MRI in practice chapter 9 instrumentation

Passive and active

Magnetic shielding are achieved in what 2 ways?

Balanced gradients

- During slice selection the RF EP is only applied during the first lobe of the gradient, ensuring that RF only excites the spins at a matching fq along the gradient slope. ○ The undesirable dephasing of the MM of spins caused during this process is then corrected by the second lobe.

GUI

- Graphical user interface of the system is used to identify the patient, usually via a network link to the radiology information system. ○ The age, weight, gender, and physical orientation of the patient may also be recorded. This ensures that the system is prepared to calculate the amplitude of the RF pulses and label the physical orientation of anatomy shown on the images

Ferro (Ferromagnetism)

- compounds are said to have a large positive magnetic susceptibility and are powerfully attracted to an external magnetic field ○ Magnetic domains (whereby the atomic MM are aligned parallel by an external magnetic field) cause ferromagnetic elements to retain their MM when removed from an external field. ○ The flux lines of an external magnetic field are powerfully distorted by a _______magnetic object, and this causes geometric distortion of images in MRI patients with ______magnetic implants ○ 4 elements that are _______magnetic at a normal ambient temp: iron, nickel, cobalt, gadolinium ○_______magnetic objects powerfully attract the flux lines.

Para (Paramagnetism)

- compounds characterized by the fact that they exhibit a weak attraction to an external magnetic field. ○ Has a small magnetic susceptibility ○ _____magnetic elements increase the strength of an external magnetic field into which they are introduced ○ The effects are due to the presence of unpaired electrons, which generates a net magnetic moment. ○ However removal from an external field, the electron paths lose alignment ○ The ______magnetic material does not retain any net magnetism and does not possess any magnetic moment of its own. ○ When placed in an external field, the lines of magnetic flux converge toward a ________magnetic object. ○elements: Calcium, O2, aluminum, titanium, and platinum.

Dia (Diamagnetism)

- compounds that exhibit a weak repulsion to an external magnetic field. ○ Has small magnetic susceptibility ○ Have atoms in which all electrons are evenly paired. ○ As a fast spinning negatively charged particle, a single unpaired electron induces a powerful MM. however when the spins are paired, their magnetic fields cancel each other out. This is because the electrons spin in opposite directions. ○ As a result a _____magnetic material does not retain any net magnetism when removed from the external field and does not possess any MM of its own spin. ○ Total of 31 elements: hydrogen, Helium, gold, silver, and lead. ○ ____magnetic substances slightly oppose (decrease) the applied magnetic field.

Gradient coil

- cylindrical solenoid electromagnet ○ Rather than using conductive windings, the modern gradient coil is copper plated cylinder with the conductive elements etched into the surface of the metal plating. ○ Fingerprint design- the etched pattern looks like a human fingerprint ○ The copper etching is supported by a shell to prevent physical vibration or distortion of the coil by Lorentz forces during use. ○ The elements of the _________ _______ are supplied by 3 separate powered sources called gradient amplifiers allowing manipulation of the main MF in any direction. ○ As the gradient set operates at room temp, application of the current causes heating due to resistivity. This usually necessitates the use of a water cooling circuit, especially in high power gradient sets. ○ Another consideration in gradient coil design is that the gradient MF should be applied across the imaging vol., but should not extend further into the cryostat. For this reason the outer layer of the gradient coil provides active shielding. ○ The principle is similar to that of the active shielding of the static field whereby an equal but opposite current is applied to the shielding coil. ○ The main difference is that active gradient shielding operates at room temp. The reason the gradient field should not be permitted to extend into the main magnet solenoids is that the resulting interaction causes unawakened currents (known as eddy currents) that can affect the quality of the main MF.

RF system

- the purpose of the ___ ________ is to transmit and receive electromagnetic radiation during image acquisition ○ RF used in MRI is often referred to as B1 ( a secondary magnetic field, in addition to B0 the main magnetic field) ○ The purpose of the transmitted RF is to target a volume (slice) of tissue within the region to be examined and increase the energy level of a proportion of the hydrogen nuclei within that slice. ○ All those fq's lie in the RF portion of the electromagnetic spectrum, specifically the VHF (very high fq) band, which is shared with many other applications such as nondigital FM radio and TV broadcasts, maritime communication, and air traffic control. ○ For this reason, the magnet room is shielded from external radio sources.

Active shimming

- uses electromagnets instead of ferromagnetic shims and is used in addition to passive shimming. □ Usually performed by additional superconducting solenoids inside the cryostat. □ Their advantage is that no additional electrical power is required. □ Like passive shimming, this procedure is usually carried out once, at the time of installation, as manipulating the current flow in superconducting coils is time-consuming. □ The advantage of using resistive shim coils is that the shim can be manipulated at any time by adjusting the current flowing through the windings. □ Often positioned close to the gradient coils.

Passive shimming

- uses shims to adjust for large changes in MF homogeneity. □ The inner circumference of the warm bore of the cryostat is lined with several long plastic trays that fit along the full length of the bore □ These shim trays have around 14-16 spaced compartments designed to accommodate the small ferromagnetic shims. □ The shims must be strategically positioned to balance out any inhomogeneity present in the bare filed by manipulating the lioness of flux.

Cryostat

-cold and stable ○ Somewhat larger version of a thermal vacuum flask that you might use to keep your wine chilled. ○ Contains the cryogen liquid helium, which has a boiling point of just 4.2 K (-268.9 deg C) ○ Primary function is to prevent heat transfer from the adjacent system components (particularly the gradient coils) to the cryogen. ○ This thermal insulation reduces the rate at which the liquid helium boils off to the atmosphere. ○ The area inside the cylinder of the cryostat is known as the warm bore.

Magnetic System

-creating the magnetic field required for high quality anatomical imaging Is demanding task. There Are 6 main requirements, each of which presents technological challenges. ○ The field strength (flux density) must be high, typically between 1.0 and 8.0 T ○ The Fringe Field having a strength of 0.5 mT (5 G) or greater must not extend outside of safety zones III and IV, and should ideally be contained within the magnet room ○ The field must be spatially homogenous to a very high degree. ○ The homogeneity must extend over a large spherical imaging volume (40 cm) to accommodate the required anatomical FOV's. ○ The field must be temporally stable. This means that the flux density must not vary over time (for ex., if the environmental temp fluctuates. ○ The weight and bulk of the magnet must be kept in a level that does not pose any problems with installation in a normal imaging dept.

Ferromagnetic discs

-known as pole shoes and are typically mounted on a yoke that positions them directly above and below the imaging volume.

Closed bore systems

-most popular; tunnel shaped and resemble a larger version of CT scanner. -Encloses front, back, and sides, but sill allows limited access. -Generated the magnetic field using toroidal superconducting solenoid electromagnets positioned in circumference to the cylindrical bore. -Can generate very high magnetic field strengths, typically between 1 and 3 T for clinical use and 8 T and above for research studies. -Highest currently used is 21 T for animal research.

Open systems

-pt's are positioned on a wider imaging table that is maneuvered between 2 magnetic poles that are located above and below the imaging volume. -This only encloses the pt. Above and below, leaving a relatively unobstructed view from all sides -Useful when imaging lateral body structures such as the shoulder or elbow, as it allows the region of interest to be positioned closer to the isocenter of the magnet rather than at the edge of the imaging volume where there may be poorer field inhomogeneity. -Open scanners use large permanent magnets or superconducting solenoids to generate the main magnetic field. -The max currently available field strength for an open superconducting MRI system is 1.2 T.

Resistive electromagnets

-the flux density of these magnets is determined by the number of windings in the solenoids and the magnitude of the current flowing through them. ○ If we assume that current flows around the solenoid windings in a clockwise direction, and we stand at the front end of a closed bore MRI scanner, the flux lines run parallel to the bore, with the Roth pole of the magnet at the far end. ○ They employ copper wound solenoids that operate just below normal temp. ○ The principle advantage of this type of system is that field strength can be adjusted and the magnet switched off safely after use. ○ Industrial resistive magnets can achieve an ultra high field strength; however they typically feature very narrow magnet bores. ○ To attain a max flux density Of around 0.4 T in a. Solenoid of a size required for human scanning, a current of over 10 (kW) is required ○ Like an electric bar fire, the resistivity in the windings produces significant heat and water cooling is required to prevent damage to the system (which would otherwise become incandescently hot). This is achieved by sitting the solenoid magnets inside a water filled vessel through which chilled water is constantly circulated. ○ Superconductive magnets were introduced to avoid resistivity issues.

Solenoid magnets

-the main magnetic field is induced using this. ○In practice, several separate solenoid segments are employed. ○They are typically wound onto a former (or bobbin) ○Windings (turns) ○The end _________ __________ are responsible fro generating the bulk of the main magnetic filed, while 4 evenly spaced solenoids ensure that the homogenous imaging volume is large enough to cover the requisite 40 cm sphere. ○the commonest type of magnet used in clinical MRI

Field strength and SNR

-the principle advantage of a high field MRI magnet is an inherent higher SNR. ○Signal and noise both increase with filed strength. ○Signal- increases in proportion to the square of the external field strength ○noise increases in a linear Relationship. ○A potential disadvantage of an ultra high field. Strength is the corresponding increase in energy deposition to the patient. ○Much of the RF energy used to flip the NMV is also dissipated as heat in the patients body tissues, and this increases with the square of the field strength ( it is the electric field component of an RF pulse that causes heating) ○The corresponding SAR (specific absorption rate) may have consequences for the TR in spin echo sq's and the flip angle in gradient echo pulse sq's.

Permanent magnets

-these scanners do not employ electromagnets. Instead they are equipped with large discs of a ferromagnetic alloy such as neodymium, boron, and iron, or aluminum, nickel and cobalt (alnico) -The magnetic field is created by the inherent ferromagnetism of the alloy, namely the combined forces of unpaired electrons in the atoms of the metal that create a macroscopic magnetic field. -The advantages of this type of magnet are that it does not require electrical power or cryogenic cooling. -These advantages are somewhat offset by the fact that these magnets are unable to generate a flux density of more than 0.5 T, are typically very heavy (17 US tons), and cannot be switched off in an emergency. -The flux density of a permanent magnet is not stable and may change with the environmental temp of the magnet room.

Superconducting magnets

-this creates a magnetic field in the same way as a resistive magnet; however the windings of the solenoid are spun from a type of metal alloy that is superconductive, typically Niobum/Titanium. ○This means the resistivity of the metal decreases to 0 when the metal is cooled below a certain critical temp (Transition temp) ○ These devices use Cryogens (coolants) to reduce the temp of the windings to within 4 deg. Of absolute zero (4 kelvin (K)). This enables a substantially higher flux density using a solenoid large enough to fit a patient inside.

Shim

-used in carpentry and engineering and refers to a narrow strip of material (often tapered) that might be used to level a shelf or make minor adjustments to components. ○ Typically small ferromagnetic plates that are positioned around the inner circumference of the cryostat. ○ The task is not to physically adjust the magnet solenoids, but rather to compensate for inhomogenity ○ To determine the optimum placement of shims, an MRI engineer must 1st measure the flux density of the magnet over the entire 40 cm imaging vol. this is done with a piece of equipment known as a plotting-rig. ○3 types of shim- passive, active, gradient offset (Dyanmic)

acquisition

During the __________, each of the elements of the gradient coil is activated thousands of times. Every time the coil is activated, power is applied to conductive coil elements until the gradient slope reaches the maximum required amplitude in a particular direction. The element is then briefly deactivated before the next stage of the ____________. Some gradient functions (such as phase encoding) require gradients to be switched on at different amplitudes and polarities (direction) throughout the _____________. While a gradient coil is activated, the MM of the H nuclei within the imaging volume experience slightly different flux densities along the length of the slope of the induced gradient. The Larmor fq's of these MM therefore vary in a linear way along the axis of the gradient slope. When the gradient coil is deactivated, MM of H nuclei return to the center fq, but exhibit diff phase positions. The vectors that momentarily experienced a higher flux density during the gradient application are phase advanced compared to those that were at a lower flux density at a different location along the gradient slope.

higher field strengths

High magnetic strengths permit fast imaging and produce images with a high SNR. However some artifacts (ex chemical shift and magnetic susceptibility artifact) are more evident at _______ ________ ________.

gadolinium

In the field of MRI contrast agents, _____________ is always considered to be paramagnetic, but in its refined state, gadolinium is a silver metal that has unimpaired electrons and magnetic domains, which are the characteristics of ferromagnetism. Ferromagnetism have a "curie temp", above which they cease to exhibit ferromagnetic properties. On the case of gadolinium this is 20 deg C. Below this temp, gadolinium exhibits a MM of its own, which does not rely on the presence of an external magnetic field.

smaller

The________ the coil, the better your SNR.

x,y and z

There are 3 gradients that perform several functions during a sequence

Surface and volume coils

Types of RF receive coils

Super paramagnetic

______ __________substances have a magnetic susceptibility that is greater than that of paramagnetic substances but less then that of ferromagnetic materials.

Gradients

___________ causes changes in the magnetic flux density over a certain distance.

Paramagnetic

_________________ substances add to (increase) the applied magnetic field.

Neodymium magnets

are also known as rare earth magnets.

Magnetic susceptibility

refers to how susceptible (responsive) a material is to an applied magnetic field.

in

○ Moving electrical charge___ a conductor induces a magnetic field around it.

RF receive system

the aim is to receive signal returning from the pt at time TE.

pulse generator module (PGM)

• In addition to the host computer, the system uses dedicated modules to control various aspects of the MRI process. The most important of these is a ________ ________ _________, an independent computer that is responsible for sending instructions relating to the timing, amplitude, and shape of the transmitted RF pulses and the timing and duration of the sampling window. ○ The _____ _____ _____ is also connected to the gradient amplifiers and generates pulses used to determine the shape and duration of the gradients used during the scan. For these reasons, the PGM also needs to be connected to any physiological sensors that are monitoring the breathing rate or pulse rate of the patient.

Gradient offset (dynamic) shimming

□ final method for shimming Uses the ______ ____, which is another electromagnet that is designed to manipulate the MF during image acquisition. □ In terms of shimming it is possible to apply a current to the gradient coils that offsets any minor inhomogeneity in the main MF. □ As stated earlier, it is desirable to achieve a homogeneity of better than 10 ppm for good image quality. By introducing a lg pt into the magnet bore, the homogeneity can be changed by up to 9 ppm by diamagnetic repulsion. □ Dynamic shimming can help to correct for this. The primary purpose of the gradient system is to create linear slopes along the main MF.

In an open bore magnet

□ pt positioned in the supine position □ X axis- lies horizontally from left to right (always lies across the pt left to right) □ Y axis- lies vertically (posteroanteriorly) □ Z axis- lies horizontally along the length of the magnet bore (inferosuperiorly or head to foot) □ Always in the direction of the main magnetic field.

In an open MRI system

□ the flux lines of B0 run vertically between the poles of the magnet and the z and y gradient directions are therefore reversed in comparison to the closed bore configuration.

simple fq encoding

○ In ______ __________ _________- the gradient is activated during sampling. The timing is therefore determined by the required digital sampling rate and the number of samples to be acquired. ○ These factors are influenced by the receive bandwidth and the required spatial resolution of the image in the fq direction. ○ If a corrective lobe were to be applied at the same amp. And timing as the lobe applied during readout, the resulting TR may have to be lengthened. This is undesirable as it may reduce the max number of slices that can be acquired. ○ To avoid this, the corrective lobe is applied at a higher amp but for a shorter time, the area under each lobe remaining equal but opposite.

spin echo pulse

○ In _______ _______ _______sq's- the corrective lobe is applied first, and sampling takes place during the second lobe of the FEG. This means the first lobe may be applied at any convenient time before the spin echo. ○ The only caveat is that if that the first lobe is applied before the 180 deg RF RP essentially reverses the effect of the gradient (called the Stejskal-Tanner scheme)

Field strength (flux density)

○ Magnetic fields are measured 2 main ways. Strictly speaking measured in amperes (A) per m and is given the symbol H. ○Visualized as having lines of Flux that are seen when iron fillings are sprinkled around a bar magnet. ○describes the number of flux lines passing through a given area. ○Affected by the permeability of the medium through which the line pass. ○Measured in Teslas and is given the symbol B ○ The earth's magnetic field is usually around 0.5 G depending on one's position in relation to the equator (0.000 05 T)

through

○ Moving a conductor a magnetic field induces an electrical charge in it.

RF transmit system

○ RF pulses are transmitted and received during image acquisition. ○ The transmitted RF is an electromagnetic wave, which contains narrow range of fq's, centered around the Larmor fq of the MM of H nuclei at the external field strength of the system. This narrow range of fq's is known as the transmit bandwidth and in turn, determines the duration of the transmitted pulse. ○ To excite a rectangular slice profile, the pulse should ideally only contain the narrow range of fq's imposed by the slice select gradient across the thickness of the slice and that these fq's should be at the same amplitude. ○ In the field of signal processing the shape of the RF pulse used in MRI is known as a sinc pulse (sine cardinal). After FFT this type of pulse creates a slice profile with well defined boundaries and an approx. rectangular profile when viewed from the edge.

polarity

○ The _________ of a gradient is determined by the direction of the current flowing through the coil.

Resistivity

○ The ______________ of the electromagnetic coils is zero because they are immersed in a cryogen that reduces their temp to almost absolute zero. ○Current therefore flows, and the magnetic field is retained as long as the electromagnetic coils have no resistance.

Receiver coils

○ are a critical part of image optimization. ○ The selection of the appropriate receiver coil for the area under examination is very important

Gradient characteristics

○ during data acquisition, the gradient coils are required to ensure that the transmitted RF is targeted at the volume of tissue required in each individual slice. ○ Additionally, the gradient set is also responsible for determining the spatial origin of signal returning from each slice along the two remaining axes of anatomy. ○ to accomplish the goals of spatial encoding, refocusing and other tasks during image acquisition in acceptable imaging times, gradient systems need to be fast and strong. To evaluate speed and strength of gradients, gradient characteristics need to be understood. They include: gradient amplitude, gradient rise time, slew rate and duty cycle.

RF transmit coils

○ known as resonator coils ○ When this type of coil is caused to resonate at the required fq, large oscillations in voltage and current are produced in the coil. These in turn, generate radio waves at the desired fq, which are transmitted into the anatomical area under investigation. ○ The aim of the transmit coil is to transfer energy to the hydrogen nuclei. The most efficient way to do this is to generate a field that not only oscillates at the Larmor fq but also rotates in the same orientation as the spins. ○ The term receiver is used to denote the fact that this coil transmits RF and if needed, also receives RF. In closed bore MRI systems, the main RF transceiver is colloquially known as the body coil and typically features what is known as a birdcage.

Small coils

○ provide small coverage but relatively good SNR. This is because the sensitive volume of the coil will be filled with tissue. ○ The filling factor is high. However, aliasing is more likely

RF shielding

○ the RF produced during the scanning process may cause interference with other nearby electrical equipment. To circumvent this problem, it is necessary to shield the entire scan room using what is known as a Faraday cage. ○ It is accomplished by lining the walls, floor, ceiling, and door with any conductive metal ○ Copper was the original material of choice, but is expensive and heavy. ○ Aluminum panels are now favored as they are easier to handle during equipment upgrades and are suitable for mobile installations where weight is a prime concern.

SNR- (sound to noise ratio)

○ the increase in ______ permits techniques such as fMRI (functional MRI), where signal changes in the brain cortex might otherwise be difficult to distinguish from background noise at a lower field strength. ○BOLD (Blood oxygen level dependent) techniques such as fMRI rely on magnetic susceptibility, so the effect becomes more pronounced at high field ○ The increase in _______ also permits the use of smaller voxels, which in Turn, increases the spatial resolution of the image.

Phased array surface coils

○ these are now offered as standard and are ubiquitous. ○ They offer the SNR advantages of single-loop coils, but with a greatly extended FOV and many different configurations. ○ Phased array coils of the linear and volume type are the best option, as they combine the benefits of using small coils with those of using large ones.

Surface coils

○-used to image anatomy ○ Not typically used as RF transmitters because of RF homogeneity. ○ They are described as receive only coils. ○ Their design, usually consists of a single conductive loop or an array of loops that are either connected or overlapped and operated independently via separate channels. ○ The latter configuration is known as a phased array. ○ In normal use, surface coils are positioned flat, in proximity to the anatomy under examination. This provides an optimum filling factor whereby the entire sensitive volume of the coil contains tissue. ○ _______ _______ receive a high signal from the surface anatomy, offering a two to fivefold increase in sensitivity compared to volume coils. ○ An additional factor that contributes to the high SNR from this type of coil is that they only receive noise from a small region of the anatomy

greater

○Diamagnetic effects appear in all substances. However, in materials that possess both diamagnetic and paramagnetic properties, the positive paramagnetic effect is_______ than the negative diamagnetic effect, and so the substance appears paramagnetic.

substances

○Ferromagnetic ____________ are strongly attracted to, and align with the applied magnetic field. They are permanently magnetized even when the applied field is removed.

Acoustic gradient noise

○In the gradient system rapid pulsing of electric current through the conductive elements of the coil results in rapidly oscillating mechanical forces on the coil structure. ○ When the coil vibrates, it results in _________ ________ ________. ○ Pulse sequences that require rapid gradient switching (such as EPI) tend to cause the highest levels of acoustic noise with studies reporting intensities of up to 115 decibels (dB) or higher.

amplitude

○The ____________ of a gradient is determined by the amount of current passing through the coil. ○ The ____________ of a gradient determines the spatial resolution.

Inhomogeneity

○______________of the field results in a change in precessional fq of MM of H within imaging volume, and this has implications for the SNR and techniques that require spectral fat suppression. ○Such sq's employ an additional RF pulse each TR, tuned to the fq of the MM of H nuclei in fat at the field strength in use. ○______________ causes an uneven phase shift across the FOV, and this is shown on a MF map that is created using data comparing both echoes. ○Any variation in fq due to harmonics of the magnetic field prevent the saturation pulse from achieving its purpose. Fat remains relatively hyper-intense In areas where fq of the saturation pulse does not match the fq of the MM of H nuclei in fat.

Homogeneity

○a homogenous MF is desirable for 2 main reasons. ○Firstly, any distortions in the magnetic field lead to geometric distortion of images ○Secondly, excitation of the hydrogen nuclei is fq dependent. ○To create resonance in a group of H nuclei, the transmitted RF piles must match the precessional fq of their magnetic moments. ○Typically described in ppm over the 40 cm (diameter) spherical imaging volume. ○To receive high image quality, imaging at 1.5 -3 T requires a homogeneity of at least 10 ppm or better. ○A procedure known as shimming the magnet is employed to achieve this. ○Following the shimming procedure, the ____________ of the MF is tested by comparing the phase evolution of the MM of the spins between the 1st and 2nd echoes acquired using a dual echo, gradient-echo pulse sequence.

Duty cycle

○defines the %age of time that the gradient is permitted to work. ○Expressed in units of percentage.

Gradient amplitude (strength)

○defines the power of a gradient, specifically how steep or strong a particular gradient is. ○ Measured in milliTesla per meter (mT/m) or gauss per centimeter (G/cm) ○ determined by the slice thickness, phase resolution and FOV.

Gradient rise time (speed)

○defines the time it takes for a given gradient to reach max amp. ○ Measured in microseconds (us) ○ Rise time cannot be used as an indicator of overall gradient performance per se, because a low a,plitude gradient is likely to have a shorter rise time than a high amp gradient.

Slew rate

○defines the time it takes for a given gradient to reach maximum amplitude and what that amplitude is. ○The speed and strength of the gradient and is measured in units of milliTesla per meter per second (mT/m/s) ○determines how fast data can be acquired.

Extremity systems

○designed to scan limbs and are smaller in size that their whole body counterparts. -Approx. the size and shape of a washing machine having a narrow aperture in the center that is large enough to accommodate an arm or leg. -Typically generated by permanent magnets and is therefore restricted to below 1 T.

Shim system

○first component to be encountered within the warm bore of the cryostat. ○Operates at room temp and is located around the circumference of the inner wall of the cryostat. ○The function is to ensure homogeneity of the magnetic field within the imaging volume. ○One of the 1st tasks after installation of a new MRI scanner is to assess the homogeneity of B0 with the scanner in situ.

Sinc pulse

○generated by a digital waveform generator and is immediately converted into an analog waveform. This is then processed by a synthesizer that ensures that the fq's in the pulse are centered around the Larmor fq of the MM of the spins. ○ In addition, the synthesizer determines the phase of the waveform, as it may be desirable in some sq's to adjust the phase of the transmitted RF pulses. The waveform is then through an RF amplifier to provide enough energy to couple with the nuclei in the region of interest. ○ The RF pulses thus generated by the coil are transmitted into the pt repeatedly throughout the data acquisition. ○ The pulses tend to be described according to their function (excitation pulses, rephasing pulses, inversion pulses) and are timed to occur at certain time periods throughout each repitition (e.g. TR, tau, and TI)

Gradient system

○moving toward the center of the bore, the next component to be found In the gradient system. ○Determines scan time, slice location, spatial encoding, and (indirectly) image contrast in studies using gradient echo pulse sequences, perfusion and diffusion imaging. ○The accuracy of the location of data samples in k space relies on the use of high precision magnetic filed gradients. This task is achieved by the gradient system by creating linear magnetic field gradients (slopes) along the 3 orthogonal axes of the imaging volume. ○These mutually perpendicular axes are labeled x,y, and z according to the Cartesian geometry . ○ The primary purpose of the ________ ________ is to create linear slopes along the main MF.

Bucking coils

○oppose the effect of the main magnet windings. (Buck) ○Their function is to actively shield the local environment by constructing the 5 G footprint of the fringe field to within a short distance from the scanner. ○ To do this the _________ ________ carry a current flowing in the opposite direction to the main magnet windings, reversing the flux

Gradient amplifiers

○power is generated In 3 __________ ____________, one for each orthogonal gradient. ○ Typically housed in a cabinet remote from the scanner. ○ Their function is to supply the current required by the gradient coils during image acquisition. ○ Older models of MRI scanner used linear analog amplifiers; more recent models are equipped w/ a different type of amplifier having what is known as a pulse width modulator (PWM) design. ○ Linear amplifiers are similar in operation to a music amplifier. The waveform created by the output is identical to the input waveform, only larger. ○ In MRI they receive a comparatively low amplitude signal from a pulse generator that must be amplified to a level that is used to drive the coil. The applied current may be as high as 900 A, and the voltage ranges between 1000 and 2000 volts (V) ○ The main drawback of this linear method of amplification is that it causes power looses (heating) in the gradient coil, but also in other parts of the circuit, including amplifiers ○ Efficient gradient operation is problematic in linear amplifiers because of losses in the components that perform this task (semiconductors) ○ Put simply, the higher the %age of time that a gradient spends at maximum amplitude (duty cycle), the more heating occurs. PWM amplifiers help reduce this burden by applying voltage to the coil in short closely spaced discrete bursts. ○ This technique improves the efficiency of the gradient coil, and, if the pulses are of sufficiently high fq (closely spaced in time), the coil interprets the supply as smooth and continuous.

Large coils

○provide large coverage but a relatively poor SNR. This is because the anatomical area may not fill the entire sensitive area of the coil. ○ The filling factor is low.

Volume coils

○required when a large volume of tissue is examined. ○ A volume coil (especially the integrated RF transceiver) typically contains areas of the body other than the region of interest, and although signal only returns from a specific thin volume of tissue (slice), noise may be detected from all other anatomy inside the coil. ○ The trade-off for this high SNR is that the sensitivity decreases with the distance from the coil. Useful signal is only returned from a tissue depth equal to the diameter of the conductive loop. This makes the coils unsuitable for imaging anatomy deep inside the patient. ○ Most common is the birdcage design. Others are transverse electromagnetic resonator and the saddle shaped design. ○ volume coils are typically employed to transmit RF, but there are also receive-only volume coils. ○ The primary advantage of a volume coil is that, unlike surface coils, they offer a comparatively homogenous RF field over a large imaging volume. This means that volume coils can be used to image anatomy at any location within the region of interest, at any depth. Signal from any anatomical structure enclosed by the coil is received with equal sensitivity. ○ Volume coils are now offered with phased array capability. ○ The only potential disadvantage to using a volume coil is that the filling factor tends to be lower than that of a surface coil. The coil is not totally filled by the anatomy and contains empty space, reducing the SNR. In addition, areas of anatomy that are contained within the coil contribute to the thermal noise, but may not contribute to signal. ○ This SNR issue is largely compensated for by the fact that these coils are multichanneled. Importantly, manufacturers ensure that volume coils are designed to fit closely around the desired anatomical area.

Passive shielding

○requires the scanner to be surrounded by large steel plates ○Reroutes the fringe field away from the outside environment and back toward the scanner ○Has several ajar disadvantages ○The iron cladding can weigh over 20 tons, it is very expensive, and the proximity of ferromagnetic metal can adversely affect the homogeneity of the scanner that it is intended to shield. ○For these reasons PS has now been replaced by active shielding in most clinical scanners and in some ultra high field research systems. ○ Causes lines of flux to pass through steel cladding in preference to air.

Lorentz force

○the force exerted on a charged particle moving with velocity through an electric and magnetic field ○If a current carrying conductor is placed perpendicular to an external magnetic filed, the ________ _______ acts on the moving charges in the wire and causes physical deflection of the conductor.

Active shielding

○uses solenoid magnets. ○Allows convenient sitting of an MRI scanner, it does not remove the safety risk from projectiles ○Causes a very steep magnetic field. ○In modern ultra high field strength systems, it is permitted even at 8 T. ○In addition to the main magnet solenoids, the larger diameter solenoids positioned at the need of the bobbin are colloquially known as Bucking coils (they oppose buck)