Mri physics ii

D R. A RC H A N A KO S H Y

MRI PHYSICS II

OVERVIEW

1. MRI Instrumentation 2. Sequences 3. Artefacts 4. MR safety

RADIOFREQUENCY COIL • RF coils are used to transmit RF pulse into the

patient and to receive the signals from the patient .

• Energy is transmitted in the form of short intense bursts of radiofrequencies known as RF PULSES .

• Causes phase coherence and flip some of the protons from a high energy to a low energy state .

• Rotating TM vector induces current in the receiver coil ,which forms signal .

SURFACE COILS • Surface coils are the simplest design of coil.

• A loop of wire, either circular or rectangular, that is placed over the region of interest

• The depth of the image of a surface coil is generally limited to about one radius.

• Commonly used for spines, shoulders, temporomandibular joints, and other relatively small body parts.

PAIRED SADDLE COIL

• Commonly used for imaging of the knee.• Provide better homogeneity of the RF in the area of interest and

are used as volume coils, unlike surface coils. • Paired saddle coils are also used for the x and y gradient coils. • By having current flow in opposite directions in the two halves of

the gradient coil, the magnetic field is made stronger near one and weaker near the other.

• The Helmholtz pair coils consist of two circular coils parallel to each other. They are used as z gradient coils in MRI scanners.

• They are also used occasionally as RF coils for pelvis imaging and cervical spine imaging.

HELMHOLTZ COIL

BIRDCAGE COIL• Provides the best RF homogeneity of all the RF coils.• Has the appearance of a birdcage.• This coil is commonly used as a transceiver coil for imaging

of the head. • This type of coil is also used occasionally for imaging of the

extremities, such as the knees.

MAGNETS • Magnetic susceptibility Is the ability of the substance to get

affected by external magnetic field .

1. Paramagnetism –Unpaired electrons within the atom -Results in a small magnetic field around them called magnetic moment . -Gadolinum , Oxygen,Melanin

2. Diamagnetism- React in opposite way when external magnetic field is applied .

3. Ferromagnetism- Strongly attracted to a magnetic field . (Fe,Co,Ni)

-Used to make permanent magnets

PERMANENT MAGNETS

• Do not require power supply and are of low cost . • Magnetic field is directed vertically .

• Open MRI is possible with permanent magnets , useful for claustrophobic patients .

• Magnetic field strength achievable with permanent magnet is low in the range of 0.2-0.5 Tesla .

• Low Spatial noise resolution hence higher applications like spectroscopy cannot be done on them .

ELECTROMAGNETS • Based on the principle of

electromagnetism –Moving electric charge induces a magnetic field around it .

• When a wire is looped like a spring coil , the magnetic field generated is directed along the long axis of the coil.

• Capital cost is low ,operational cost

is high for electromagnets because enormous power is required .

• Easy to install and can be readily turned on and off inexpensively.

SUPERCONDUCTING MAGNETS

• Higher field strength is achieved by eliminating resistance.• Once the wires/coils are energized, the current continues in a

loop as long as it is maintained below the critical temperature.• No power loss • Continuous power supply is not required to maintain a

magnetic field .

SEQUENCES

SPIN ECHOThe MR scanner can only detect signal in the transverse plane.

• There are two disadvantages of this approach:

(1) The signal decays very rapidly, requiring an extremely fast scanner to detect it before it dies out

(2) The signal depends on T2*, not T2, so it is very susceptible to local magnetic field inhomogeneity.

In order to combat both of these challenges, we can add another pulse to generate an "echo" that occurs later in time and happens to reverse the T2* effects to leave us with T2 only. This is referred to as the spin echo.

• 3 main phases

(1)90-degree pulse followed by free induction decay;

(2) 180-degree pulse followed by rephasing

(3) The echo that occurs at TE, when the scanner acquires the signal -Readout.

- We need to repeat this sequence a number of times to acquire the entire image.

- The time between each repetition of the sequence is referred to as the repetition time or TR

FORMATION OF A SPIN ECHO

• In terms of k-space representation of the spin-echo sequence, the application of phase encoding and read dephase gradients results in movement from the centre of k-space A to position B.

• The 180º pulse reverses the k-space position in both phase and frequency directions, resulting in movement from B to C.

• This is followed by frequency encoding from C to D via the centre of k-space.

• Each line of data is Fourier transformed to extract frequency information from the signal and the process is repeated for different phase encode steps.

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GRADIENT COILS• Induce small linear changes in magnetic field along one or more

dimensions.• Consists of three sets of coils that produce field with changing

strength in X,Y,Z directions . • Produces two types of spatial encoding referred to as Frequency

and Phase Encoding.

• Three basic differences between Spin echo and gradient echo sequence imaging :

1. There is no 180 degree pulse in GRE.-Rephasing of TM in GRE is done by gradients, particularly by reversal of frequency encoding gradient .

2. Flip angle in GRE is smaller,usually less than 90 degree .

GRADIENT ECHO SEQUENCE • Consists of a series of excitation pulses, each

separated by a repetition time TR.• Data is acquired at some characteristic time after

the application of the excitation pulses and this is defined as the echo time TE.-Time between the mid-point of the excitation pulse and the mid-point of the data acquisition.

INVERSION RECOVERY SEQUENCE• A variant of a SE sequence in that it begins with a

180º inverting pulse. • This inverts the longitudinal magnetisation vector

through 180º. • When the inverting pulse is removed, the

magnetisation vector begins to relax back to B0.

Uses of Inversion Recovery SequenceContrast is based on T1 recovery curves following the 180º inversion pulse. Inversion recovery is used to produce heavily T1 weighted images to demonstrate anatomy.

The 180º inverting pulse can produce a large contrast difference between fat and water because full saturation of the fat or water vectors can be achieved by utilising the appropriate TI.

STIR (SHORT T1 INVERSION RECOVERY ) • The inversion recovery sequences are spin echo sequences

with a 180° preparation pulse to flip the longitudinal magnetization into the opposite direction (i.e. spins are flipped to the 180° position).

• To generate an MR signal, the longitudinal magnetization is then converted to transverse magnetization through the application of a 90° pulse.

• The interval between the 180° pulse and the 90° stimulation pulse is known as inversion time (TI).

• As the spins relax back to their equilibrium configuration the signal for each spin group will evolve from a negative signal which is zero (null point) to a positive signal. This rate is determined by the T1 of the spin group.

• Since at 1.5 T, T1 fat= 260 ms and for most other tissues T1 > 500 ms,there fore the null point for the fat signal will occur sooner than the other tissues.

• Therefore an inversion recovery sequence with a short inversion time (TI) of 130-150 is used for fat suppression.

FAT SATURATION• Normal MR imaging methods visualize protons from both

water and fat molecules within the tissue.

• Fat and water have a chemical shift difference of approximately 3.5 ppm in their resonant frequencies.

• • Fat saturation uses a narrow-bandwidth rf pulse centered

at the fat resonant frequency applied in the absence of a gradient .

• The resulting transverse magnetization is then dephased by spoiler gradients.

• A standard imaging sequence may then be performed that produces images from the water protons within the slice.

• TWO main advantages over STIR imaging for fat suppression.

1. It may be incorporated into any type of imaging sequence.

2. T1 fat saturation sequences may also be used with gadolinium based T1 contrast agents since the contrast agent shortens the T1 relaxation times of only the water protons

3. Enables the enhanced tissues to generate significant signals while the fat signal remains minimal in the presence or absence of the contrast agent

PROTON DENSITY IMAGE • Tissues with the higher concentration or density of

protons produce the strongest signals and appear the brightest on the image.

• Proton density weighted sequence produces contrast mainly by minimizing the impact of T1 and T2 differences with long TR (2000-5000ms) and short TE (10-20).

DIFFUSION WEIGHTED IMAGING (DWI)

• MR imaging based upon measuring the random Brownian motion of water molecules within a voxel of tissue.

• Densely cellular tissues or those with cellular swelling exhibit lower diffusion coefficients

• Particularly useful in tumour characterisation and cerebral ischaemia

• The motion of these water molecules can be restricted by the presence of barriers, principally cell membranes.

• The degree of diffusion restriction can be quantified by a diffusion coefficient, which reflects the average distance a particle will move in a second

• Typical DWI sequences are spin echo sequences, with 90- and 180-degree pulses.

• The diffusion gradients are turned on before and after the 180-degree pulse.

• DWI sequences need to be extremely fast in order to eliminate any motion within the body part .

• The TR is long in order to reduce T1 effects and improve signal.

• The TE is kept as short as possible, but the insertion of the diffusion gradient after the 180 pulse necessitates a longer TE.

• Therefore, DWI images are also T2 weighted .

FLAIR ( FLUID ATTENUATED INVERSION RECOVERY )

• Another variation of the inversion recovery sequence.

• The signal from fluid e.g. cerebrospinal fluid (CSF) is nulled by selecting a TI corresponding to the time of recovery of CSF from 180º inversion to the transverse plane.

• The signal from CSF is nullified and FLAIR is used to suppress the high CSF signal in T2 and proton density weighted images so that pathology adjacent to the CSF is seen more clearly.

USES :1. To assess the extent of

perilesional edema 2. Infarcts are better

appreciated 3. Bright lesions of Multiple

Sclerosis 4. Increased signal in mesial

temporal sclerosis 5. Fast FLAIR – subarachnoid

haemorrhage

ARTEFACTS

1. MR HARDWARE AND ROOM SHIELDING

• Zipper artifact• Zebra stripes• Moire fringes• Central point artifact• RF overflow artifacts• Inhomogeneity artifacts• Shading artifact

2. MR SOFTWARE• Slice-overlap artifact- cross-

talk artifact• Cross excitation

3. PATIENT AND PHYSIOLOGIC MOTION• Phase-encoded motion artifact

4. TISSUE HETEROGENEITY AND FOREIGN BODIES• Black boundary artifact• Magic angle effect• Susceptibility

artifact/magnetic susceptibility artifact

• Chemical shift artifact

5. FOURIER TRANSFORM AND NYQUIST SAMPLING THEOREM• Gibbs artifact/truncation

artifact• Aliasing/wrap around

artefact

GHOSTS • Replica of a structure in the image .

• Produced by anatomy moving along a gradient during pulse sequence resulting into phase mismapping .

• Can originate from any structure moving during the acquisition of data .

• Always occur along the phase encoding axis

• Can be caused by mechanical vibration ,temporal variation or receiver coil sensitivity ,fluctuations of magnetic field and simulated echoes .

Appearance of ghosting on final clinical image depends on where in k-space such phase errors occur:

• If along the x-axis of k-space : Frequency encoding direction

• If along y-axis : Phase encoding direction

• If in middle of k-space : Smearing appearance

• If phase errors are periodic (as in pulsatile motion)-Periodic ghosting.

Relatively more common in the phase encoding direction.

ALIASING/WRAPAROUND • In aliasing, anatomy that

exists outside the field of vision appear in an image . • Produces a signal if it is in

close proximity to the receiver coil • During signal

encoding ,signals from this outside field of view structures are also allocated pixel positions . • Aliasing along frequency

encoding axis is called frequency wrap • Along phase encoding axis is

called phase wrap

CHEMICAL SHIFT RELATED ARTEFACTS • Due to the different chemical environments , protons

in water and fat precess at different frequencies –CHEMICAL SHIFT

• Frequency of water protons is about 3.5 ppm greater than that of fat protons .

• Chemical shift forms the basis of MR spectroscopy ,however some chemical shifts become source of artefacts in MR imaging .

(i) Chemical shift misregistration artefact (ii) Interference from chemical shift (in phase/out phase)

TRUNCATION ARTEFACTS • Produce low intensity bands running through a high

intensity area .

• Caused by under sampling of the data so that interfaces of high and low signals are incorrectly represented on the image .

• Can be misleading in long narrow structures, such as spinal cord or intervertebral disc .

GIBBS’ ARTEFACT : In T1 weighted sagittal image of the cervical spine,CSF in central canal appear dark as compared to spinal cord May be misinterpreted as syringomyelia

MAGNETIC SUSCEPTIBILITY ARTEFACT

• These artefacts results from local magnetic field inhomogeneities introduced by a metallic object into the otherwise homogeneous external magnetic field B0.

• These local magnetic field inhomogeneities are known as magnetic susceptibility and are a property of the object being imaged

• More prominent in Gradient echo sequence than Spin echo sequence .

• Corrected by removing all metals and use of soin echo sequence .

05/03/2023 MRI artifacts-sudil 47

ZIPPER ARTIFACTS• Lines with alternating bright and dark pixels

propogating along the frequency encoding direction . • Can be eliminated by spoiler gradients in special

pattern to remove simulated echoes .


CROSS-EXCITATION ARTIFACTS• The imperfect shape of RF slice profiles leads to the

unintended excitation of adjacent tissue.• This excitation results in the saturation of such tissue• Manifest as decreased signal intensity and decreased

contrast that can hinder lesion detection.


SPIKE ARTIFACT• Caused by one defective data point in k-space. • The resulting image show diagonal lines

throughout the image.


ZEBRA STRIPES• Observed along the periphery

of gradient-echo images (abrupt transition in magnetization at the air-tissue interface)

• Increased by aliasing that results from the use of a relatively small field of view.

• May also occur when pt. touches the coil or a result of phase wrap.

CORRECTIVE MEASURES • Expanding the FOV, using SE

pulse sequences.• Using oversampling techniques

to reduce aliasing.


RF OVERFLOW ARTIFACTS (CLIPPING)Causes a nonuniform, washed-out appearance to an

image.Occurs when the signal received from the amplifier

exceeds the dynamic range the analog-to-digital converter causing clipping.


MOIRE FRINGESAn interference pattern most commonly seen when doing

gradient echo images.One cause is aliasing of one side of the body to the other

results in superimposition of signals of different phases that add and cancel.

Can also be caused by receiver picking up a stimulated echo.

MRI SAFETY

FORCES IN THE MR ENVIRONMENT

• There are two types of effects the magnet will have on Ferromagnetic substances

• Translation: The “Missile Effect”

• Rotation/Torque: The “Rotational Effect”

TRANSLATIONAL FORCE: THE MISSILE EFFECT

• Also referred to as the “Projectile Effect” and is used to describe the attraction of the object to the center of the magnetic field

• This transforms objects into projectiles as they accelerate toward the magnet

• These items can become airborne, accelerating at speeds of up to 40 miles per hour.

• This effect causes accidents jeopardizing the safety of patients and staff, as well as the MRI equipment itself.

ROTATIONAL/TORQUE FORCE: ROTATIONAL EFFECT

• This force relates to the North and South pole orientation of the scanner’s magnetic field

• Ferrous objects will attempt to align their long axes with this orientation

• This force will rotate objects until they are aligned with the magnetic field.

• Translational and Rotational Force happen at the same time making objects projectiles and dangerous!

METAL OBJECTS BECOMING PROJECTILES

• All persons that have reason to enter the MR suite area should be trained in MR safety procedures. These include but are not limited to:

• MR technologists, researchers, research assistants• Research volunteers and test subjects• Maintenance and janitorial personnel

• Public safety forces (i.e. law and fire personnel) that would respond to the MR suite for an emergency must also know the potential hazards of the MR equipment.

ITEMS THAT CAN BE DAMAGED BY THE MAGNET

• The magnetic field can seriously damage or impair the following personal items:

• Cameras

• Watches

• Credit /Bank cards

• Hearing Aids

• Hair Accessories, Belt Buckles, Shoes

• I-pods

NOISE

• The MR scanner can produce very high acoustic noise levels.

• Some patients may experience discomfort from the associated noise of the scanner.

• Prior to scanning ear plugs and a head set is provided to the patient to reduce the noise level.

ABSOLUTE CONTRAINDICATIONS• Cardiac Pacemakers

• Cochlear (inner ear) implants

• Ferromagnetic or unidentifiable aneurysm clips of the brain

• Implanted neuro stimulators

• Metal or unidentifiable foreign bodies in the eyes

• Implanted pumps to deliver medicine that cannot be removed

• Full mouth braces or retainers that cannot be removed

This could be you!

FUNCTIONAL MRI • Hemoglobin has different

magnetic properties when bound to oxygen, that can be distinguished by fMRI.

• Areas of brain activity have a surge of oxygenated blood.

• fMRI can identify areas of the brain with high oxyhemoglobin content, which correlates to areas of heightened brain activity.

Mri physics ii

Healthcare

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