High Field MRI Advantages & Disadvantages PDF

High Field MRI, its Advantages & disadvantages Dr.Amal Alorainy Lecture 1 1 Objectives  To Identify the benefits and challenges of using higher fields - 3T.  To explore T1 Recovery times, T2 and T2* relaxation times at 3T.  To discuss the affe...

High Field MRI, its Advantages & disadvantages Dr.Amal Alorainy Lecture 1 1 Objectives  To Identify the benefits and challenges of using higher fields - 3T.  To explore T1 Recovery times, T2 and T2* relaxation times at 3T.  To discuss the affects of magnetic field inhomogeneities at 3T.  To describe a variety of artefacts at 3T. 2  Introduction  The main aim of introducing high MRI scanners is to boost the MR signal / SNR.  However, added to the advantages of higher field strength, there are several disadvantages & sequence parameters used will also affect the signal gain: T1 & T2 times of the tissues being imaged. Image artefacts. The SAR limitations (flip angle being used).  The question is: How do we use this increased signal to improve the quality of images or the service provided? 3 SNR at high fields Moving from a 1.5T to a 3T magnetic field will This increase in signal can be used to reduce double the SNR, why? the scan time / improve the temporal resolution, and / or to improve the spatial resolution. 4  Comparison of 1.5T and 3T axial 2D FLAIR MR images in a 50-year-old healthy woman. M. Neema et al. AJNR Am J Neuroradiol 2009;30:911-916 ©2009 by American Society of Neuroradiology  SNR at high fields 3T T2 WI - MRCP 1.5 T 6 Spatial resolution at high fields  The SR can be increased without an increase in the number of averages (NEX) or the SR can be the same with a reduced NEX & therefore shorter time, why?  The inherent signal is high enough to reduce the voxel size, and thereby increasing the resolution without a time penalty.  MR scans at 3T can be undertaken with much higher resolution and thinner slices. 7 Spatial resolution at high fields 8  Spatial resolution at high fields 1.5 T - 256 x 256 matrix 3T - 512 x 512 matrix 9 Scan time / Temporal resolution at high fields  The high SNR can be used to reduce sequence acquisition time: 1. Can be applied for dynamic scanning such as imaging of the liver, pelvis, breast, prostate or brain, improve the detection for some disease processes. 2. Acquisition times can be reduced to increase patient throughput on the scanner & also improve patient compliance & decrease the possibility of motion. 10 Pre contrast 10 s post Gd 20 s post Gd Temporal resolution for pathology T1WI + FS 11  T1 relaxation time at higher fields An increase in magnetic field strength generally increases T1 relaxation times in tissues. The rate at which protons precess is determined by the Larmor Equation: ω = ∂ B◦ If B○ is increased, the precessional frequency is increased. When T1 relaxation takes place, the spins give up their energy back to the surrounding (spin-lattice relaxation). Therefore, the higher the precessional frequency  the lower the efficiency of T1 relaxation (spin-lattice relaxation)  the longer the spins take to transfer their energy to the surrounding tissue & to relax back to B○  slower relaxation of tissue - the T1 recovery / time is longer. 12  T1 relaxation time at higher fields T1 Recovery Curves for 3T vs 1.5T e.g. Brain Tissue T1 @3T- 1820 ms e.g. Brain Tissue T1@ 1.5T – 950 ms TR For T1-Weighting, TRs at 3T should be increased from 500ms to 700ms to achieve comparable signal and contrast 13 characteristics as in 1.5T.  1.5T 3T T1WI of brain @3T: CNR between grey and white matter is reduced @3T: CNR between white matter and basal ganglia is reduced 14  T2 relaxation times at higher fields  T2 relaxation times mostly independent on Bo as they rely on interactions between individual spins (spin-spin interaction). Generally T2 relaxation times are shortened by ~ 10%.  For soft tissue where protons are nearer each other, the effect of interactions are stronger  there is more dephasing resulting in a slightly shorter T2 time.  For adipose tissue where protons are less tightly bound, the drop in T2 times is not as much as in soft tissues.  For fluid the interactions remain the same at higher field strengths so there is relatively no change in T2 times.  Further reduction in SNR are usually seen in longer TEs protocols  TE values need to be reduced for T2WI @ 3T to compensate for signal dephasing. 15  T2* relaxation times at higher fields  It is of a greater concern due to its faster decay at 3T compared to 1.5T, as it depends on the external magnetic field homogeneity.  T2* decay times are much shorter due to increased inhomogeneity in the magnetic field, increased susceptibility effects, & spin-spin interactions.  Mainly seen on GRE sequences, WHY ?! 16  T1 and T2 Relaxation Times T1 Relaxation (ms) T2 Relaxation Times (ms) Tissue 1.5T 3T 1.5T 3T Grey Matter 950 1820 100 80 White Matter 780 1084 80 70 Liver 586 809 46 34 Kidney 966 1142 85 81 Spleen 1057 1328 79 61 Pancreas 584 725 46 43 17  Susceptibility effects  Magnetic susceptibility is the extent to which a material becomes magnetized when placed in a magnetic field.  Susceptibility artefacts occur due to microscopic variations in magnetic field strength near interfaces of different materials of different susceptibilities, leading to frequency variations, e.g. air-soft tissue or air- bone interface.  The effects of susceptibility are proportional to field strength. At higher field strengths it is harder to have a homogenous field  Larger frequency variations at 3T, so susceptibility appears more pronounced.  Since the susceptibility of metals is much higher than soft tissues, metal objects produce more susceptibility artefacts with blooming & distortion effects, which may obscure findings at 3T that may be visualized at 1.5T. 18  Susceptibility effects  E.g. Prostate imaging where patients have hip replacement, the artefact appears much worse than on the 1.5T scanner.  GRE- & EPI- related sequences are mostly affected by susceptibility (T2*) effects & lower SNR. This is because GRE sequences dose not have 180º refocusing pulses, & EPI acquisitions are made up of a constantly declining echo (long ETLs) & apply fast changing of gradients.  A reduction in TE will give the spins less time to dephase  susceptibility will be less evident on the images.  However, the susceptibility effect can be helpful to see iron deposition in tissue (haemorrhage). As T2* affects are directly related to magnetic field inhomogeneity, images are greatly affected by this dephasing. 19  1.5T 3T Susceptibility effect from bleeding on Gradient Echo (T2*)Images 20  1.5T 3T Susceptibility artefact (air-tissue interface) on DWI-EPI sequence 21  1.5T 3T Susceptibility artefact on single shot DWI-EPI sequence 22  Chemical shift artefact of the 1ST kind at higher fields  C.S artefact of the first kind appears due to the differences in precessional frequency between fat and water (220Hz @ 1.5T - 440Hz @ 3T). Fat precesses at a lower frequency than water.  The problem exists when fat and water live in the same voxel, the computer cannot register the differences in frequencies, leading to spatial mismapping of fat and water pixels (signals).  It occurs along the F.E direction and appears as a bright line around the organ towards the higher part of the frequency gradient & a dark line towards the lower part.  Chemical shift more apparent at 3T. 23 C.S of the 1st kind appears around the kidneys & orbital area on T2WI @ 3T 24 Chemical shift of the 2ND kind at higher fields  C.S of the 2nd kind / phase cancellation /chemical misregistration / in and out of phase imaging is not limited to F.E direction.  It can be seen based on intra-voxel phase-cancellation effect, when fat & water exist in the same voxel out of phase to each other by 180º.  Size of the effect dose not increase with Bo.  However, TEs need to be adjusted at 3T because fat and water precess faster at Fat 3T  and water the times when they are Fat and in and outwater of phase out of phase - TE (ms) inphase - TE (ms) will come around faster & therefore become shorter at 3T. 1.5T 2.2 ….. 4.4 ….. 3T 1.1, 3.3 ….. 2.2, 4.4 ….. 25  In-phase Out-of-phase A doubling of field strength will half the in and out of phase TE values. 26 Benefits of increased chemical shift  The wider chemical shift separation between fat and water at 3T will increase the efficiency of spectral fat suppression (a 90º excitation selective pulse with a narrow bandwidth centred on the resonant frequency of fat) why?  as there is greater distance between water peak and the fat peak, there is very little overlap of signals and fat is more selectively suppressed.  E.g. Breast imaging, MSK, carotid plaque imaging.  Increased differences in resonant frequency between fat & water at 3T allows for better separation between peaks on MRS. 27  1.5T 3T At 3T the images are more susceptible to susceptibility artefact at the air-tissue interface, leading to inadequate fat 28 suppression. Specific Artefacts to 3T When electromagnetic waves come in contact with the human body it results in certain phenomenon: a. The wavelength decreases. b. Electrical currents are generated. c. Wave refraction may occur as it enters the body. 29 Specific Artefacts to 3T 1. Dielectric effect: as Larmour frequency increases from 64MHz @ 1.5T to 128MHz @ 3T, the wavelength decreases from 52cm @1.5T to 26 cm @ 3T  This results in areas of high S.I and low S.I. Can be most prominent in smaller, athletic built patients with large FOVs scans. 30 Specific Artefacts to 3T Low S.I High S.I areas of high and low signal intensities due to dielectric effect. 31 Specific Artefacts to 3T 2. Electrical currents: are generated in the presence of a conducting medium (patients with ascites, pregnant patients), there is significant induction of an electric current  this acts as an electromagnet that opposes the main magnetic field & leads to signal loss in the image. 32  T2 SS-FSE @ 3T T2 SS-FSE @ 1.5T Specific Artefacts to 3T Signal loss shown in the anterior abdomen due to Electrical currents 33  Clinical Consideration & limitation of 3T: Specific Absorption Rate (SAR) level  SAR is a measure of RF energy deposition within body.  RF pulses induce an electric field in the patient which interacts with ions & molecules, resulting in acceleration & rotation of these molecules  this leads to heat up of tissue  increase the SAR level.  At 3T, SAR limits are reached more quickly  By doubling the field strength, SAR is quadrupled. 34  In conclusion  Doubling the SNR at 3T will not generally be effective without additional sequence modifications.  Generally speaking, every artefact presents at 1.5T is also presents at 3T, but in some situations higher field strength causes artefacts to be more problematic & prominent.  Devices containing metal & may considered safe at 1.5T are not necessarily safe at 3T & need to be tested. 35  Summary  3T provides double the SNR of 1.5T.  T1 recovery times of tissues increases, especially in brain parenchyma.  There is a slight reduction on T2 relaxation times @ 3T.  Susceptibility effects are more obvious due to inhomogeneities within the magnetic field and interactions between tissues.  Chemical shift artefact is larger but results in more effective fat suppression. 36 Homework  What pathologies are seen better in high or low filed and why? 37 REFERENCES  Article : 6 Feb 2023, Andrew Murphy https://radiopaedia.org/articles/mri-artifacts-1  Kamel I, Merkle, E. (2011) Body MR Imaging at 3T, Cambridge University Press.  The Physics of Clinical MR Taught Through Images 4th Edition.by Wolfgang R. Nitz (Author), Miguel Trelles (Author), Frank Goerner (Author) 38