MRI 2024 Student Notes - Topic 1 & 2 PDF

Magnetic Resonance Imaging MRI is used for prescribing slices (MRI) is a computerized cross-sectional imaging modality that uses a: ▪ strong magnetic field and axials : up-down...

Magnetic Resonance Imaging MRI is used for prescribing slices (MRI) is a computerized cross-sectional imaging modality that uses a: ▪ strong magnetic field and axials : up-down peronals: posterior- anterior ▪ radiofrequency (RF) pulses trasnverse: side to side coils or antenna gives off RF pulses and is placed around the ROI - need both magnetic field and RF pulses for an image - can cause sparking if there is metal to create cross-sectional images of the human body in a clinical setting. The use of x-rays (ionizing radiation) is not necessary for the production of images. if metal is fixed to the body (i.e titanium, screws, stabilization) fibrotic tissues form around and wont pull out of the body anything that is loose and feromagnetic gets pulled out (phones, change, cards demagnitized) - metal will be attracted and can cause pulling, discomfort, artifacts, remind patients why it is necessary to take off metal and clothing can be made with metalic thread which can cause burning and fires, no makeup of false eyelashes non-feromagnetic wont get pulled - we use non-feromagnetic equipment like wheelchairs and stretchers to do patient transfers in MRI MRI is very different from CT and provides a number of important advantages: considered gold standard for finding what is wrong best modality for brain, spine/ spinal chord and MSK (musculoskeletal), meniscus, abdomen ▪ No ionizing radiation ▪ Superior soft tissue contrast resolution signal is better, see the anatomy in better detail ▪ Direct multiplanar imaging ▪ No bone or gas artifacts ▪ Chemical composition analysis is possible ▪ No known biological hazards - slices/ cut thickness cannot be changed, the exam must be redone for thinner cuts for example - MRI doesnt work well with air (i.e lungs or pneumonia) we would use CT instead - takes a long time to scan - there is no emergency MRI scan because of this (only x-ray, CT-especially for MVA-, and ultrasound) CT is also a cross-sectional modality but only in axial direction but we can reconstruct for sagital and coronal CT and MRI go hand in hand for imagign joints in the body ▪ CT produces images based on the x-ray attenuation (absorption) of body parts/tissues. CT shows bleeding in the brain great and is a fast modality making it great for MVA or strokes for appendicitis: CT or ultrasound to rule out echo cardiography: ultrasound of the heart whc isnt always definitive, so many MRIs are used to include more information mammography also used in MRI to deceifer pathology ▪ MRI is not based on x-ray attenuation of tissues demonstrating very different relationships between tissues in the body.  ▪ MRI has the ability to distinguish soft tissue structures based on their response to radio- frequency pulses while exposed to a strong / main magnetic field. RF pulses are given off from antennas ▪ These responses are demonstrated as high or low intensity signals that appear as bright or dark areas on an image. ▪ Physicians are able to view & differentiate between high detailed normal anatomic structures and pathology. CSF is fluid and shows brighter in parameters and other stuff in other shades of gray parts of the brain is ade of white (more fat) and gray matter ▪ The contrast in between body tissues (especially soft tissues) in MRI is based on: - the differences in their biological composition - on their appropriate response to RF pulses while exposed to a strong magnetic field. ▪ The degree of brightness, darkness, signal and resolution of MR images can be controlled through the adjustment of a variety of MRI parameters. we adjust the parameters to see CSF as dark OR bright  This gives MRI superior flexibility than CT Signal strength is dependent on factors such as: number of precessing nuclei in a given volume of tissue relaxation rates of nuclei flow stationnary tissue or is there flow (i.e blood vessels, CSF from brain and spinal chord, bile from the liver is stored in the galbladder and pushed bile into duodenum to breakdown to be absorbed in small intestine, lymphatic fluid Disadvantages of MRI over CT: ▪ Expensive maintenance and equipment (MSCT scanners’ costs are high as well). magnetic field is created with electricity which forms heat and we need to cool it with helium ▪ Data acquisition time is longer. ▪ Not everyone can tolerate the exam due to its confinement (claustrophobia) and loud noise—open magnets have addressed the confinement issue. two way mirrors for patients to see you when laying down ▪ Not everyone can have an MRI; ie: patients with certain pacemakers (contraindication) welders, and ophtamologist has to remove metal in the eye, need orbit xray first cochlear implants that work with magnets  Resolution Spatial resolution = CT Contrast resolution > CT Temporal resolution < CT based on the timing of the scan, MRI scans are longer - patients cant always stay still and causes motion artifacts Temporal resolution relates to the duration of time for acquisition of a single frame of a dynamic process, i.e., cine imaging (Radiology Reference Article | Radiopaedia.org) Basic MR Components superconducting magnet, work with electromagnetism ▪ Magnet and shim coils makes homogeneous signals ▪ Gradient coils (XYZ) gives us the planes x (sagital), y (transverse), z (axial) ▪ RF coils (Transmit/Receive) ▪ Computer and operator’s console  MR images are created by: ▪ Transmitting radio-frequency pulses (RF) directly to a specific anatomical part while the patient is lying in an external magnetic field. entire body is affected by main magnetic field RF is what provides signal from knee coil for example, only in that ROI ▪ The RF pulses are emitted via an RF antenna in order to disturb/stimulate the protons that are aligned parallel to the external magnetic field. 80% of body is hydrogen (found in fluid and fat H protons spin around its own axis randomly the spins cause little magnets / dipoles (2 directional) when placed into a strong magnetic field, some protons are attracted (most) or repelled by the magnetic field when RF pulses are at same frquency there will be absorption of the radiofrquencies by that tissues and will move away from paralel of main magnetic field ▪ The anatomical part that is surrounded by the antenna absorbs the energy from the RF pulses. parameters tell RF pulses to stop when you determine, the body will release the absorbed energy and has different relxation rates bones, meniscus, ligaments, tendons, fluid, fat ect have different relaxations ▪ When the radio-frequencies are turned off, the absorbed energy/signal is released (relaxation rates) from the tissues of the anatomical part. ▪ The signal/energy released is measured and reconstructed into a cross-sectional image of that anatomical part.  MRI Signal ▪ Anatomical tissue must be able to respond to an external magnetic field and to RF in order for an MR signal to be produced. ▪ There are several chemical substances within the human body that generate measurable signals in MRI. ▪ Since water is the most abundant substance within the human body and it has one proton, it is useful in generating MR images; hydrogen makes up about 80%-90% of all the atoms in the human body. ▪ Clinical MRI is based on the production of signal from hydrogen (H) in the body. ▪ Since many pathologic conditions (diseases) contain large amounts of fluid (hydrogen), these pathologies have strong signals and thus are easily identifiable. edema and inflammation around pathologies MR Physics Review Hydrogen protons in the body ▪ The atom consists of a central nucleus, containing protons and neutrons and orbiting electrons. ▪ Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. ▪ Each element is exclusive because it has a different number of protons in its nucleus. ▪ Hydrogen has the simplest atomic structure -- one proton and one electron (no neutron) therefore it is often referred to as a proton. ▪ Water (H2O) is used to generate MR images; H2O is made up of two hydrogen atoms bound to one oxygen atom.  There are three types of motion present within the atom that can create a magnetic field: ▪ Negatively charged electrons spinning on their own axis ▪ Negatively charged electrons orbiting the nucleus ▪ The nucleus spinning on its own axis. (p. 3) page 4 from text book even number of protons and nutrons have rotation forces that cancel out and has no net spin, no signal nuclei with odd number of P and N, have an angular momentom or a net spin, i.e H (1) other elements (odd elements) are used in spectrosity, only H is used in clinical MRI Hydrogen rotates at a different frquency than the other elements (RFantenna is sent at same RF as H) hydrogen can then pickup that energy the other elements spin are not at the same frquency as the antenna pulses The laws of electro-magnetism state that: Charged particles in motion generate an electric current; a magnetic field is created when a charged particle moves. Protons have a positive electrical charge and a spin. H protons are "magnets" Faraday’s Law: A charged particle in motion (moving electrical field) produces a magnetic field and vice versa (p.4) ▪ This spin produces an electrical current, which in fact creates a magnetic force or magnetic field. ▪ The proton thus has magnetism and is considered as a little bar magnet with north and south poles. ▪ MR active nuclei are elements that have an odd number of protons and are the only elements that can interact with an external magnetic field because they have angular momentum or spin; they possess an electrical charge. ▪ MRI is based on the spinning motion and creation of magnetic fields of MR active nuclei in tissues only. ▪ Examples of MR active nuclei include: hydrogen, phosphorus, sodium, fluorine, etc. (p.4) hydrogen has a high percentage and thus is used to target H at the same frequencies as Hydrogen to give resonance  The hydrogen nucleus containing a spinning positively charged proton has a magnetic field induced around it and therefore acts like a little bar magnet or magnetic moment. The magnet of each spinning hydrogen nucleus has a north and south pole; like every other magnet—this is called a magnetic dipole. ▪ This dipole is represented by a magnetic moment. ▪ A magnetic moment can be represented as a vector and is symbolized by an arrow; it has vector properties, (it has magnitude/size and direction). ▪ The direction of the arrow represents the orientation of the magnetic moment, whereas the length of the arrow represents the strength of the magnetic moment in main magnetic field aka net magnetization angular moment vectors are all directions and sizes ▪ MR active nuclei have a tendency to align their axis of rotation to an applied magnetic field. all in same direction of main magnetic field, all vectors add up ▪ The vector sum of the individual aligned magnetic moments of all the protons in the nucleus is called the total magnetic moment. ▪ Hydrogen is the MR active nucleus used in MR; Its single proton gives it a large magnetic moment.  In the absence of an applied magnetic field, these dipoles are randomly distributed, canceling each other out; the patient does not possess net magnetization. (p.6) When placed in an external magnetic field, the magnetic moments align with it when we approach the main magnetic field our prrotons also align in the coils of wire creates the superconduction same direction (both aptient and tech) - larger scanners are not made of iron face different directions and spinning B0= main magnetic field cardiac exams can be upwards of 1.5hrs non-ionizing radiation  net magnitization beta sub-zero or B-not An external magnetic field can be represented by B0. The clinical MRI magnet is much larger and more powerful than a little bar magnet. The magnetic force it produces ranges from 0.035 to 3.0 Tesla (T). One Tesla is equal to 10,000 Gauss. higher the gauss, the higher the strength This stationary magnet produces a strong magnetic field within the tunnel-shaped gantry in which the patient is positioned during the examination. protons (H) align with the magnetic field direction of pull is positive (due to net magnetization) but some repel  Resonance ▪ In a normal environment where the magnetic field is absent, hydrogen nuclei are randomly distributed. ▪ When these hydrogen nuclei are placed in a strong external magnetic field, the magnetic moments of the hydrogen nuclei align themselves in one of two directions:  Nuclei with a high energy level have magnetic moments opposed or anti-parallel to the external magnetic field. defy the energy of the magnet, they are able to oppose the magnet Nuclei with a lower energy level have magnetic moments aligned parallel to the external magnetic field. Low-energy nuclei outnumber high-energy nuclei by only a few protons. (p. 7)  Parallel and anti-parallel protons cancel each other’s magnetic effects. There is a larger number of low energy nuclei aligned parallel; this excess produces the net magnetic moment (sum of magnetic effects) that is used in MRI. (p.8) An overall net magnetization occurs in the parallel direction. no signal yet, protons are aligned The individual magnetic moments of the hydrogen nuclei oriented parallel to the external magnetic field are added together resulting in the total net magnetization which is symbolized by the vector NMV (net magnetization vector). p. 9  The patient has net magnetization. NMV represents the sum of many small magnetic moments, generated by the slight majority of hydrogen nuclei oriented parallel to the external magnetic field B0. As this magnetization is in the direction along/longitudinal to the external magnetic field (z-axis), it is also called longitudinal magnetization.  net magnitization vector The interaction of the NMV with B0 and RF pulses is the fundamental premise of MRI. The influence of an external magnetic field, B0, produces an additional phenomenon; the NMV starts to wobble around B0; this is called precession. the wobble is not synched up yet, not in phase yet The magnetic moments precess around the axis of B0, (similar to a spinning top). Hydrogen nuclei are not aligned perfectly parallel or anti- parallel to the magnetic field; they are slightly angled to B0 and precess around it. RF pulses are given off at 90 degree to this main vector comign from the antenna around the body part being imaged - RF pulses must be the strength or rotation frequency of the hydrogen protons local frequency for aborption to take place and to acquire signal every element in the body precesses at a different frequency  This path around the axis of B0 is called the precessional path and the rate at which the NMV precesses around B0 is called the precessional frequency. The unit of precessional frequency is the megahertz (MHz). as magnetization increases (teslas), the precessional frequency will also increase The precessional frequency is very fast; for hydrogen protons it is around 42 MHz in a magnetic field strength of one Tesla. This means that the protons precess more than 42 million times per second. (p.10) at 1 tesla: 42 MHz will precess more if we increase this  The frequency of precession is represented in the Larmor equation. (p. 10) omega sub-zero gamma The Larmor equation states that: (ω0) = B0 x γ where: beta sub-zero ω0 = the precessional frequency of a given element (H) (expressed in MHz) antenna will match this frequency B0 = the external magnetic field strength strength of magnet (expressed in Tesla) γ = the gyromagnetic ratio (this is a constant of a specific nucleus at 1.0 Tesla—MHz/T) (Hydrogen has a gyromagnetic ratio of 42.57 MHz/T) every element or nucleus has a different gamma number if B0 increased, the precessional frequency will increase, gamma is a constant and won't change when hydrogen protons absorb the same frequency, they will flip to transverse plane and become in phase ▪ The precessional frequency is also called the Larmor or resonant frequency. ▪ This equation demonstrates that as B0 changes, the rate at which nuclei precess (precessional frequency) changes proportionally. alll H and odd numbered elements will change precessing according to magnet strength ▪ Within a magnetic field, all hydrogen protons precess at the same rate or frequency. ▪ Other MR active nuclei have different precessional frequencies because they have different gyro-magnetic ratios. Hydrogen has a very high gyromagnetic ratio. ▪ The precessional frequency of hydrogen is 63.86 MHz at 1.5 Tesla ▪ The precessional frequency of hydrogen is 42.57 MHz at 1.0 Tesla ▪ The precessional frequency of hydrogen is 21.28 MHz at 0.5 Tesla  ▪ Once exposed to a magnetic field, the magnetic moments all precess at the same frequency but precess randomly and are thus incoherent; in other words, they are out-of-phase. ▪ They have different locations around the precessional path. ▪ Precessional phase: the position of the magnetic moments in their respective precessional path. law of electromagnetism states, signal is picked up only is the NMV if in the transverse plane and the precessional path of the hydrogen moments must be in phase RF pulse must be the same as hydrogen for them to pickup energy and they get excited that they arent as attracted to the magnet and flip 90 degrees ▪ At this point, we cannot measure any signals stemming from the magnetization of the patient because it is in the same direction, parallel to the external magnetic field, B0. ▪ A disturbance in the form of an RF pulse must be applied to the NMV. ▪ Radio waves are low-energy electromagnetic waves within the electromagnetic spectrum. High radio- frequency pulses (RF pulses) are used in MR imaging. ▪ These pulses are transmitted and received by an antenna (RF coil) found around the body (inside the gantry) or placed around a specific region of the body, (ie: knee coil). the antenna is inside main magnetic field or around ROI anatomy ▪ After a patient is positioned within the MRI scanner, RF pulses are emitted into the patient’s body part via the RF coil (antenna) at a right angle to the system’s magnetic field, B0. ▪ The purpose of these RF pulses is to disturb the protons, which are peacefully precessing in alignment with the external magnetic field. but out of phase ▪ The objective is to differentiate between tissues in that particular region of interest ▪ ***If the frequency of the RF pulse matches the Larmor frequency of the of the hydrogen NMV and the pulse is applied at 90º to the NMV and B0, resonance will occur. *** ▪ Hydrogen protons absorb the energy from the RF pulses and move out of alignment from B0. (p. 14) The other MR active nuclei do not resonate due to their different rates of precession. Resonance is often referred to as excitation because it results in the nuclei absorbing energy (transfer of energy from one object/system to another). resonance is when hydrogen protons precesss in phase in the transverse plane other elements have different yromagnetic frequencies and are still unaffected ▪ As a result of this absorption, some protons are lifted to a higher level of energy (aligned in the anti-parallel direction) resulting in more canceling out of protons in the parallel direction. ▪ This in effect decreases the magnetization in the longitudinal direction ▪ This energy absorption also forces the magnetic moments within the NMV to flip or rotate away from B0 towards the transverse plane (change direction) and get into sync or are considered coherent; in other words, precess in-phase.  The first result of resonance: ▪ The NMV flips/spirals away from B0; flipped to the transverse plane at the Larmor frequency. ▪ The extent of flip is dependent on the strength and duration of the RF pulse applied; usually 90º. ▪ The energy is so strong that the longitudinal NMV completely turns into a transverse NMV.  The second result of resonance: ▪ The hydrogen magnetic moments within the transverse NMV precess in- phase; same location on the precessional path at the Larmor frequency. ▪ As a result of resonance, a new form of magnetization is created in the x-y plane called transverse magnetization. RF pulses have to be constant to keep them in transverse and in phase, when rf pulses stop, they come back to the main magnetic field and are attracted tot he magnet we read the signal when we stop the RF pulses and measure recovery ▪ The laws of electro-magnetism state that: “if a receiver coil (antenna) is placed in the transverse plane (xy- plane), an electrical signal is generated in this receiver coil when in- phase magnetization cuts across the coil producing magnetic field fluctuations inside the coil.” signal = brightness or white ▪ The transverse magnetic vector can induce an electrical current in an antenna because it is in constant motion/precession (moving magnetic field) ▪ This current makes up the MR signal emitted by the patient. ▪ The resulting MR signal has the same precessional frequency as the Larmor frequency. left to right: x direction anterior to posterior: y direction whent eh rf pulses are absorbed and flip they are now in transverse of xy plane  ▪ The flip angle represents the amount of net magnetization that is flipped/tilted away from B0. ▪ The flip angle can have any value (chosen by the technologist) and this value is responsible for the generation of the MR signal. ▪ A smaller flip angle would only move a part of the net magnetization into the transverse plane; a larger flip angle would move a larger part of the net magnetization into the transverse plane. the smaller the flip angle, the less signal we can get, so we aim for 90 if not all vectors are in the same direction, the arrows will form a smaller vector  Relaxation independant processes Type text here As soon as the RF pulses are turned off, two processes start to take place: after RF is stopped, the protons return to be out of phase and in the alignment of B0 - the timing depends if the hydrogen is in fat or fluid ▪ The hydrogen nuclei start to lose phase coherence ▪ The NMV starts to re-align with B0 due to its force of attraction. It loses the energy given to it by the RF pulse. fat which has hydrogen recovers faster in the longitidinal plane than fluid the recovery period is different based on if hydrogen is in fat, fluid or tissue we take the signal when we turn off RF pulses  These two processes are referred to as relaxation and are independent processes. Longitudinal magnetization increases and transverse magnetization decreases. ▪ Magnetization in the longitudinal plane increases/recovers— recovery. returning to B0 plane ▪ Magnetization in the transverse plane gradually decreases/decays--- decay. dephasing of precessing corpus collosum is fat fat has a fast recovery and decay phase grey matter is not fluid has a slow recovery/decay phase when we scan the brain, the fat recovers and decays faster than grey matter allows differentiation in tissue in the images hydrogen is bound tightly to fluid decay happens a little bit faster than recovery ▪ As transverse magnetization decreases, the signal induced in the receiver coil decreases. ▪ This decrease is called Free Induction Decay (FID) signal. ▪ The MRI signal is of greatest magnitude immediately after the RF pulse is switched off and decreases with time. we take signal when there is differentiation so we need to wait a bit to see the differences in recovery and decay for contrast resolution knows when to pickup signal if we want fluid dark or fat ▪ The strength of the signal also depends on the size of the transverse NMV. (p.20) T1 weighted image for fat seen brighter and fluid dark T2 weighted image for fat seen darker and fat dark ▪ The FID signal is composed of many different frequencies. ▪ The FID by itself is not used for clinical imaging because it lacks information for the generation of images. ▪ Relaxation occurs as a result of the NMV giving up absorbed RF energy and returning to B0 as well as the magnetic moments of the NMV losing transverse magnetization due to dephasing ▪ The signal in the antenna decreases as a result of relaxation. ▪ The recovery of longitudinal magnetization is referred to as T1 recovery (p. 26-27) ▪ The rate of recovery is an exponential time constant. ▪ On a plotted curve, longitudinal magnetization is represented as increasing with time after the RF pulse is turned off. This curve is called a T1-curve. ▪ T1 recovery occurs when nuclei release the energy they absorbed from the transmitted RF pulse to the surrounding environment/molecules or lattice; referred to as: spin lattice relaxation. energy is released as lattice when the RF pulse is stopped and is given off throughout the body this is why patients feel warm in the sk, especially with larger tesla ▪ The hydrogen nuclei are forced to return to equilibrium and recover their longitudinal magnetization. ▪ The rate at which the nuclei dissipate the energy to the surrounding lattice is unique for each type of tissue. fat vs. fluid vs tissues ▪ Each tissue experiences T1 relaxation at a different rate; this is one way tissues can be differentiated on an MR image. ▪ The decay of transverse magnetization is referred to as T2 decay. (p. 27-30) T1 recovery and T2 decayse ▪ T2 decay involves the energy exchange among neighboring nuclei due to the interaction of their magnetic fields. ▪ This process causes dephasing to occur (loss of phase coherence/fanning out). decay causes the protons to hit into each other ▪ Nuclei move further and further out of phase with one another (loss of phase coherence) until they are precessing randomly again. ▪ In order to collect MR signal from magnetization in the transverse plane and differentiate between tissues, magnetization must be (at the very least) partially in-phase ▪ The decay of transverse magnetization is referred to as spin spin relaxation. interactions where protons smack into each other as they dephase ▪ The rate of decay is an exponential time constant. ▪ On a plotted curve, transversal magnetization is represented as decreasing with time after the RF pulse is turned off; this curve is called a T2-curve. ▪ Each tissue in the body experiences T2 relaxation at a different rate thus can be differentiated on an MR image. ▪ Longitudinal relaxation occurs because of dissipation of energy; transverse relaxation occurs because of loss of phase coherence. They are different independent processes that occur simultaneously. T1 recovery is longer than T2 decay. T1 is about 300 to 2000 msec, while T2 is about 30 to 150 msec. Relaxation in Different Tissues fat has a lower energy= easily absorbs energy = slower molecular tumbling =faster T1 recovery period and T2 decay fluid takes its time (pina colada) Fat & Water provided the RF is continuous, the vectors would stay in the transverse plane, but when it stops, the energy must be given off ▪ Fat is composed of hydrogen linked to carbon. ▪ Water is composed of hydrogen linked to oxygen. ▪ Fat’s molecular motion is slow because it consists of large closely packed lipid molecules. ▪ The molecular motion of hydrogen in water is higher than hydrogen in fat because the molecules are spaced apart and molecule tumbling rate is easier and faster. p.32 recovery is faster because tumbling rate is easier and lower energy bonds  book page 34 T2 decay occurs due to interaction with each protons spin-spin interractions occurs slower in fluid ▪ It is difficult for small water molecules to get rid of their energy, they move too rapidly and cannot hand their energy over to the lattice quickly and efficiently. ▪ It thus takes a longer time for the recovery of longitudinal magnetization in liquids/water. ▪ Water thus has a long T1 and fat has a short T1 (p.33) magnet has harder time manipulating higher energy  if we wait for fat recovery and decay where water is still recovering and decaying fat will be dark as it now has no signal (aligned with B0) and the fluid will have signal ▪ As water molecules move around very fast, their local magnetic fields fluctuate fast and energy exchange with neighboring nuclei is less efficient. ▪ The hydrogen protons in water thus stay in step (coherence) longer than fat. ▪ Water thus has a long T2 and fat has a short T2. TR time (repitition time of RF) there are multiple TRs and the protons keep flippng with the P RFULSES TE time: time we pick up the signal if we wanat to see the water as bright we want the Rf pulses to be short If we want fat as bright, our TR must be short, multiple RF pulses If we want fluid to be bright we want short TE We have to wait until the dephasing of water just starts to take the signal The TR time to see water as bright must be long and TE time needs to also be long to allow fat to relax ▪ Hydrogen in fat recovers faster longitudinally than water and loses transverse magnetization faster than water due to its efficient energy exchange stemming from its slower molecular motion. to pickup signal, need to be in transverse and in phase, fluid is slapped againa nother 90 and wont give signal but the fat will give off signal now because its is in the transver plane ▪ Fat and water thus appear differently in MR images. every sequence has its parameter which determines if fluid or fat is seen dark and bright ▪ Images obtain contrast mainly through T1 recovery, T2 decay or proton density. fat as bright (i.e fatty liver) wait for T1 recovery of fatty strcutures, then apply another Rf pulse again fluid recovers at 2000ms TR: 400ms TE: 15 ms pure fat recovers at 300ms 400 ms fo the rest of fat = TR time, the time between the RF pulses decay happens 15 ms for fat, TE time/echo time is picked up at 15 ms because thats when fat is abaout to start decaying ▪ The proton density of a tissue is the measure of the number of hydrogen nuclei within a unit volume of that tissue. ▪ In a proton density weighted image, any tissue that is rich in hydrogen will produce a Type text here signal that is bright. pure fat is 300ms, if we choose TR of 300, the image will be very black and white and have high contrast TR are usually 500-650ms fatty structures, some muscle then we flip it again with another RF pulse which flips the fluid another 90 = T1 weighted image fluid is in phase but no longer in the transver plane  Summary ▪ T1 recovery takes place when nuclei release the absorbed energy to the surrounding environment/lattice. ▪ The magnetic moments of fat nuclei have a slow molecular mobility and thus recover rapidly. ▪ The T1 time of fat is short. Type text here ▪ The magnetic moments in water nuclei have a high molecular mobility, thus recovery of longitudinal magnetization is slower than that of fat. ▪ The T1 time of water is long. ▪ T2 decay occurs when the magnetic fields of nuclei interact with each other and exchange energy with each other resulting in dephasing (loss of phase coherence). fluid is bright ▪ Since energy exchange is more efficient in the hydrogen in fat, the T2 time is short. ▪ Since energy exchange in water is less efficient than in fat, the T2 time of hydrogen in water is long. - TE controls T2 weigthed images - Wait for fatty structures to decay, and we choose the TE time right before fluid dephasing - Fluid dephases at a 90TE and + https://www.youtube.com/watch?v=Khn-azofAD4 https://www.youtube.com/watch?v=p3WnFYBnghU&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj https://www.youtube.com/watch?v=XCrpMwz6Z1g https://www.youtube.com/watch?v=9uCcumdaDIE&feature=emb_rel_pause https://www.youtube.com/watch?v=Ovo2LxBVQzs&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=3 https://www.youtube.com/watch?v=-Rr1D5m78BU&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=4 https://www.youtube.com/watch?v=HLkFA6bOnL8&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=5 https://www.youtube.com/watch?v=_kDbjXncgXo&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=6 https://www.youtube.com/watch?v=w41ErQS3mfY&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=7 https://www.youtube.com/watch?v=cLiV5APlUOo&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=8 https://www.youtube.com/watch?v=kbNha0e2Sk4&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=9 https://www.youtube.com/watch?v=gLnGNIwCQLo&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=10 https://www.youtube.com/watch?v=U0Y2LJiXQRw&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=11 https://www.youtube.com/watch?v=JOliKAry-DI&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=12 https://www.youtube.com/watch?v=9uCcumdaDIE&list=PLp_h8BySIVcuMQFG-vA1IteLLez3-kVwj&index=13 https://www.youtube.com/watch?v=qBCcYntX5oc https://www.youtube.com/watch?v=tsHzj8IdEHA https://www.youtube.com/watch?v=wq6MwwHfR1c