Questions and Answers ​in MRI
  • Home
  • Complete List of Questions
  • …Magnets & Scanners
    • Basic Electromagnetism >
      • What causes magnetism?
      • What is a Tesla?
      • Who was Tesla?
      • What is a Gauss?
      • How strong is 3.0T?
      • What is a gradient?
      • Aren't gradients coils?
      • What is susceptibility?
      • How to levitate a frog?
      • What is ferromagnetism?
      • Superparamagnetism?
    • Magnets - Part I >
      • Types of magnets?
      • Brands of scanners?
      • Which way does field point?
      • Which is the north pole?
      • Low v mid v high field?
      • Advantages to low-field?
      • Disadvantages?
      • What is homogeneity?
      • Why homogeneity?
      • Why shimming?
      • Passive shimming?
      • Active shimming?
    • Magnets - Part II >
      • Superconductivity?
      • Perpetual motion?
      • How to ramp?
      • Superconductive design?
      • Room Temp supercon?
      • Liquid helium use?
      • What is a quench?
      • Is field ever turned off?
      • Emergency stop button?
    • Gradients >
      • Gradient coils?
      • How do z-gradients work?
      • X- and Y- gradients?
      • Open scanner gradients?
      • Eddy current problems?
      • Active shielded gradients?
      • Active shield confusion?
      • What is pre-emphasis?
      • Gradient heating?
      • Gradient specifications?
      • Gradient linearity?
    • RF & Coils >
      • Many kinds of coils?
      • Radiofrequency waves?
      • Phase v frequency?
      • RF Coil function(s)?
      • RF-transmit coils?
      • LP vs CP (Quadrature)?
      • Multi-transmit RF?
      • Receive-only coils?
      • Array coils?
      • AIR Coils?
    • Site Planning >
      • MR system layout?
      • What are fringe fields?
      • How to reduce fringe?
      • Magnetic shielding?
      • Need for vibration testing?
      • What's that noise?
      • Why RF Shielding?
      • Wires/tubes thru wall?
  • ...Safety and Screening
    • Overview >
      • ACR Safety Zones?
      • MR safety screening?
      • Incomplete screening?
      • Passive v active implants?
      • Conditional implants?
      • Common safety issues?
      • Projectiles?
      • Metal detectors?
      • Pregnant patients?
      • Postop, ER & ICU patients?
      • Temperature monitoring?
      • Orbital foreign bodies?
      • Bullets and shrapnel?
    • Static Fields >
      • "Dangerous" metals?
      • "Safe" metals?
      • Magnetizing metal?
      • Object shape?
      • Forces on metal?
      • Most dangerous place?
      • Force/torque testing?
      • Static field bioeffects?
      • Dizziness/Vertigo?
      • Flickering lights?
      • Metallic taste?
    • RF Fields >
      • RF safety overview?
      • RF biological effects?
      • What is SAR?
      • SAR limits?
      • Operating modes?
      • How to reduce SAR?
      • RF burns?
      • Estimate implant heating?
      • SED vs SAR?
      • B1+rms vs SAR?
      • Personnel exposure?
      • Cell phones?
    • Gradient Fields >
      • Gradient safety overview
      • Acoustic noise?
      • Nerve stimulation?
      • Gradient vs RF heating?
    • Safety: Neurological >
      • Aneurysm coils/clips?
      • Shunts/drains?
      • Pressure monitors/bolts?
      • Deep brain stimulators?
      • Spinal cord stimulators?
      • Vagal nerve stimulators?
      • Cranial electrodes?
      • Carotid clamps?
      • Peripheral stimulators?
      • Epidural catheters?
    • Safety: Head & Neck >
      • Additional orbit safety?
      • Cochlear Implants?
      • Bone conduction implants?
      • Other ear implants?
      • Dental/facial implants?
      • ET tubes & airways?
    • Safety: Chest & Vascular >
      • Breast tissue expanders?
      • Breast biopsy markers?
      • Airway stents/valves/coils?
      • Respiratory stimulators?
      • Ports/vascular access?
      • Swan-Ganz catheters?
      • IVC filters?
      • Implanted infusion pumps?
      • Insulin pumps & CGMs?
      • Vascular stents/grafts?
      • Sternal wires/implants?
    • Safety: Cardiac >
      • Pacemaker dangers?
      • Pacemaker terminology?
      • New/'Safe" Pacemakers?
      • Old/Legacy Pacemakers?
      • Violating the conditions?
      • Epicardial pacers/leads?
      • Cardiac monitors?
      • Heart valves?
      • Miscellaneous CV devices?
    • Safety: Abdominal >
      • PIllCam and capsules?
      • Gastric pacemakers?
      • Other GI devices?
      • Contraceptive devices?
      • Foley catheters?
      • Incontinence devices?
      • Penile Implants?
      • Sacral nerve stimulators?
      • GU stents and other?
    • Safety: Orthopedic >
      • Orthopedic hardware?
      • External fixators?
      • Traction and halos?
      • Bone stimulators?
      • Magnetic rods?
  • …The NMR Phenomenon
    • Spin >
      • What is spin?
      • Why I = ½, 1, etc?
      • Proton = nucleus = spin?
      • Predict nuclear spin (I)?
      • Magnetic dipole moment?
      • Gyromagnetic ratio (γ)?
      • "Spin" vs "Spin state"?
      • Energy splitting?
      • Fall to lowest state?
      • Quantum "reality"?
    • Precession >
      • Why precession?
      • Who was Larmor?
      • Energy for precession?
      • Chemical shift?
      • Net magnetization (M)?
      • Does M instantly appear?
      • Does M also precess?
      • Does precession = NMR?
    • Resonance >
      • MR vs MRI vs NMR?
      • Who discovered NMR?
      • How does B1 tip M?
      • Why at Larmor frequency?
      • What is flip angle?
      • Spins precess after 180°?
      • Phase coherence?
      • Release of RF energy?
      • Rotating frame?
      • Off-resonance?
      • Adiabatic excitation?
      • Adiabatic pulses?
    • Relaxation - Physics >
      • Bloch equations?
      • What is T1?
      • What is T2?
      • Relaxation rate vs time?
      • Why is T1 > T2?
      • T2 vs T2*?
      • Causes of Relaxation?
      • Dipole-dipole interactions?
      • Chemical Exchange?
      • Spin-Spin interactions?
      • Macromolecule effects?
      • Which H's produce signal?
      • "Invisible" protons?
      • Magnetization Transfer?
      • Bo effect on T1 & T2?
      • How to predict T1 & T2?
    • Relaxation - Clincial >
      • T1 bright? - fat
      • T1 bright? - other oils
      • T1 bright? - cholesterol
      • T1 bright? - calcifications
      • T1 bright? - meconium
      • T1 bright? - melanin
      • T1 bright? - protein/mucin
      • T1 bright? - myelin
      • Magic angle?
      • MT Imaging/Contrast?
  • …Pulse Sequences
    • MR Signals >
      • Origin of MR signal?
      • Free Induction Decay?
      • Gradient echo?
      • TR and TE?
      • Spin echo?
      • 90°-90° Hahn Echo?
      • Stimulated echoes?
      • STEs for imaging?
      • 4 or more RF-pulses?
      • Partial flip angles?
      • How is signal higher?
      • Optimal flip angle?
    • Spin Echo >
      • SE vs Multi-SE vs FSE?
      • Image contrast: TR/TE?
      • Opposite effects ↑T1 ↑T2?
      • Meaning of weighting?
      • Does SE correct for T2?
      • Effect of 180° on Mz?
      • Direction of 180° pulse?
    • Inversion Recovery >
      • What is IR?
      • Why use IR?
      • Phase-sensitive IR?
      • Why not PSIR always?
      • Choice of IR parameters?
      • TI to null a tissue?
      • STIR?
      • T1-FLAIR
      • T2-FLAIR?
      • IR-prepped sequences?
      • Double IR?
    • Gradient Echo >
      • GRE vs SE?
      • Multi-echo GRE?
      • Types of GRE sequences?
      • Commercial Acronyms?
      • Spoiling - what and how?
      • Spoiled-GRE parameters?
      • Spoiled for T1W only?
      • What is SSFP?
      • GRASS/FISP: how?
      • GRASS/FISP: parameters?
      • GRASS vs MPGR?
      • PSIF vs FISP?
      • True FISP/FIESTA?
      • FIESTA v FIESTA-C?
      • DESS?
      • MERGE/MEDIC?
      • GRASE?
      • MP-RAGE v MR2RAGE?
    • Susceptibility Imaging >
      • What is susceptibility (χ)?
      • What's wrong with GRE?
      • Making an SW image?
      • Phase of blood v Ca++?
      • Quantitative susceptibility?
    • Diffusion: Basic >
      • What is diffusion?
      • Iso-/Anisotropic diffusion?
      • "Apparent" diffusion?
      • Making a DW image?
      • What is the b-value?
      • b0 vs b50?
      • Trace vs ADC map?
      • Light/dark reversal?
      • T2 "shine through"?
      • Exponential ADC?
      • T2 "black-out"?
      • DWI bright causes?
    • Diffusion: Advanced >
      • Diffusion Tensor?
      • DTI (tensor imaging)?
      • Whole body DWI?
      • Readout-segmented DWI?
      • Small FOV DWI?
      • IVIM?
      • Diffusion Kurtosis?
    • Fat-Water Imaging >
      • Fat & Water properties?
      • F-W chemical shift?
      • In-phase/out-of-phase?
      • Best method?
      • Dixon method?
      • "Fat-sat" pulses?
      • Water excitation?
      • STIR?
      • SPIR?
      • SPAIR v SPIR?
      • SPIR/SPAIR v STIR?
  • …Making an Image
    • From Signals to Images >
      • Phase v frequency?
      • Angular frequency (ω)?
      • Signal squiggles?
      • Real v Imaginary?
      • Fourier Transform (FT)?
      • What are 2D- & 3D-FTs?
      • Who invented MRI?
      • How to locate signals?
    • Frequency Encoding >
      • Frequency encoding?
      • Receiver bandwidth?
      • Narrow bandwidth?
      • Slice-selective excitation?
      • SS gradient lobes?
      • Cross-talk?
      • Frequency encode all?
      • Mixing of slices?
      • Two slices at once?
      • Simultaneous Multi-Slice?
    • Phase Encoding >
      • Phase-encoding gradient?
      • Single PE step?
      • What is phase-encoding?
      • PE and FE together?
      • 2DFT reconstruction?
      • Choosing PE/FE direction?
    • Performing an MR Scan >
      • What are the steps?
      • Automatic prescan?
      • Routine shimming?
      • Coil tuning/matching?
      • Center frequency?
      • Transmitter gain?
      • Receiver gain?
      • Dummy cycles?
      • Where's my data?
      • MR Tech qualifications?
    • Image Quality Control >
      • Who regulates MRI?
      • Who accredits?
      • Mandatory accreditation?
      • Routine quality control?
      • MR phantoms?
      • Geometric accuracy?
      • Image uniformity?
      • Slice parameters?
      • Image resolution?
      • Signal-to-noise?
      • Ghosting?
  • …K-space & Rapid Imaging
    • K-space (Basic) >
      • What is k-space?
      • Parts of k-space?
      • What does "k" stand for?
      • Spatial frequencies?
      • Locations in k-space?
      • Data for k-space?
      • Why signal ↔ k-space?
      • Spin-warp imaging?
      • Big spot in middle?
      • K-space trajectories?
      • Radial sampling?
    • K-space (Advanced) >
      • K-space grid?
      • Negative frequencies?
      • Field-of-view (FOV)
      • Rectangular FOV?
      • Partial Fourier?
      • Phase symmetry?
      • Read symmetry?
      • Why not use both?
      • ZIP?
    • Rapid Imaging (FSE &EPI) >
      • What is FSE/TSE?
      • FSE parameters?
      • Bright Fat?
      • Other FSE differences?
      • Dual-echo FSE?
      • Driven equilibrium?
      • Reduced flip angle FSE?
      • Hyperechoes?
      • SPACE/CUBE/VISTA?
      • Echo-planar imaging?
      • HASTE/SS-FSE?
    • Parallel Imaging (PI) >
      • What is PI?
      • How is PI different?
      • PI coils and sequences?
      • Why and when to use?
      • Two types of PI?
      • SENSE/ASSET?
      • GRAPPA/ARC?
      • CAIPIRINHA?
      • Compressed sensing?
      • Noise in PI?
      • Artifacts in PI?
  • …Contrast Agents
    • Contrast Agents: Physics >
      • Why Gadolinium?
      • Paramagnetic relaxation?
      • What is relaxivity?
      • Why does Gd shorten T1?
      • Does Gd affect T2?
      • Gd & field strength?
      • Best T1-pulse sequence?
      • Triple dose and MT?
      • Dynamic CE imaging?
      • Gadolinium on CT?
    • Contrast Agents: Clinical >
      • So many Gd agents!
      • Important properties?
      • Ionic v non-ionic?
      • Intra-articular/thecal Gd?
      • Gd liver agents (Eovist)?
      • Mn agents (Teslascan)?
      • Feridex & Liver Agents?
      • Lymph node agents?
      • Ferumoxytol?
      • Blood pool (Ablavar)?
      • Bowel contrast agents?
    • Contrast Agents: Safety >
      • Gadolinium safety?
      • Allergic reactions?
      • Renal toxicity?
      • What is NSF?
      • NSF by agent?
      • Informed consent for Gd?
      • Gd protocol?
      • Is Gd safe in infants?
      • Reduced dose in infants?
      • Gd in breast milk?
      • Gd in pregnancy?
      • Gd accumulation?
      • Gd deposition disease?
  • …Cardiovascular and MRA
    • Flow effects in MRI >
      • Defining flow?
      • Expected velocities?
      • Laminar v turbulent?
      • Predicting MR of flow?
      • Time-of-flight effects?
      • Spin phase effects?
      • Flow void?
      • Why GRE ↑ flow signal?
      • Slow flow v thrombus?
      • Even-echo rephasing?
      • Flow-compensation?
      • Flow misregistration?
    • MR Angiography - I >
      • MRA methods?
      • Dark vs bright blood?
      • Time-of-Flight (TOF) MRA?
      • 2D vs 3D MRA?
      • MRA parameters?
      • Magnetization Transfer?
      • Ramped flip angle?
      • MOTSA?
      • Fat-suppressed MRA?
      • TOF MRA Artifacts?
      • Phase-contrast MRA?
      • What is VENC?
      • Measuring flow?
      • 4D Flow Imaging?
      • How accurate?
    • MR Angiography - II >
      • Gated 3D FSE MRA?
      • 3D FSE MRA parameters?
      • SSFP MRA?
      • Inflow-enhanced SSFP?
      • MRA with ASL?
      • Other MRA methods?
      • Contrast-enhanced MRA?
      • Timing the bolus?
      • View ordering in MRA?
      • Bolus chasing?
      • TRICKS or TWIST?
      • CE-MRA artifacts?
    • Cardiac I - Intro/Anatomy >
      • Cardiac protocols?
      • Patient prep?
      • EKG problems?
      • Magnet changes EKG?
      • Gating v triggering?
      • Gating parameters?
      • Heart navigators?
      • Dark blood/Double IR?
      • Why not single IR?
      • Triple IR?
      • Polar plots?
      • Coronary artery MRA?
    • Cardiac II - Function >
      • Beating heart movies?
      • Cine parameters?
      • Real-time cine?
      • Ventricular function?
      • Tagging/SPAMM?
      • Perfusion: why and how?
      • 1st pass perfusion?
      • Quantifying perfusion?
      • Dark rim artifact
    • Cardiac III - Viability >
      • Gd enhancement?
      • TI to null myocardium?
      • PS (phase-sensitive) IR?
      • Wideband LGE?
      • T1 mapping?
      • Iron/T2*-mapping?
      • Edema/T2-mapping?
      • Why/how stress test?
      • Stess drugs/agents?
      • Stress consent form?
  • …MR Artifacts
    • Tissue-related artifacts >
      • Chemical shift artifact?
      • Chemical shift in phase?
      • Reducing chemical shift?
      • Chemical Shift 2nd Kind?
      • In-phase/out-of phase?
      • IR bounce point?
      • Susceptibility artifact?
      • Metal suppression?
      • Dielectric effect?
      • Dielectric Pads?
    • Motion-related artifacts >
      • Why discrete ghosts?
      • Motion artifact direction?
      • Reducing motion artifacts?
      • Saturation pulses?
      • Gating methods?
      • Respiratory comp?
      • Navigator echoes?
      • PROPELLER/BLADE?
    • Technique-related artifacts >
      • Partial volume effects?
      • Slice overlap?
      • Aliasing?
      • Wrap-around artifact?
      • Eliminate wrap-around?
      • Phase oversampling?
      • Frequency wrap-around?
      • Spiral/radial artifacts?
      • Gibbs artifact?
      • Nyquist (N/2) ghosts?
      • Zipper artifact?
      • Data artifacts?
      • Surface coil flare?
      • MRA Artifacts (TOF)?
      • MRA artifacts (CE)?
  • …Functional Imaging
    • Perfusion I: Intro & DSC >
      • Measuring perfusion?
      • Meaning of CBF, MTT etc?
      • DSC v DCE v ASL?
      • How to perform DSC?
      • Bolus Gd effect?
      • T1 effects on DSC?
      • DSC recirculation?
      • DSC curve analysis?
      • DSC signal v [Gd]
      • Arterial input (AIF)?
      • Quantitative DSC?
    • Perfusion II: DCE >
      • What is DCE?
      • How is DCE performed?
      • How is DCE analyzed?
      • Breast DCE?
      • DCE signal v [Gd]
      • DCE tissue parmeters?
      • Parameters to images?
      • K-trans = permeability?
      • Utility of DCE?
    • Perfusion III: ASL >
      • What is ASL?
      • ASL methods overview?
      • CASL?
      • PASL?
      • pCASL?
      • ASL parameters?
      • ASL artifacts?
      • Gadolinium and ASL?
      • Vascular color maps?
      • Quantifying flow?
    • Functional MRI/BOLD - I >
      • Who invented fMRI?
      • How does fMRI work?
      • BOLD contrast?
      • Why does BOLD ↑ signal?
      • Does BOLD=brain activity?
      • BOLD pulse sequences?
      • fMRI Paradigm design?
      • Why "on-off" comparison?
      • Motor paradigms?
      • Visual?
      • Language?
    • Functional MRI/BOLD - II >
      • Process/analyze fMRI?
      • Best fMRI software?
      • Data pre-processing?
      • Registration/normalization?
      • fMRI statistical analysis?
      • General Linear Model?
      • Activation "blobs"?
      • False activation?
      • Resting state fMRI?
      • Analyze RS-fMRI?
      • Network/Graphs?
      • fMRI at 7T?
      • Mind reading/Lie detector?
      • fMRI critique?
  • …MR Spectroscopy
    • MRS I - Basics >
      • MRI vs MRS?
      • Spectra vs images?
      • Chemical shift (δ)?
      • Measuring δ?
      • Backward δ scale?
      • Predicting δ?
      • Size/shapes of peaks?
      • Splitting of peaks?
      • Localization methods?
      • Single v multi-voxel?
      • PRESS?
      • STEAM?
      • ISIS?
      • CSI?
    • MRS II - Clinical ¹H MRS >
      • How-to: brain MRS?
      • Water suppression?
      • Fat suppression?
      • Normal brain spectra?
      • Choice of TR/TE/etc?
      • Hunter's angle?
      • Lactate inversion?
      • Metabolite mapping?
      • Metabolite quantitation?
      • Breast MRS?
      • Gd effect on MRS?
      • How-to: prostate MRS?
      • Prostate spectra?
      • Muscle ¹H-MRS?
      • Liver ¹H-MRS?
      • MRS artifacts?
    • MRS III - Multi-nuclear >
      • Other nuclei?
      • Why phosphorus?
      • How-to: ³¹P MRS
      • Normal ³¹P spectra?
      • Organ differences?
      • ³¹P measurements?
      • Decoupling?
      • NOE?
      • Carbon MRS?
      • Sodium imaging?
      • Xenon imaging?
  • ...Artificial Intelligence
    • AI Part I: Basics >
      • Artificial Intelligence (AI)?
      • What is a neural network?
      • Machine Learning (ML)?
      • Shallow v Deep ML?
      • Shallow networks?
      • Deep network types?
      • Data prep and fitting?
      • Back-Propagation?
      • DL 'Playground'?
    • AI Part 2: Advanced >
      • What is convolution?
      • Convolutional Network?
      • Softmax?
      • Upsampling?
      • Limitations/Problems of AI?
      • Is the Singularity near?
    • AI Part 3: Image processing >
      • AI in clinical MRI?
      • Super-resolution?
  • ...Tissue Properties Imaging
    • MRI of Hemorrhage >
      • Hematoma overview?
      • Types of Hemoglobin?
      • Hyperacute/Oxy-Hb?
      • Acute/Deoxy-Hb?
      • Subacute/Met-Hb?
      • Deoxy-Hb v Met-Hb?
      • Extracellular met-Hb?
      • Chronic hematomas?
      • Hemichromes?
      • Ferritin/Hemosiderin?
      • Subarachnoid blood?
      • Blood at lower fields?
    • T2 cartilage mapping
    • MR Elastography?
    • Synthetic MRI?
    • Amide Proton Transfer?
    • MR thermography?
    • Electric Properties Imaging?
  • Copyright/Legal
    • Copyright Issues
    • Legal Disclaimers
  • Forums/Blogs/Links
  • What's New
  • Self-test Quizzes - NEW!
    • Magnets & Scanners Quiz
    • Safety & Screening Quiz
    • NMR Phenomenon Quiz
    • Pulse Sequences Quiz
    • Making an Image Quiz
    • K-space & Rapid Quiz
    • Contrast & Blood Quiz
    • Cardiovascular & MRA Quiz

Pulsed Arterial Spin Labeling (PASL)

What is PASL and how does it differ from CASL?  
PASL overview
Continuous Arterial Spin Labeling (CASL) techniques, described in the prior Q&A, are now primarily of historical and didactic interest, having fallen out of favor by the late 1990s. The main problems with CASL were its high tissue energy deposition and transmitter duty-cycle requirements accompanying the use of continuous RF-pulses. Thus came PASL, a family of ASL methods using pulsed (rather than continuous) RF-excitation. Three PASL variants are illustrated in the diagram below.
PASL techniques
Pulsed Arterial Spin Labeling (PASL) methods
The first PASL method was developed by Edelman and colleagues in the mid-1990s and called EPISTAR (Echo-Planar Imaging-based Signal Targeting by Alternating Radiofrequency pulses). The tagging sequence of EPISTAR begins with a slice-selective 90°-pulse and spoiler gradient to initially saturate magnetization in the imaged slice. (The profile of this pulse is usually slightly wider than the imaged slice to minimize artifacts from RF-side lobes and ensure uniform saturation). A wide inversion slab is then applied proximally using a 180° adiabatic RF-pulse that inverts inflowing spins. The control sequence repeats the 90°-spoiler saturation of static tissue in the imaged slice, followed by a mirror-image distally placed thick inversion slab to create MT effects equivalent to those of the first slab. Subtraction of the tagged from control images produces a perfusion-weighted image where MT effects have been minimized.
Single-slice EPISTAR has subsequently evolved into several multi-slice variants (STAR, PULSAR, and QUASAR), some of which have been offered as commercial products by Philips. These are described briefly in the Advanced Discussion. 
PICORE (Proximal Inversion with Control of Off-Resonance Effects) is another asymmetric multislice PASL technique with similarities to EPISTAR. In fact, the tagging sequence is identical to EPISTAR with imaging slice saturation and proximal inversion pulses. The control sequence differs in that the 180°-inversion pulse is applied at the same frequency offset relative to the imaging slice as the tagging sequence, but in the absence of a spatial gradient. 

FAIR (Flow-sensitive Alternating Inversion Recovery) uses a slightly different pulsed labeling strategy that is symmetric with respect to the imaging slice. The tagged sequence begins with a spatially selective inversion pulse limited to a small region in and around the imaged slice. (The inversion pulse is usually slightly wider than the imaged slice to minimize artifacts from RF-side lobes and ensure uniform inversion). After this inversion pulse, "fresh" (fully magnetized) blood flows into the slice which serves as the "tag". This is followed by a non-slice-selective inversion pulse to the entire imaged volume as the "control". Versions of both PICORE and FAIR with Q2-TIPS bolus saturation (see Advanced Discussion) are currently offered as Siemens ASL products.
The choice between EPISTAR, PICORE, and FAIR methods should depend in part on the geometry of expected blood flow to the imaging volume. PICORE tags blood exclusively from one side and is thus a reasonable choice for axial brain perfusion imaging where all supply is coming from the neck. In cases where inflow may be from multiple or unknown directions (such as for sagittal brain perfusion imaging or in a complex organ such as the liver), FAIR may be a better choice since it tags both sides of the imaging volume. Conversely, FAIR may be more sensitive to contamination by unwanted venous inflow and is somewhat more difficult to implement in multi-slice mode due to inversion slice-profile imperfections. In some situations EPISTAR may be advantageous to the other methods having fewer eddy current artifacts due to its balanced slab-select gradients between control and tagged sequences.

Advanced Discussion (show/hide)»

An additional difference between the three PASL techniques concerns the polarity of the signals between the tagged (St) and control (Sc) states. For EPISTAR and PICORE, inflowing arterial spins are inverted and thus St < Sc. For FAIR the tagged magnetization is not inverted while the control state is; hence St >Sc.

EPISTAR Variants Mentioned Above

  • Multi-slice STAR (Signal Targeting by Alternating Radiofrequency pulses)  replaces the single distal inversion slab of the EPISTAR Control sequence with two proximal inversion slabs each at half amplitude applied in rapid succession. This has the same effect as CASL Solution #2 to create identical MT effects on the imaged slice but no net effect on flowing spins.
  • 

  • PULSAR (PULsed StAR) uses the same inversion strategy as STAR but replaces the slice-selective 90°-pulse and spoiler gradient presaturation of the imaged slice using a more sophisticated cluster of 4 spectroscopic water-suppression pulses.

  • QUASAR (QUantitative Star labeling of Arterial Regions) uses PULSAR labeling and control modules followed by a Look-Locker strategy sampling multiple time points for T1 estimation plus a repetitive Q2-TIPS-like saturation scheme for sharper definition of the arterial bolus. Other advanced features include perfusion quantification using an arterial input function (AIF) convolution method.

"Sharpening the Bolus"

Both the leading and trailing edges of the bolus may be somewhat ill-defined due to transit delays and dispersion. Several pulse sequence modifications have been proposed to "sharpen" and thus improve edge definition of the bolus. One of the first was named QUIPSS (Quantitative Imaging of Perfusion using a Single Subtraction). QUIPSS involved adding an additional slab-select 90º, 18-lobe sinc saturation pulse just before readout in a PASL sequence applied either to the imaging volume (QUIPSS I) or labeling volume (QUIPSS II). A popular variant we use, Q2TIPS (QUIPSS II with Thin-slice TI1 Periodic Saturation), replaces this single thick-slab saturation pulse with a train of thin-slice saturation pulses at the distal end of the tagged region. Two operator-selectable parameters are available that determine the start and end times of this saturation train (and hence the edges of the bolus) Our Q2TIPS sequence uses 16 very selective suppression (VSS) pulses applied every 25 ms during a window 800-1200 ms after the initial inversion. These saturate a 2 cm slab of tissue leaving about a 1 cm gap proximal to the first imaging slice.


References
     Edelman RR, Chen Q. EPISTAR MRI: Mulitslice mapping of cerebral blood flow. Magn Reson Med 1998; 40:800-805. (Later iteration of EPISTAR with modifications and multi-slice acquisition)
     Edelman RR, Siewert B, Darby DG, et al. Qualitative mapping of cerebral blood flow and functional localization with echo-planar MR imaging and signal targeting with alternating frequency. Radiology 1994; 192:513-520. (one of the early descriptions of EPISTAR)  
     Golay X, Hendrikse J, Lim TCC. Perfusion imaging using arterial spin labeling. Top Magn Reson Imaing 2004; 15:10-27. (Good review plus a description of ASL of variants and acronyms such as FAIRER, TILT, BASE, and others).
     Golay X, Peterson ET, Hui F. Pulsed Star labeling of arterial regions (PULSAR): a robust regional perfusion technique for high field imaging. Magn Res Med 2005; 53:15-21. 
     Kim S-G. Quantification of relative cerebral blood flow change by Flow-sensitive Alternating Inversion Recovery (FAIR) technique: application to functional mapping. Magn Reson Med 1995; 34:293-301. (original description of FAIR technique)
     Luh W-M, Wong EC, Bandettini PA, Hyde JS. QUIPSS II with thin-slice TI1 periodic saturation: a method for improving accuracy of quantitative perfusion imaging using pulsed arterial spin labeling. Magn Reson Med 1999; 41:1246-1254 (description of the Q2TIPS method).
     Petersen ET, Lim T, Golay X. Model-free arterial spin labeling quantification: approach for perfusion MRI. Magn Reson Med 2006; 55:219-232. (QUASAR method)
     Wong EC, Buxton RB, Frank LR. Implementation of quantitative perfusion imaging techniques for functional brain mapping using pulsed arterial spin labeling. NMR in Biomed 1997; 10:237-249. (description of the PASL/PICORE method)

Related Questions
     Can you briefly explain the difference between the various ASL methods? Which is the best?
     What is pCASL and how does it differ from CASL and PASL?   

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