improved signal to noise ratio. Faster imaging is
  possible.

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Field Strength

• Computer software and imaging sequence
  improvements have made low to middle field
  strength more competitive.
• The open MRI units are low to middle field
  strength units with permanent magnets. This
  design is often preferred by patients.
• High field strength machine are super conducting
  design so they have a long bore that large
  patients find very restrictive.

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Gradient Magnetic Coils

• Gradient magnets are located inside the gantry
  bore and allow for slicing the patients anatomy
  along the sagittal, coronal or transverse planes.
• They switch on and off very rapidly during the
  exam resulting in the quite load tapping noise of
  the examination.

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Radiofrequency Coils

• Radiofrequency (RF) coils are placed close the
  the area of the body being imaged.
• They transmit and receive RF information
  pertaining to the location of the hydrogen
  nuclei.
• They come in various designs to most
  appropriately image the region of anatomy in
  question.

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Image Production

• MR image production is based upon the systems
  ability to spatially localize hydrogen atoms
  within body tissue.
• In the nucleus are charged particle that generate
  small magnetic fields like tiny bar magnets.
  Normally they are randomly oriented so their
  magnetic fields cancel out.
• Hydrogen is useful because 80 of the atoms are
  hydrogen.

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Image Production

• The MR unit creates a strong magnetic field.
  Field strength is measured in gauss and tesla.
• 10,000 gauss 1 Tesla
• Earth gravity is about 0.5 g. A 1.5 T magnet
  produces 30,000 times the strength of the
  earths magnetic field.

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Image Production

• When the patient is placed inside the strong
  magnet field, some of the hydrogen atoms align
  with themselves with the magnetic field.
• Hydrogen atoms can not be statically polarized,
  rather they wobble like a top.

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Image Production

• The wobble is called precession.
• A linear relationship exists between the
  frequency of precession and the strength of the
  magnetic field called the gyromagnetic ratio.
• It is measured in MHZ/T.

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Image Production

• The relationship between the gyromagnetic ratio
  and magnetic field strength is described by the
  Larmor equation and is the basis of MRI.
• Frequency of precession gyromagnetic ratio time
  the strength of the external magnetic field.

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Image Production

• To obtain images, an RF identical to the Larmor
  frequency of precession is pulsed into the
  patients body.
• The RF is precisely the frequency of precession
  of the hydrogen atom and will excite the atom.
• This is the concept of resonance.

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Resonance

• This is similar to using an appropriately tuned
  tuning fork and a guitar.
• When struck, the vibration of the fork is
  propagated through the air to cause selective
  vibration of the correct string of the guitar.

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Resonance

• With the pulses transmitted by RF coils the
  hydrogen proton magnetic fields deviate from the
  plane of the main magnetic field with the same
  phase.

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MR Image Production

• When the RF is turned off, the excited nuclei
  undergo longitudinal relaxation back to the
  parallel plane of the magnet.
• During relaxation, the accumulated energy is
  released in the form of RF.

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MR Image Production

• The RF is detected by the surface coil acting as
  an antenna.
• The RF signal received is used to reconstruct an
  image.

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MR Image Production

• The dynamics of MRI can be summarized in four
  steps.
• Resting
• Magnetism
• Excitation
• Relaxation

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Imaging Techniques

• In MRI, image appearance is manipulated by
  controlling the timing of the RF pulses sent to
  the patient (repetition time TR) and the echo of
  the signal from the patient (echo time TE).
• The most common is the spin-echo sequence.
  Typically the are designed to flip or reorient
  the hydrogen magnetic field vectors 90 to 180
  degrees.

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Imaging Techniques

• The appearance of the image reflects the
  intensity of the emitted signal from the body.
• High signal strength appears bright
• Low signal strength appears dark.
• Signal intensity is dependent upon the population
  of hydrogen atoms and the environment in which
  they are found.

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Imaging Techniques

• How the atoms are bound also influences the
  signal.
• Tightly bound atoms like ligament emanates
  minimal signals.
• Loosely bound atoms like within fluid has the
  potential to emanate bright signals.

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Imaging Techniques

• The ability to evaluate Hydrogen atoms in varying
  chemical and structural environments is
  accomplished by evaluating T1 and T2 relaxation
  times.
• T1 reflects a short TR and TE
• T2 reflects a long TR and TE
• Spin Density has a Long TR and a short TE .

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Imaging Techniques

• T1 Hydrogen atoms in fat appear bright.
• T2 Hydrogen atoms in water appear bright.
• Both fat and water appear bright in spin density
  sequences.

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Imaging Techniques

• Alternative sequences are available and used to
  enhance certain clinical circumstances to enhance
  delineation of pathologic processes. Examples
  are
• Gradient echo where the protons are flipped less
  than 90 degrees. As the flip angle tend toward 0
  degrees, images creates an increase in the signal
  intensity from fluids.
• STIR( short tau inversion recovery) is used to
  suppress fat signals while maintaining water and
  soft tissue. Good for bone marrow.

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Typical Lumbar Spine Exam

• Starts with a scout coronal image of the area.
  Localized line are placed on this image noting
  the subsequent sagittal T1 and T2 sequences.
• The sagittal sequences extend from the left to
  right neuroforamina or depending upon the
  radiologists preferences.
• The direction always needs to be checked.

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Typical Lumbar Spine Exam

• A middle sagittal image is used to plan the
  subsequent axial images. The axial images can be
  printed in to formats
• Axial with each slice parallel to the disc
  interspaces for L3, L4 and L5.
• Continuous set of sequences extending from L3 to
  S1.

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Bright Signal Tissue Types

• T1
• Fat
• Yellow bone marrow
• White matter of brain

• T2
• Cerebrospinal fluid-water
• Cysts
• Edema
• Normal nucleus pulposus
• Tumor

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Medium Signal Tissue Types

• T1
• Fluid
• Intravenous pyelogram
• Muscle
• Red bone marrow
• Spinal cord
• Tumor

• T2
• Dehydrated nucleus pulposus
• Fat
• Gray matter of brain
• Muscle
• Spleen

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Dark Signal Tissue Types

• T1
• Air
• Calcification
• Cerebrospinal fluid
• Cortical bone
• Fast moving blood
• Fibrous tissue
• Ligaments, tendons

• T2
• Air
• Calcification
• Cortical Bone
• Fast moving blood
• Fibrous Tissue
• Ligaments, Tendons

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Patient Preparation

• Exams take from 30 to 90 minutes. Average is 45
  minutes.
• Time varies with the type and number of sequences
  ranging from 2 to 10 minutes each.
• Patient must lie absolutely still for these long
  sequences.
• The referring doctor should be familiar with the
  facility so they can properly explain the
  procedure to the patient. Well informed patients
  are less anxious about the exam.

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Patient Preparation

• The patient should dress comfortably without any
  metal. It is very important that no ferrous
  metallic object go into the unit.
• Horrible accident have happened when metal goes
  into the magnet room. A young child was killed by
  an oxygen tank at Stanford a few years ago in
  their 1.5 T unit.

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Patient Preparation

• Credit cards should not be taken to the facility
  as the information may be erased by the magnet.
• The patient is placed on the MRI table.
• Surface coils are placed on the area being
  studies.
• The patient is the guided into the gantry.
• The technologist will tell the patient when the
  sequence will begin and how long it will last.

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Patient Preparation

• At this point, the patient will hear the sounds
  of the gradient magnets switching on a off. It
  will sound like load knocking noises or rhythmic
  beating noise emanating from the magnet.
• Many facilities provide earphone and a choice of
  music for the patient or noise suppression
  equipment.

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Patient Preparation

• Claustrophobia is the main complication of the
  MRI exam, occurring about 5 of the time.
• A mild tranquilizer may alleviate the problem.
• Sometime placing the patient prone in the scanner
  may help.

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Contraindications

• Patients with interventional surgeries and
  patients who work around metals are at risk of
  injury in the strong magnetic field.
• Patients with implanted electronic equipment such
  as pacemakers or ear implants.
• Surgical clips such as aneurysm clips.
• Heart valves

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Contraindications

• Any metal surgical clip
• Shrapnel
• Bone or joint replacement
• Tattoos or permanent eyeliner

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Contrast agents

• Paramagnetic agents such as gadolinium is used as
  contrast media. It produces strong relaxation
  that appears bright on T-1 weighted images.
• Used in MR angiography and in lumbar spine and
  brain exams.
• Does not contain iodine so reactions are rare.
  Recent report of kidney problems associated with
  gadolinium.

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End of Lecture

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