MR pulse sequences.

Dr/ ABD ALLAH NAZEER. MD.

MRI pulse sequences:An MRI pulse sequence is a programmed set of changing magnetic gradients. Each sequence will have a number of parameters, and multiple sequences are grouped together into an MRI protocol.

Pulse sequences can be broadly grouped as follows:Spin echo sequencesInversion recovery sequencesGradient echo sequencesDiffusion weighted sequencesSaturation recovery sequencesEcho-planar pulse sequencesSpiral pulse sequences

Functional techniques.Diffusion-Weighted imaging.Perfusion weighted imaging.Spectroscopy.fMRI.Diffusion tensor imaging (DTI). High angular resolution diffusion imaging (HARDI),

ParametersA pulse sequence is generally defined by multiple parameters, including:

Time to echo (TE)Time to repetition (TR)Flip angleField of view and matrix sizeInversion pulse(s)Spoiler gradient(s) (crusher gradients)Echo train length (ETL)The spatial acquisition of k-space3D acquisition vs. 2D acquisition vs. multiple overlapping slab acquisitionpost contrast imaging with gadolinium contrast agentsDiffusion weighting (b values)Different combinations of these parameters affect tissue contrast and spatial resolution.

Pulse Sequences.Spin Echo Pulse SequencesT1 weighted imagesShort TR(300ms-700ms).Short TE (10ms-30ms).T2 weighted ImagesLong TE(>80ms).Long TR(>2000ms).

Spin Echo(SE)(Conventional Spin Echo).RF pulse sequences with a 90° excitation rephrasing or refocusing pulse to eliminate field inhomogeneity and chemical shift effect.Main Points:90° excitation pulse.180°rephrasing pulse.

Gss (Slice select gradient).Gpe (phase encode gradient).Gfe (frequency encode gradient).

T1WI of the brain is obtainedby short TR and TE.

Images demonstrate good contrast between soft tissue types(because different tissues have different T1 values).Fat appears bright at the T1WI and the fluid appears dark.T1-weighted sequences provide the best contrast for para-magnetic contrast agent(e. g a gadolinium containing compound.

T2WI of the brain can be obtained by long TR(>1000ms) and Long TE(>60ms). Images demonstrate good contrast between normal tissue and pathology(because many pathologies have elevated T2 value due to increased free water content).Fat appears intermediate to bright at the T2WI and the fluid appears bright.

Pulse SequencesRapid Acquisition with Relaxation Enhancement Also Known as RARE

Fast Spin Echo(FSE)-GE, Toshiba and Hitachi.Turbo Spine Echo(TSE)-Siemens and Philips.One advantage is speed without loss of S/N.In CSE, if acquisition time is reduced by 50%, the S/N is reduced by 40%. Fast(Turbo) Spin EchoEcho Train Length(ETL) or Turbo factor(TF)Effective Echo Time(ETE)Echo Train Spacing(ETS).

GRE Sequences(GES).Generally uses an excitation flip angle (FA) of less than 90°degree and a gradient reversal to rephrase the protonsMain Points:Variable Flip Angle(FA)Gradient reversal.Advantage of Gradient Echo:Much shorter scan times the SE pulse sequencesLow FA allows for faster recovery of longitudinal magnetizationGradients rephrase faster than 180° RF pulsesTR and TE values are shorter than spin echo pulse sequences.Disadvantages of Gradient Echo:Susceptible to magnetic field inhomogeneitiesContain magnetic susceptibility artifacts.T2* weighting.

Gradient echo imagesof the brain are an alternative techniques to spin echo sequences,Differing from it in two principle points.Utilization of gradient field to generate transverse magnetization.Flip angle of less than 90 degree.It take short time than spin echo and it helpful in breath hold techniques . It is one of the fat suppression techniques. It is susceptible to the blood product and para-magnetic artifacts(Blooming effect).

Inversion Recovery sequences(IR).Inversion recovery pulse sequences are a type of MRI sequences used to selectively null the signal of certain tissues(e.g. fat or fluid).A basic IR spin echo pulse sequence consists of a 180° inversion pulse, followed by an inversion time T1, then a 90° RF pulse. The time elapsed between the preparatory 180° pulse and the90° readout pulse is termed time to inversion.By choosing the appropriate T1, suppression of different tissuesis possible:Short tau inversion recovery(STIR): fat is nulled.Fluid attenuated inversion recovery(FLAIR) fluid is nulled.In spin echo inversion recovery imaging sequences, the 90°pulse is followed by a 180° pulse in order to produce a spin echo at time TE following the 90° pulse.

Pulse SequencesEcho Planar Imaging(EPI)(single shot techniques).Allows data to be collected and reconstructed in less than a second.Stronger and faster gradients (slew rate) requiresData collected along GFE (readout) directionEcho-planar imaging is more vulnerable to magnetic susceptibility effects and provides greater tissue contrast than does imaging with standard GRE sequences; therefore, echo-planar imaging sequences are widely used to assess cerebral perfusion.

Functional techniques.Diffusion-Weighted imaging(DWI)• Technique used to measure the motion of water molecules.• Areas with increased diffusion within a tissue show greater

signal loss• Terms such as-b-value, Apparent diffusion coefficient(ADC).• Clinical application-acute cerebral infarcts(ischemic areas)

appear bright.• Perfusion-weighted image(PWI).• Technique used to measure the flow of blood through the capillary

region of an organ or tissue.• Dynamic susceptibility contrast(DSC) uses gadolinium as a tracer.• Arterial Spin labeling(ASL) noninvasive .• Terms-cerebral Blood Flow(CBF),Mean transit Time(MTT) and

cerebral Blood Volume (CBV).

Functional techniquesSpectroscopy(MRS)• Technique that provides chemical

information about a tissue.• Nuclei such as 1H, 31P, 13C, 23Na, 39K,19F. • Methods used:• Stimulated echo acquisition mode(STEAM).• Point-resolved spectroscopy(PRESS).• Image-selected in vivo Spectroscopy(ISIS).• Chemical shift imaging(CSI)a.k.a. magnetic

resonance spectroscopic imaging(MRSI).

Functional techniquesFunctional MRI(fMRI).Term used to described any technique that evaluates brain physiology rather than anatomy.fMRI specifically used to map brain activity .Areas of interest-motor cortex, visual cortex.Blood oxygenation level-dependant (BOLD).BOLD Blood Oxygen Level dependence fMRI I the most common for research purposes.Measures the hemodynamic response(amount of blood flow) related to neural activity in the brain.Increase in magnetic susceptibility when blood is oxygenated.

Direct correlation between brain activity and cerebral blood flow has been observed, but unsure of exact underlying mechanisms.Usually uses a T2* weighted contrast.TR: 2-4s.2-4 mm spatial resolution(better with 3-9 T).Although commonly thought of as a directed measure of brain activity, fMRI usually identifies relative differences in brain activity.fMRI is not currently used for diagnostic purposes.Used to identify functional maps of neural networks & research relative deficiencies in brain function.New software allows for the quantification of network activity via Independent Components Analysis.

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