Overview. Optimizing MRI Protocols. Image Contrast. Morphology & Physiology. User Selectable Parameters. Tissue Parameters

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1 Overview Optimizing MRI Protocols Clinical Practice & Compromises Geoffrey D. Clarke, Radiology Department University of Texas Health Science Center at San Antonio Pulse timing parameters for adjusting image contrast Effects of spatial resolution and imaging speed on signal-to-noise ratio Effects of magnetic field strength Adapting MRI pulsing protocols to physiology under investigation Image Contrast Basic image contrast is effected by the amplitude and timing of the RF pulses used to excite the spin system. More advanced methods may use gradient pulses (to modulate motion) and alter tissue properties with exogenous contrast agents Morphology & Physiology Chemical Shift (water vs. fat) Blood motion (macroscopic & microscopic) Diffusion of water Gross motion (peristalsis, respiration) Tissue Susceptibility Tissue Parameters Tissue Water Density Proton Density (PD) Longitudinal Relaxation Time (T1) Transverse Relaxation Time (T2) Magnetization transfer rate constants User Selectable Parameters Magnetic Field Strength (B o ) RF Pulse timing (TR, TE, TI) RF pulse amplitude - flip angles (α) Gradient Amplitude & Timing (b-value) RF pulse excitation frequency & bandwidth Receiver bandwidth (BW) G.D. Clarke, UT HSC San Antonio 1

2 Contrast-to-Noise Ratio Contrast relationships are determined by the RF pulse timings used and the relaxation properties of tissues under investigation. CNR = n ( ) I a I b σ σ = standard deviation of the signal (the noise) n = number of signals acquired I a,i b = signal intensities from tissues a and b Relaxation Times T1: longitudinal relaxation time defines recovery of potential for next signal T2: transverse relaxation time defines rate of dephasing of MRI signal due to microscopic processes T2*: transverse relaxation time with B o inhomogeneity effects added; defines rate of dephasing of MRI signal due to macroscopic and microscopic processes T 1, T 2 and for Various Tissues Tissue T 1 (ms) T 2 (ms) Liver Kidney Muscle Gray Matter White Matter Akber,, 1996 (at 63 MHz) Manipulating Contrast The weighting of image contrast is related to delay times, TR (repetition time) & TE (echo time) Spin Echo manipulates image contrast with 180 o refocusing pulses (insensitive to B o inhomogeneities) Gradient Echo manipulates image contrast by varying the excitation flip angle (fast scans) Inversion Recovery manipulates image contrast with 180 o inversion pulses Pulse Sequence Classifications Name Spin Echo Gradient Echo Inversion Recovery RF Pulses Two or more One Three Contrast Weighting T1, PD or T2 T1 or T2* T1 and T2 Application Conventional Fast imaging (3DFT) Exclude certain tissues Spin Echo - Rules of Thumb TE controls T2 dependence TR controls T1 dependence T1 competes with T2 and proton density PD Weighted = long TR, short TE T1 Weighted = short TR, short TE T2 Weighted = long TR, long TE G.D. Clarke, UT HSC San Antonio 2

3 Spin-Echo Imaging Sequence TX RX G sl RF 1 =90 o RF 2 =180 o Spin Echo time time time Multi-Echo Acquisitions Image 1 Image 2 Image 3 G ro time G pe time SNR and Imaging Parameters Matched Bandwidth SNR FOV M ro ro FOV M ph ph z NSA BW rx 1 2 FOV = field of view M = matrix size NSA = number of signals averaged BW rx = receiver bandwidth ro = read out (frequency encoding) direction z = slice thickness ph = phase encoding direction Mugler & Brookeman, Magn Reson Imag 1989; 7: Matched Bandwidth Bandwidth on ADC 1 is limited by finite T max & desire to minimize TE 1. T1W Matched Bandwidth = Unmatched Distortion T2W T 1 weighting and SNR is sacrificed more by T 2 decay than by BW T 2 -weighted image - lots of time for ADC 2, poorer SNR. It would be nice to decrease BW (increase T max ) Mugler & Brookeman, Magn Reson Imag 1989; 7: Smaller BW on T2W image leads to increased image distortion G.D. Clarke, UT HSC San Antonio 3

4 MR Brain Imaging SE Effect of TR TR = 63 ms, NSA = 16 TR =125 ms, NSA = 8 TR = 250 ms, NSA = 4 TR = 500 ms, NSA = 2 AX T1W AX T2W FLAIR Diffusion Gadolinium Gray-white matter contrast See inside bony structures High spatial resolution Depicts white matter lesions Evaluate cerebral blood flow B o = 1.5 T FOV = 230 mm 256 x 256 st = 4 mm TE = 15 ms TR = 1 s, NSA = 1 TR = 2 s, NSA = 1 TR = 4 s, NSA = 1 Vlaardingerbroek & den Boer, 1999 Multiecho Image Matrix Most T 1 Weighted Most Proton Density Weighted Most T 2 Weighted TE ms TR (s) Paramagnetism Oxygen Metal ions with unpaired electrons resulting in positive susceptibility -Fe -Gd -Mg Paramagnetism has less than 1/1000th effect of ferromagnetism Shortens T1 and T2 times Gd-DTPA Typical dose of ~0.1 mm/kg Dalton seven unpaired electrons high magnetic moment unpaired electrons react with protons in adjacent water molecules shortening their relaxation time Gadolinium shortens T1 relaxation of water high SI on T1 weighted images in lower concentrations shortens T2 relaxation of water lowers SI on T1 weighted images in high concentrations G.D. Clarke, UT HSC San Antonio 4

5 Gadolinium Nonuniform distribution of Gd-DTPA increases magnetic susceptibility differences Decreases MR signal on T2 * -weighted images Gadolinium Contrast Agents Normal T 1 -weighted Images Meningioma With Gd Contrast Commercial Contrast Agents Inversion Recovery FLAIR (FLuid Attenuated Inversion Recovery) Multiple Sclerosis Proton Density T2-Weighted FLAIR M z Brain M xy Lesion Lesion 0 time CSF TI Brain T eff time TR = 2350 TE = 30 TR=2350 TE= 80 Rydberg JN et al. Radiology 1994; 193: TR=2600 TE=145 TI= 11,000 G.D. Clarke, UT HSC San Antonio 5

6 FSE Pulse Sequence FSE Point Spread Function Depends on T 2 Effective TE ETL = Echo Train Length Effect of Echo Spacing Spin Echo vs. Fast Spin Echo TE = 30 ms T 1 -weighted (TR = 500) Signal-to-noise decreases for short T2 tissues (gray & white matter) leading to a decrease in spatial resolution Vlardengerbrook & den Boer, 1999 TE = 120 ms T 2 -weighted (TR= 2000) Spin Echo Fast Spin Echo (Echo Train Length = 4) Inversion Recovery FSE TR =1400 TI = 100 TR =1400 TI = 280 TR =2000 TI = 280 FLAIR Imaging Phase Compensated Modulus Vlaardingerbroek & den Boer, 1999 T2W-FSE TE/TR = 98/3500ms, Slice 5/1.5mm, ET:8 (split) 256x224, 1 NEX, 20x20 cm FOV, 3:23 FLAIR-FSE TI/TE/TR = 2200/147/10000ms, Slice 5/1.5mm, 256x160, 1 NEX, 20x20cm FOV, 3:40 G.D. Clarke, UT HSC San Antonio 6

7 Magnetization Transfer Magnetization Transfer Contrast Multislice FSE: Magnetization Transfer Contrast Enhances T 2 - Weighted Appearance Attenuation Due to Diffusion A( TE) = A(0) exp[ γ G Dappδ α ( )] 4 2 δ Where: α=π/2; G is amplitude of diffusion sensitive gradient pulse; δ is duration of diffusion sensitive gradient; is time between diffusion sensitive gradient pulses; D app is the apparent diffusion coefficient Hahn E., Phys Rev 1950 Diffusion Weighted Imaging: Prototype Pulse Sequence Stejskal EO & Tanner JE, : b = γ G δ ( ) 2 δ 3 Diffusion Echo-Planar Imaging Signal has to be acquired in time ~T2 Images in less than 100 ms but poor spatial resolution ( ~ 3mm x 3mm pixel) Requires very good B o homogeneity big susceptibility artifacts The b-value Controls amount of diffusion weighting in image The greater the b-value the greater the area under the diffusion-weighted gradient pulses longer TE stronger and faster ramping the gradients G.D. Clarke, UT HSC San Antonio 7

8 Diffusion Imaging of Infarcts SAG T1W SAG T2W STIR Gadolinium Spine Imaging SE Echo Planar TE = 80 ms SE Echo Planar b = 1205 s mm -1 Left: Coronal T2W (one slice) Right: MIP See inside bony structures Surface coil almost always used Evaluate nerve root compression Gradient-Echo Imaging Partial (<90 o ) flip angle keeps all longitudinal magnetization from being used up Short TE values preserve SNR Short TR values allow for fast scanning (~ 1 sec/image) Poor T2-weighted contrast Sensitive to susceptibility Spoiled Gradient Echo Also called spoiled GRASS, fast field echo, FLASH, etc. Magnetic Susceptibility Effects Gradient-Echo Imaging The susceptibility-induced artifacts in GRE images: Increase with TE - limiting the utility of GRE T 2 *-weighted images in many cases. Are worst for tissue/air interfaces, but noticeable at tissue/bone interfaces. Are usually a detriment, but are useful in some circumstances (e.g., blood-sensitive imaging, BOLD contrast functional imaging, etc.). G.D. Clarke, UT HSC San Antonio 8

9 Gradient-Echo Imaging Reference: Wehrli, Fast-Scan Magnetic Resonance. Principles and Applications Spatial Saturation Spatial presaturation pulses (slabs) can be applied in oblique coronal planes and are used to suppress signal intensities of tissue adjacent to spine and reduce breathing artifacts Phased Array Surface Coils STIR (Short TI Inversion Recovery) M z Fat Surface receiver coils are used to maximize signal from regions of interest while minimizing noise from volumes outside of the region of investigation. 0 time Liver TI SAG T1W SAG T2W STIR Gadolinium Spine Imaging SAG T1W SAG T2W STIR Gadolinium Spine Imaging Right: Saggital T1W image of T11 L1 demonstrates minimal wedging of the vertebral bodies associated with intravertebral disc herniations (arrows) T1-weighted MRI CT T2-weighted Heavily T2- weighted images show CSF as bright, creating myelographic effect G.D. Clarke, UT HSC San Antonio 9

10 SAG T1W SAG T2W STIR Gadolinium Spine Imaging SAG T1W SAG T2W STIR Gadolinium Spine Imaging Top: Sag T1W Bottom: STIR Short TI Inversion Recovery STIR Left Axial T1W image of S1 (note nerve root) Right Gd-enhanced image confirms abnormal soft tissue is scar Contrast Agent AX T2W AX EPI AX T1W Ferumoxide Body MR Imaging requires: Control of respiratory an dother motion artiafcts Identification and/or elimination of fat signals Avoidance of wrap-around (aliasing) artifact Liver Imaging Advantages: Soft tissue contrast High degree of contrast manipulation Lesion characterization High sensitivity to iron Chemical Shift Artifact Occurs in Readout direction Conventional SE Phase encode direction Echo-Planar Controlled by Fat Pre-Saturation STIR sequence Chemical shift artifact seen in axial image of kidney Chemical Shift Chemical shift (fat-water) ~3.5 ppm At 1.5T: MHz f water = T T fat 220 At 0.5T: MHz f fat water = T 73 Hz T field strength. chemical shift Hz CHESS This is typically accomplished by preceding a SE or FSE sequence with a 90 o pulse that is frequency, not spatially, selective. G.D. Clarke, UT HSC San Antonio 10

11 T2W T2W PD/T1W PD/T1W Contrast Contrast agents agents Optimizing MRI Protocols 7/27/2005 A Liver Imaging Chemical Shift C Liver Imaging AX FSE AX EPI AX T1W Ferumoxide T2-Weighted FAST SPIN ECHO is often used to reduce motion artifact & scan time B A. In phase-spoiled FFE image w/ TE= 4 ms B. Out of phase spoiled FFE images w/ TE = 2 ms C. T2 breath-hold FSE with fatsat pulse Liver Imaging Liver Imaging AX T2W T2W EPI AX T1W Ferumoxide Echo planar is the fastest imaging sequence and can be used to minimize motion artifacts Long TE Gradient Echoes produce T2* contrast to identify tumors Echo Planar Increasing the number of shots increases imaging time but decreases geometric distortions 8 Shot 4 Shot 2 Shot 1 Shot Medhi P-A et al. Radiographics 2001; 21:767 AX T2W AX EPI AX T1W Ferumoxide PDW /T1W SGE photo Very fast (EPI or Gradient Echo) T1 weighted images allow effective management of respiratory motion. AX T2W AX EPI AX T1W Ferumoxide Super Paramagnetic Iron Oxide is the contrast agent of choice in liver imaging T2W Fast Spin Echo Liver Imaging T2*W Gradient Echo Changes in MRI with B o Field-Strength The Good Increased signal Increased chemical shift? The Bad Poorer T1W contrast due to longer T1 Increases in magnetic susceptibility The Ugly Increased absorption of RF power Less uniform images G.D. Clarke, UT HSC San Antonio 11

12 Contrast Agents & B o Strength Due to increases in tissue T1 s, Gdbased contrast agents are more effective at 3T compared to 1.5T Use less contrast agent to get same tissue contrast or Achieve much higher tissue contrast for the same dose Nobauer-Huhmann IM, Invest Radiol 2002; 37: What is SAR? The patient is in an RF magnetic field that causes spin excitation (the B1 field) The RF field can induce small currents in the electrically conductive patient which result in energy being absorbed. The RF power absorbed by the body is called the specific absorption rate (SAR) SAR has units of watts absorbed per kg of patient If the SAR exceeds the thermal regulation capacity the patient s body temperature will rise. Scan Parameters Effecting SAR Patient size: SAR increases as the patient size increases directly related to patient radius Resonant frequency: SAR increases with the square of the Larmor frequency (ω o ) therefore with B o 2 RF pulse flip angle: SAR increases as the square of the flip angle (α 2 ) Number of RF pulses: SAR increases with the number of RF pulses in a given time Parallel Imaging Uses spatial information obtained from arrays of RF coils Information is used to perform some portion of spatial encoding usually done by gradient fields and RF pulses Multiplies imaging speed without needing faster-switching gradients without additional RF power deposited Generalized Projections 3 RF coils SENSE Imaging (SENSitivity Encoding) SENSE X-ray CT Parallel MRI Parallel imaging can be thought of as being analogous to x-ray CT Data acquired from each PA coil element goes into reconstruction of whole image reduces imaging time, reduces SNR, reduces uniformity. G.D. Clarke, UT HSC San Antonio 12

13 Conventional breath-hold cardiac MRI Requires 14 heartbeats. SENSE breathhold cardiac MRI Requires 3 heartbeats. Image Acceleration What We Haven t Covered MR Angiography In-flow enhancement Magnetization transfer contrast MR Perfusion Imaging Dynamic contrast uptake Presaturating inversion slabs (FAIR) Functional MRI Echo-planar imaging Blood oxygen level dependent contrast Resolution FOV & matrix size Slice thickness FSE inter-echo spacing Motion artifact Chemical shift artifact Summary Signal-to-Noise FOV & matrix size RF Pulse flip angles & timing B o field strength Receiver bandwidth RF coil sensitivity Contrast Relaxation times RF Pulse flip angles & timing Preparation pulses Gradient timing (b-value) Magnetization transfer Suggested Reading (In order of increasing complexity) MRI: From Picture to Proton McRobbie DW, Moore EA, Graves MJ & Prince MR. Cambridge Univ. Press, 2003; ISBN: Magnetic Resonance Imaging 3 rd ed. Vlaardingerbroek MT, den Boer JA, Luiten A. Springer 2002; ISBN: Handbook of MRI Pulse Sequences Bernstein MA, King KF, Zhou XJ. Elsevier, 2004; ISBN: G.D. Clarke, UT HSC San Antonio 13