- "superparamagnetic" - augment the external magnetic field substantially - exhibit "self-magnetism" - can significantly distort the acquired signals

Ferromagnetic materials materials containing iron, cobalt, nickel

***Chapter 12 - Magnetic Resonance Basics Flashcards by Enrhico Rivera

The spectroscopic study of the magnetic properties of the nucleus of the atom

Nuclear magnetic resonance (NMR)

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An energy coupling that causes the individual nuclei , when placed in a strong external magnetic field, to selectively absorb, and later release, energy unique to those nuclei and their surrounding environment

Resonance

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A fundamental property of matter ; it is generated by moving charges, usually electrons

Magnetism

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Smallest entities of magnetism

Domains

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Number of magnetic lines of force per unit area; decreases roughly as the inverse square of the distance from the source

SI Unit: Tesla (T)
1 T = 10,000 G (gauss)

Magnetic field strength,B (also called the magnetic flux density)

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Earth’s magnetic field strength in mT

0.05 mT

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____ of the current in the coil determines the overall magnitude of the magnetic field strength

Amplitude

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Magnetic field lines extending beyond the concentrated field

Fringe fields

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Heart of the MR system

Magnet

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Performance criteria for magnets(3)

Field strength
Temporal stability
Field homogeneity

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Magnets which have a HORIZONTAL main field produced in the bore of the electrical windings, with the Z axis (B0) along the bore axis

Air core magnets

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Magnet with a vertical field, produced between the metal poles of a permanent or wire-wrapped electromagnet; Fringe fields are confined with this design

Solid core magnet

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Interact with main magnetic field to improve homogeneity over the volume used for patient imaging

Shim coils

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Exists within the main bore of the magnet to transmit energy to the patient as well as to receive returning signals

Radiofrequency (RF) coils

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Contains within the main bore to produce a linear variation of magnetic field strength across the useful magnet volume

Gradient coils

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Obtained by superimposing the magnetic fields of two or more coils carrying a direct current of specific amplitude and direction with a precisely defined geometry

Magnetic field gradient

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Describes the extent to which a material becomes magnetized when place in a magnetic field

Magnetic susceptibility

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3 categories of magnetic susceptibility

Diamagnetic, paramagnetic and ferromagnetic

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slightly negative susceptibility

- oppose the applied magnetic field, due to PAIRED electrons in the surrounding orbital electrons

Diamagnetic elements

*calcium, water and most organic materials (Carbon, Hydrogen)

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UNPAIRED electrons
slightly positive susceptibility
enhance the local magnetic field
no measurable self-magnetism

Paramagnetic materials

*molecular oxygen, deoxyhemoglobin, methemoglobin and gadolinium-based contrast agents

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“superparamagnetic”
augment the external magnetic field substantially
exhibit “self-magnetism”
can significantly distort the acquired signals

Ferromagnetic materials

materials containing iron, cobalt, nickel

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For a given nucleus , it is determined thru the pairing of the constituent protons and neutrons

Nuclear magnetic moment

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The principal focus for generating MR signals

Nucleus of the hydrogen atom, the proton

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Describes the dependence between the magnetic field and the angular precessional frequency

Larmor equation

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A stationary reference frame from the observer's point of view

Laboratory frame

A spinning axis system whereby the x'-y' axes rotate at an angular frequency equal to the lateral frequency

Rotating frame

Along the z direction, is the component of the magnetic moment parallel to the applied magnetic field, B0

Longitudinal magnetization *at equilibrium, the longitudinal magnetization is maximal and is denoted as M0, EQUILIBRIUM MAGNETIZATION

The component of the magnetic moment perpendicular to B0, Mxy, in the x-y plane

Transverse magnetization

Magnetic component of the RF excitation pulse

B1 field

Corresponds to the energy separation between the protons in the parallel and antiparallel directions

Resonance frequency

Considers the RF energy as PHOTONS (quanta) instead of waves

Quantum mechanics model

Represent the degree of Mz rotation by the B1 field as it is applied along the x'-axis (or the y'-axis) perpendicular to Mz

Flip angles

A damped sinusoidal electrical signal

Free induction decay (FID)

Caused by loss of Mxy phase coherence due to intrinsic micro magnetic inhomogeneities in the sample's structures - individual protons in the bulk water and hydration layer coupled to macromolecules process at incrementally different frequencies arising from the slight changes in local magnetic field strength

FID amplitude decay

Elapsed time between the peak transverse signal (e.g. Directly after a 90-degree RF pulse) and 37% of the peak level (1/e) - decay time resulting from INTRINSIC magnetic properties of the sample

T2 relaxation time

Contain mobile molecules with fast and rapid molecular motion

Amorphous structures (e.g. CSF , highly edematous tissues)

Decay time resulting from both INTRINSIC and EXTRINSIC magnetic field variations

T2*

Term describing the release of energy back to the lattice (the molecular arrangement and structure of the hydration layer), and the regrowth of Mz

Spin-lattice relaxation

Time needed for the recovery of 63% of Mz after a 90-degree pulse

FREE INDUCTION DECAY

T2 relaxation

RETURN TO EQUILIBRIUM

T1 relaxation

Effective in decreasing T1 relaxation time of local tissues

Gadolinium chelated with complex macromolecules

The period between B1 excitation pulses - T2 decay and T1 recovery occur in the tissues

Time of repetition (TR)

Time between the excitation pulse and the appearance of the peak amplitude of an induced echo, which is determined by applying a 180-degree RF inversion pulse or gradient polarity reversal at a time equal to TE/2

Time of echo (TE)

Time between an initial inversion/excitation (180 degrees) RF pulse that produces maximum tissue saturation and a 90-degree readout pulse

Time of inversion (TI)

A state of tissue magnetization from equilibrium conditions

Saturation

3 major pulse sequences that perform the bulk of data acquisition (DAQ) for imaging:

1. Spin echo (SE) 2. Inversion recovery (IR) 3. Gradient echo (GE)

Describes the excitation of the magnetized protons in a sample with a 90-degree RF pulse and production of a FID, followed by refocusing 180-degree RF pulse to produce an echo

Spin echo (SE)

Pulse which converts Mz into Mxy, and creates the largest phase coherent transverse magnetization that immediately begins to decay at a rate described by T2* relaxation

90-degree pulse

Applied TE/2, inverts the spin system and induces phase coherence at TE

180-degree RF pulse

___ is proportional to the difference in signal intensity between adjacent pixels in an image, corresponding to different voxels in the patient

CONTRAST

Designed to produce contrast chiefly based on the T1 characteristics of tissues, with de-emphasis of T2 and proton density contributions to the signal - short TR (to maximize the differences in longitudinal magnetization recovery during the return of equilibrium) - short TE (to minimize T2 decay during signal acquisition)

T1 weighted SE sequence

Preserves the T1 signal differences by not allowing any significant transverse (T2) decay

short TE

T1 signal intensity from highest to lowest...

Fat>White matter>Gray Matter>CSF

Relies mainly on differences in the number of magnetized protons per unit volume of tissue - long TR (to reduce T1 effects) - short TE (to reduce T2 influence in the acquired signals)

Proton density contrast weighting *(CSF>fat>gray matter>white matter) *typical PDW (TR: between 2,000 and 4,000 ms; TE: between 3 and 30 ms)

Sequence which achieves the highest overall signal intensity and the largest SNR

Proton density SE sequence

Generated from the second echo produced by a second 180-degree pulse of a long TR spin echo pulse sequence, where the first echo is proton density weighted, with short TE - long TR (reduce T1 differences in tissues) - long TE (emphasize T2 differences)

T2-weighted signal * CSF is bright, gray and white matter are reversed in intensity * typical T2-weighted sequence uses a TR of approx. 2,000 to 4,000 ms and a TE of 80 to 120

Emphasizes T1 relaxation times of the tissues by extending the amplitude of the longitudinal recovery by a factor of two

Inversion recovery (IR)

***page 427

Inversion recovery spin-echo(IR SE)

The delay between the excitation pulse and conversion to transverse magnetization of the recovered longitudinal magnetization

Inversion time (TI)

Produces "negative" longitudinal magnetization that results in negative (in phase) or positive (out of phase) transverse magnetization when short T1 is used

IR sequence

A pulse sequence that uses a very short TI and magnitude signal processing, where Mz signal amplitude is always positive

SHORT TAU INVERSION RECOVERY (STIR) - reduces distracting fat signals and chemical shift artifacts * typical STIR sequence uses: TI of 140 to 180 ms; TR of approx. 2,500 ms

Materials with short T1 have a longer signal intensity (the reverse of a standard T1-weighted image), and all tissues at some point during recovery have Mz=0

Bounce point or tissue null

Reduces CSF signal and other water-bound anatomy in the MR image by using a T1 selected at or near the bounce point of CSF o permit better evaluation of the surrounding anatomy

Fluid Attenuated Inversion Recovery (FLAIR)

Uses a magnetic field gradient applied in one direction and then reversed to induce the formation of an echo, instead of the 180-degree inversion pulse - a purposeful dephasing and rephasing of the FID

Gradient Echo (GE)

Magnetic field inhomogeneities and tissue susceptibilities caused by paramagnetic or diamagnetic tissues or contrast agents are emphasized in ___ imaging .

Major variable determining tissue contrast in GE sequences

Flip Angle

GRASS

Gradient recalled acquisition in the steady state

FISP

Fast imaging with steady-state precession

FAST

Fourier Acquired Steady State

SPGR

Spoiled Transverse magnetization Gradient Recalled echo (SPGR)

2 TE values in SSFP (Steady-State Free Precession)

1. Actual TE | 2. Effective TE

The time between the peak stimulated echo amplitude and the next excitation pulse

Actual TE

The time form the echo and the RF pulse that created its FID

Effective TE

3 major GE sequences

1. Coherent GE 2. Incoherent GE 3. SSFP 4. Balanced SSFP

Uses signals food the FID and the SE to produce the image, typically with contrast dependent on T2/T1 weighting and low contrast

Coherent GE

Eliminates the detection of the SE, thus, providing a means to generate T1-weighted contrast from the FID signal

Incoherent GE

uses the SE signal, which provides mainly T2-weighted contrast

SSFP

Uses both the gradient and stimulated echo to produce a T2/T1 weighting with symmetrically applied gradients in three dimensions

Balanced SSFP

Typical magnetic field strengths for MR systems range from _____

0.3 to 4.0 T