Clinical Benefits and Challenges at 3T

Welcome to this web based training on the Clinical Benefits and Challenges at 3T.                You should now be able to: Identify the increase in signal-to-noise and effects Identify the benefits and challenges of T1 Relaxation Identify the benefits and challenges of Chemical Shift Identify the benefits and challenges of Magnetic Susceptibility Illustrate how to Minimize SAR Demonstrate ways to Reduce Dielectric Resonance Effects Please continue to the assessment  page to complete the course.   Signal-to-noise increases linearly at 3T SNR (signal-to-noise ratio)= S/N is proportional to B0²/B0 Does SNR on 3T increase by a factor of 2? Factor of two (2) gain in SNR requires scan at 1.5T, four times as long Benefits Increased Spatial Resolution Reduction in Scan Time Improvement in image quality               3T                                                      1.5T                                                                   50% SNR gain     Upon successful completion of this course, participants will be able to: Identify the increase in signal-to-noise and effects Identify the benefits and challenges of T1 Relaxation Identify the benefits and challenges of Chemical Shift Identify the benefits and challenges of Magnetic Susceptibility Illustrate how to Minimize SAR Demonstrate ways to Reduce Dielectric Resonance Effects                                                                                                                                                                                                                                 T1 Relaxation Time – Benefits Benefits of T1 Relaxation Time Increased TOF MRA vessel contrast Increased enhancement of T1 based contrast agents Improved CE-MRA Vessel Contrast   High field strengths (3T) Wavelength of RF approaches dimension of human anatomy Can create destructive excitation field interference and consequently non-uniform flip angles in imaging volume Signal shading and contrast variations in imaging volume become an issue Effect also referred to as B1 inhomogeneity artifact or dielectric resonance effects              Improved SNR at 3T Improved sensitivity to contrast medium Increase in spatial resolution May reduce scan time             Definition of Chemical Shift Shift in resonant frequency of an atomic nucleus Depends on chemical bonds of the atom or structure of the molecule Caused primarily by a weakening of the applied magnetic field by the electron shell, and is proportional to the magnetic field strength             T1 Relaxation Time increases at 3T Improved TOF MRA vessel contrast Increased suppression of background tissue Increased vessel to tissue contrast      Chemical Shift increases linearly with field strength Benefits Improved spectral resolution (Spectroscopy) 1H Spectroscopy displays a 5 ppm range for both 1.5T and 3T 1.5T covers ~320 Hz 3T covers ~640 Hz Results in less overlap of metabolite peaks, better resolved spectra and greater frequency separation of metabolite peaks                                                                 1.5T                                                             3T                          Specific Absorption Rate (SAR) Definition Definition – RF power absorbed per unit of mass of an object when exposed to a radio frequency (RF) electromagnetic field Specific to Body Part and tissue Absorption of RF into body and tissue Rate of RF exposure and absorption Unit of measure = Watts per kilogram (W/kg) RF Exposure Application of time varying electromagnetic fields with frequencies in MHz range generating heat within tissue being exposed Heating is a major effect of RF exposure Safety considerations focus on temperature increases Definition of Magnetic Susceptibility Degree of magnetization that a material or tissue, exhibits in response to a magnetic field   SNR and Spatial Resolution Increase in SNR will increase Spatial Resolution Thinner slices Improved spatial resolution Less partial volume effects Smaller voxelsize achieved by Reducing FOV Decreasing slice thickness Increasing matrix size SNR Increase Increased SNR provided ability Reduce number averages Integrated Parallel Acquisition (iPAT) Technique Scan time reduction Higher iPAT factors May see improved image quality Depending on parameters utilized PD TSE SPAIR 640 matrix, iPAT 3   T2 MEDIC 640 matrix, iPAT 3   NATIVE, 3D SPACE MIP 320 matrix, iPAT 4   1.5 Tesla   3 Tesla       Artifacts Increased SNR Artifacts not seen on 1.5T may now be seen on 3T Motion Artifacts Increased voluntary and involuntary motion Flow Respiratory Patient motion  Motion Reduction Technique Saturation Bands Flow Compensation Prospective Acquisition Correction (PACE) syngo BLADE Sequence Swap phase and frequency syngo BLADE is a motion insensitive multi-shot Turbo Spin Echo sequence with inter-shot motion correction for in-plane motion correction Reduces flow effects at 3T   Prospective Acquisition CorrEction (PACE) During the measurement, PACE corrects respiratory and motion artifacts in real time by reducing the offset between the slices. PACE can be acquired with a 1D, 2D, or 3D sequence            Chemical Shift increases linearly with field strength Challenges Increased Fat/Water mis-excitation (through-plane) Increased Fat/Water pixel mis-registration (in-plane) Chemical Shift Increased Fat/Water pixel mis-excitation through-plane                                     Chemical Shift Increased Fat/Water pixel mis-registration in-plane 1.5 T     Fat              Water         3T      Fat               Water                        Decrease Fat/Water pixel mis-registration (in-plane) Increase bandwidth which may decrease SNR Decrease Fat/Water mis-excitation (through-plane) Faster transmit RF Pulses but SAR increases                     Magnetic Susceptibility increases linearly at 3T Benefits Increased Bold effect for Functional MR Imaging (fMRI) Increased contrast in T2* based perfusion Increased sensitivity to iron (hemorrhage, hemosiderin) Effect of magnetic susceptibility   increases linearly with field                                                              strength             Functional Imaging displays areas of brain participating in certain motor, sensory or cognitive activities Blood Oxygenation Level Dependence (BOLD) BOLD imaging uses local changes in blood flow as an indicator of momentary activation in a particular region of the brain Display neural active regions of the brain due to stimulations Finger tapping Cognitive memory Story categories                Cerebral MR Perfusion imaging Minimally invasive technique Access differences in the hemodynamic properties of blood flow to the vascular bed of the brain By way of signal changes induced due to rapid administration of a contrast agent during data acquisition               Arterial Spin Labeling (ASL) Technique uses water in arterial blood as an endogenous contrast agent to evaluate perfusion non-invasively Non contrast                     Magnetic Susceptibility increases linearly at 3T Challenges Increased in-plane spatial distortions T2* shortening with gradient echo techniques Increased through-plane excitation errors             Minimize in-plane spatial distortions Thinner slice thickness iPAT Higher iPAT factors can minimize in-plane spatial distortions Minimize T2* shortening Gradient Echo techniques Minimize through-plane excitation errors Careful parameter selection is required Use Faster RF Pulse Type                                Low transmission bandwidth ("low SAR" mode) High transmission bandwidth ("fast" mode)   RF Power Deposition RF Power squared when field strength doubled B1 (RF) ~ B0 (physics contraint) RF power ~ B12 ~ B02 (engineering constraint) SAR ~ RF power ~ B02 (physiological constraint) B1 (RF) ~ B0 (physics constraint) RF power ~ B12 ~ B02 (engineering constraint) SAR ~ RF power ~ B02 (physiological constraint)   Empirically determined RF Power for a 180˚angle versus field strength for whole body applications SAR RF Power  (kW)   B0 (Tesla)   RF Power Deposition Increases at 3T Challenges Increased SAR Increased Patient Heating Siemens sequences Hyperecho VERSE syngo SPACE iPAT Spin Echo with Refocusing Pulse Reducing flip angle Reduce Saturation Band Thickness RF Pulse Types   Turbo Spin Echo Hyperecho– 3T     Standard RF Pulse         VERSEPulse Reduces SAR by changing shape of excitation and refocusing RF pulses time   time Fast / HIgh Power   time   VERSE RF Pulse   Fast and Low Power     Slow / Low Power             syngo SPACE 3D Turbo Spin Echo sequence with variable flip angle Figure 2: T2 W SPACE for cranial nerves.   Figure 3: MRCP with T2 W SPACE. Free Breathing 2D-PACE-Applications. Tim-TRIO, 0.9 x 0.9 x 0.9 mm3, PAT *3.            SAR Reduction Techniques Integrated Parallel Acquisition Techniques (iPAT) Two types of iPAT techniques GRAPPA mSENSE Reduces # of phase lines Scan time reduction Decrease SNR mSENSE & GRAPPA   Spin Echo with Reduced Refocusing Flip Angle on 3T Recommended refocusing flip angle: ≥160° SAR is reduced by ~ 20%, signal reduction is ~ 3% Change in SAR and SNR dependent on T1 and TR specified         Allowed Delay Maximum Delay time after end of measurement Exception: Studies using contrast agents and dynamic imaging where timing is critical, allowed delay should not be used Note: 3T systems not recommended to use allowed delay’s >60 seconds                   SAR Reduction Increased SAR with thicker sat bands Increased edge blurring with thicker sat band SAT band Thickness range (mm) Peak Pulse Voltage (relative to TRAref) Relative SAR 3-10 1.0115 1.000 11-110 2.362 5.453 111-150 2.560 6.405   NOTE: for Avanto 1.5T VB15; may vary for other systems and software configurations          RF Pulse Type Normal RF Pulse with good slice profile with optimal SAR Low SAR Extended RF Pulse duration Reduced SAR Fast Short RF Pulse duration Aera 1.5T & 3T uses VERSE for Fast  RF 1.5T (No VERSE) – Higher SAR     TrueForm™ RF Technology – available on specific MR systems Normalization Techniques B1 Filter SPAIR Dielectric Padding cushion     Innovative hardware technology, new application and processing features TrueForm design consists of TrueForm magnet design TrueForm gradient design TrueForm RF design   Cylindrical shape Gradient linearity volume Magnet homogeneity volume Magnet homogeneity and gradient linearity shaped closer to the “true form” of the human body Benefits Fewer “shark-bites” Better fat saturation Less overlap needed for large FoV exams Enables larger imaging volume and fits true form of human body         Images from a MAGNETOM Verio TrueForm Excitation, TrueFormProcessing, and B1 Filter Conventional Conventional   TrueForm RF TrueForm RF         Filter Choices dependent on Coil – select appropriate filter Prescan Normalize Filter compensates for heterogeneous brightness in images Unfiltered Images Filter and unfiltered images Normalize Reduces brightness of areas in the vicinity of the coil Increases brightness in areas further away from the coil to equalize signal intensity for array coils   B1 Filter Filter used to reduce differences in signals caused by dielectric resonances at 3T                 RF – cushion Contains ultrasound gel with Gd-DTPA Coils – Body coil or Body Array coil Improves homogeneity of signal for slim patients Service engineer can order this Dielectric pad                 Dielectric pad Position pad on abdomen          Dielectric Pad uses Obtain optimal RF homogeneity Position RF cushion with white side on patient Obtain optimized RF homogeneity Improved fat saturation position RF-cushion with grey side on patient RF Cushion (1) Encapsulated gel (white) (2) Foam pad (grey)                                                                                                        SPAIR (Spectrally Adiabatic Inversion Recovery)  Spectrally selective fat saturation pulse used to automatically calculate inversion time to minimize fat signal Alternative fat suppression technique Only fat spins are affected no STIR like contrast Insensitive to B1 inhomogeneity Local variations of actual flip angle Fat Sat Mode – Strong and Weak All body parts   RF Pulses and VERSE Available at 3T and on Magnetom Aera 1.5T VERSE RF Pulse→Fast and low power Reduce SAR by changing shape of excitation and refocusing RF Pulses VERSE pulses reduce SAR VERSE pulses are implemented RF Pulse Type Fast  RF Pulse Type Low SAR *Uses a shorter RF pulse with less perfect slice profile **Replaced with standard RF pulse if IR is selected High resolution 3D imaging with small slice thicknesses at 3T has improved diagnosis in a broad range of diseases 3D Imaging allows Increased resolution Isotropic voxels   syngo SPACE – Sampling Perfection with Application optimized Contrast using different flip angle Evolutions Evaluation of MS 3D DESS Knee with isotropic imaging 3D data can be reformatted using MPR T1 Relaxation Time increases at 3T Limitations and Challenges May need to increase TR to maintain tissue contrast Increased slice-to-slice crosstalk Potential lengthening of scan time T1 Relaxation times are longer with increasing field strength   T1 Constants (in ms)   1.5T 3T Gray Matter 1013 1445 White Matter 560 701 • Longer measurement times, due to increased TR • Cross-talk • Magnetization transfer effects (MT Effects) with multi-slice imaging • Short excitation pulse 75° instead of 90° • Short refocusing pulse 150° instead of 180° • T1 SE multi-slice   a.  Low contrast - Flip angle = 90°/180°   b.  Improved contrast - Flip angle = 75°/150° A) 2D FLASH B) 3D MPRAGE C) T1 Dark Fluid D) T1 Spin Echo Sequences Advantages Disadvantages 2D FLASH Fast scan time, no MT Effects, Low SAR, Thin slices, Good T1 contrast, No pulsatility artifacts Bright vessels 3D MPRAGE 3D Volume, Low SAR, Good T1 contrast Long scan times, susceptible to patient movement, MT Effects after contrast T1 Dark Fluid Very good T1 contrast High SAR, Pulsatility artifacts, IR pulse suppresses the signal of blood flow in vessels, longer measurement times T1 Spin Echo Good T1 contrast if using reduced flip angles High SAR, Pulsatility aftifacts, MT Effects after contrast, Long measurement times,