MRI Physics & Instrumentation PDF

MRI PHYSICS &INSTRUMENTATION{315AMRS} BSc .in sectional imaging Applied radiologic technology College of Applied Medical Sciences/University of Jeddah Dr :Dalia Bilal. assistance professor Dr :Elbager Hamza assistance professor • Objectives : By the end of this course the student will be able to:...

MRI PHYSICS &INSTRUMENTATION{315AMRS} BSc .in sectional imaging Applied radiologic technology College of Applied Medical Sciences/University of Jeddah Dr :Dalia Bilal. assistance professor Dr :Elbager Hamza assistance professor • Objectives : By the end of this course the student will be able to: 1. Define the knowledge of the students in the theoretical background of selected nuclear magnetic resonance images. 2. Provide students with the ability to interpret different radiological modalities 3. Analyzed the key physics concepts of magnetic resonance 4. Evaluate the ability of students to understand the different scanning pulse sequences 2 Course contents : 1. 2. 3. 4. 5. 6. 7. 8. the basic scientific principles of a magnetic resonance imaging. all types of pulse sequences the production of an image, image contrast &image quality. different types of artifacts the mean of angiography in MRI. contrast agent. instrumentation of an MRI system Recognize the laboratory portion of this course which enables students to identify to the instrumentation of MR department. Image Contrast and Relaxation processes Contrast mechanisms • Contrast is the difference in brightness between the light and dark areas of a picture. • An MRI image also has a contrast! • In order to make a diagnostic image, however, we need to be able to create contrast between different structures/tissues/pathologies. What is contrast? • An image has contrast if there are areas of high signal (white on the image), as well as areas of low signal (dark on the image). • Some areas have an intermediate signal (shades of gray). 5 ▪ A tissue has a high signal (white) if it has a large transverse component of magnetization; (the amplitude of the magnetization received by the coil is large, and the signal induced in the coil is also large. ▪ A tissue has a low signal (black) if it has a small transverse component of magnetization; (the amplitude of the magnetization received by the coil is small, and the signal induced in in the coil is small. ▪ A tissue gives an intermediate signal (gray) if it has a medium transverse component of magnetization; (the amplitude of the magnetization received by the coil is medium, and the signal induced in in the coil is medium. 6 Image contrast 7 Image contrast is controlled by: • Extrinsic contrast parameters (those that are controlled by the system operator). • Intrinsic contrast mechanisms (those that do not come under the operators control). Extrinsic contrast parameters includes: 1)Repetition time (TR) 2) Echo time (TE) 3) Flip angle (FA) 4) Turbo-factor or echo train length (ETL/TF) 5) Time from inversion (TI) 6) ‘b’ value 8 1)Repetition time (TR); It is the time from the application of one RF pulse to the application of the next RF pulse. • It is measured in milliseconds(ms). • The TR affects the length of a relaxation period after the application of one RF excitation pulse to the beginning of the next. 9 2) Echo time (TE);It is the time between an RF excitation pulse and the collection of the signal. • The TE affects the length of the relaxation period after the removal of an RF excitation pulse and the peak of the signal received in the receiver coil. • It is measured in ms. 10 3) Flip angle; this is the angle through which the NMV is moved as a result of a RF excitation pulse. 11 4) Turbo-factor or echo train length (ETL/TF): • The turbo factor is the number of echoes acquired after each excitation. 5) Time from inversion (TI): • Inversion recovery (IR) is a conventional spin echo (SE) sequence preceded by a 180° inverting pulse. In other words, if a SE sequence is denoted by {90°−180°−echo}, the IR sequence can be written 180° — {90°−180°−echo} • The time between the 180° inverting pulse and the 90°-pulse is called the inversion time (TI). 12 6) ‘b’ value: The b-value is a factor that reflects the strength and timing of the gradients used to generate diffusion-weighted images. The higher the b-value, the stronger the diffusion effects. 13 Intrinsic contrast mechanisms includes: • T1 recovery • T2 decay • Proton density • Flow • Apparent diffusion coefficient (ADC): is a measure of the magnitude of diffusion (of water molecules) within tissue, and is commonly clinically calculated using MRI with diffusion weighted imaging (DWI) 14 in different tissues 15 Relaxation Processes • When the RF pulse is switched off, the NMV is again influenced by B0 and it tries to realign with it. To do so, the hydrogen nuclei must lose the energy given to them by the RF pulse. • The process by which hydrogen loses this energy is called relaxation. • As relaxation occurs, the NMV returns to realign with B0 because some of the high energy nuclei return to the low energy population (their magnetic moments in the spin-up direction) 16 • The amount of magnetization in the longitudinal plane gradually increases-this is called recovery. • At the same time, but independently, the amount of magnetization in the transverse plane gradually decreases-this is called decay. • As the magnitude of transverse magnetization decreases, the magnitude of the voltage induced in the receiver coil decreases. 17 Remember • during relaxation hydrogen nuclei give up absorbed RF energy and the NMF returns to B0. • At the same time but independently the magnetic moment of hydrogen lose coherency due to dephasing. • Relaxation results in recovery of magnetization in the longitudinal plane and decay of magnetization in the transverse plane. • The recovery of longitudinal magnetization is caused by a process termed T1 recovery. • The decay of transverse magnetization is caused by a process termed T2 decay. • 18 The T1 recovery curve 19 20 • In MRI, contrast in the image is obtained through three mechanism i.e. T1 recovery, T2 decay and proton density. • The image contrast depends on how much we allow each process to happen. • The T1 time of a tissue is the time it takes for the excited spins to recover and be available for the next excitation. • The reason that this factor can be used to produce contrast on the image is that the different tissue has different rates of T1 recovery. • The most marked difference is between the recovery rates of fat and pure water. 21 • Following a 90◦ RF pulse removal, fat recovers its longitudinal magnetization quickly. • This is due to the fact that it has large molecules with relatively slow Brownian motion that can dissipate energy quickly. • This means that in fat, the spin population loses the absorbed energy quickly and regains its low-energy, spin-up condition. 22 • Pure water, on the other hand, has high energy molecules with rapid Brownian motion that cannot dissipate energy readily. • Pure water nuclei therefore retain the absorbed energy and the magnetic vector associated with pure water remains in the transverse plane for longer than that of fat. 24 25 • If we rapidly apply a second 90◦ Pulse to the sample, the fully recovered NMV from fat will once again be flipped into the transverse plane - giving maximum signal. • The partially recovered water vector, however, will be flipped into the transverse plane and will subsequently return only a limited signal. 26 Weighting • When describing the contrast of an MRI image the term ‘weighting’ is used to indicate that the contrast is weighted or heavily influenced by one of mentioned parameters. • To demonstrate either T1, proton density or T2 contrast, specific values of TR and TE are selected for a given pulse sequence. • The selection of appropriate TR and TE weights an image so that one contrast mechanism predominates over the other two 27 • • • • Repetition Time (TR) and T1 Weighting. Repetition time (TR) is the length of the relaxation period between two excitation pulses and therefore, it is crucial for T1 contrast. TR controls how far each vector can recover before it is excited by the next RF pulse. For T1 weighting, the TR must be short enough so that neither fat nor CSF has sufficient time to fully return to B0. If the TR is too long then both fat and CSF will fully recover to the longitudinal magnetization. In that case, the difference in T1 contrast can not be demonstrated in the image. • Short TR → strong T1 weighting Long TR → low T1 weighting • For T1 weighting we should chose a short TR. 28 T1 contrast A T1-weighted image uses a short TR and will exhibit bright fat and dark fluid. 29 30 Typical parameters ▪TR 300-600 ms (shorter in gradient echo sequences) ▪TE 10-30 ms (shorter in gradient echo sequences) Typical parameters for T1 WI: ▪Short TR. ▪Short TE. Sagittal T1weighted image of spine. Intraspinal lipoma is bright as it contains fat 31 Typical parameters for T1 WI: ▪Short TR. ▪Short TE. 32 T1 o The T1 time of a particular tissue is an intrinsic contrast parameter that is inherent to the tissue being imaged. o It is defined as the time it takes for 63% of the longitudinal magnetization to recover. o The period of time during which this occurs is the time between one excitation pulse and the next (RF) o The TR determines how much T1 recovery occurs in particular tissue. o T1 recovery is caused by the nuclei giving up their energy to the surrounding environment or lattice. o It is termed spin lattice relaxation. o Energy released to the surrounding lattice causes the magnetic moments of nuclei to recover their longitudinal magnetization o The rate of recovery is an exponential process, with a recovery time constant called the T1 relaxation time 33 • The T2 time determines how quickly an MR signal fades after excitation. • Following the removal of the 90º pulse, the spins dephase rapidly. This is due to two processes: 1. The main cause of dephasing is field inhomogeneity. The introduction of human body into the field will also further spoil the homogeneity as water molecules are slightly repelled by an external field (known as diamagnetism) 2. The secondary cause of dephasing is the fact that the nuclei themselves have magnetic fields and these fields interact over time, attracting and repelling each other, these are known as spin-spin interactions. 34 • The reason that T2 is an image contrast parameter is that different tissues lose phase coherence at different rates. • The most marked difference is between solids and pure water molecules. • Following the removal of the 90º RF pulse, the magnetic vectors of tightly packed nuclei in solid structures such as bone undergo many spin-spin interactions and dephase readily and quickly. 35 • The widely spaced molecules in water contain hydrogen nuclei that undergo fewer spin-spin interactions and therefore their magnetic vectors stay in- phase for longer than those in solid bone. • The rate of decay is described by a time constant, T2* • T2* characteristics dephasing due to both B0 inhomogeneity and transverse relaxation (Mx-y). 36 • Contrast is therefore obtained by waiting for a certain time period after application of the 90° Pulse before sampling the returning signal. • Any tissues that have lost phase coherence will appear darker than tissue whose spins still in phase. • Although not strictly a solid, fat contains nuclei that are more closely spaced than water and that therefore returns an intermediate signal. 37 38 Echo Time (TE) and T2 Weighting • Echo time (TE) is the interval between application of the excitation pulse and collection of the MR signal. • If a short echo time is used (25ms), the signal differences between tissues are small because T2 relaxation has only just started and there has only been little signal decay at the time of echo collection. The resulting image has low T2 weighting. 39 • If a long echo time is used (100ms), the signal differences between tissues will be large. • Tissues with a short T2 have lost most of their signal and appear dark on the image while tissues with a long T2 produce a stronger signal and thus appear bright. This is the reason why cerebrospinal fluid (CSF) with its longer T2 (200ms) is brighter on T2-weighted images as compared to fat(80ms). • Short TE → low T2 weighting Long TE → strong T2 weighting 40 41 T2 contrast A T2 weighted image uses a long TE and will exhibit bright water, very dark solid bone medium intensity from fat. 42 43 Typical parameters ▪TR 2000 ms+ ▪TE 70 ms+ Sagittal T2 weighted image of spine. Intraspinal lipoma is dark as it contains fat 44 T2 T2 decay time of a particular tissue is an intrinsic contrast parameter and is inherent to the tissue being imaged. • It is defined as the time it takes for 37% of the transverse magnetization to be lost due to dephasing. • The period of time over which this occurs is the time between the excitation pulse and the MR signal (TE) • The TE determines how much T2 decay occurs in a particular tissue. 45 • The term proton density refers to the number of hydrogen nuclei present within a given volume of tissue. • To compare extreme, think of air and water. There are more hydrogen nuclei in the fluidfilled ventricles of the brain than in the nearby air-filled paranasal sinuses. 46 • A proton density weighted image will therefore have varying degrees of signal from different tissues. • A proton density image is obtained by using parameters that reduceT1 and T2 contrast, i.e. a long TR to reduce T1 effects and a short TE to reduce T2 effects 47 • To produce contrast due to the differences in the proton densities between the tissues, the transverse component of magnetization must reflect these differences. • Tissues with a high proton density (e.g. brain tissues) have a large transverse component of magnetization (high signal) and are bright on a proton density contrast image. • Tissues with a low proton density (e.g. cortical bone) have a small transverse component of magnetization (low signal) and are dark on a proton density contrast image. 48 (PD) Contrast 49 50 References ▪ Cathrine Wesbrook.MRI at a Glance-Blackwell science 2002 ▪ Cathrine Wesbrook.MRI in Practice – second edition-Blackwell science 1993 51