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ARMRIT STUDY GUIDE PACKET WITH
COMPLETE SOLUTIONS
Atoms - answer-fundamental submicroscopic unit of all mass. Size of the atom and its weight have interested scientists for a long period. Subsequently, experiments were devised to determine the size and weight of the various atoms. The lightest of all atoms is hydrogen. Proton - answer-a subatomic particle having a positvie charge identical in magnitude to the negative charge of an electron and, together with the neutron, a component of all atomic nuclei. The proton forms, by itslef, the nucleus of the hydrogen atom. The mass of a proton is approximately 1836 times that of an electron and the mass of an atom is contained almost entirely in the nucleus. The proton has an intrinsic angular momentum or spin and thus a magnetic moment (very small magnetic field). The number of protons in the nucleus of an atom determines what element it is; the atomic number of an element denotes the number of protons in the nucleus. Electron - answer-a subatomic mparticle that, along with protons and neutrons, make up the atoms the flow of an electric current in a conductor is caused by the drifting of free electrons in the conductor. Heat conduction is also primarily a phenomenon of electron activity. The charge of the electron is the basic unit of electricity. Electrons have half- integral spin; spin is a property of subatomic particles that indicates the particle's angular momentum. Neutron - answer-a subatomic particle that is neutral therefore has no charge. It is located in the nucleus and serves as a stabilizer. The mass of a neutron is slightly greater than the proton mass and has an enormous effect on the quantum spin value of an element. The net charge on a neutron is 0 Magnetism - answer-is descried as the ability of a substance to attract iron, cobalt and nickel. It is the power or force created by a substance having the property of creating or inducing magnetic dipoles. The magnetic field of any magnet has a direction, signified north and south. This property of a magnet makes it a dipole. Dipole - a magnetic field characterized by its own magnetic north and south poles separated by a finite distance. Magnetic susceptibility - answer-defined as the tendency of a substance to attract or repel magnetic lines of force dependent on the magnetic properties of its electrons. Diamagnetic - answer-the property of most substances with paired orbital electrons that have opposite spinning and their magnetic moents cancel each other out. Substances
with diamagnetic susceptibility weakly attract or repel magnetic lines of force. Magnetic susceptibility of most human body tissues are described as diamagnetic. Paramagnetic - answer-the property of substances with positive magnetic susceptibility that attract and repel magnetic lines of force. These substances (gadolinium, methemoglobin, free radicals) contain one or more unpaired orbital electrons and produce marked reductions in the t1 and t2 relaxation times. Unpaired orbital electrons spin in the same direction and have the tendency to line up with the magnetic field. Therefore producing additional magnetic fields. Super-paramagnetic - answer-the property of substances with large positive magnetic susceptibility which strongly attract or repel magnetic line sof force. These substances (hemosiderin and superparamagnetic iron oxide -spio) have a single magnetic domain and are magnetized in a magnetic field. Their magnetic susceptibilities are much larger than those of paramagnetic substances and similar to those of ferromagnetic materials. Unlike ferromagnetic substances, superparamagnetic materials do not exhibit residual magnetism when the external field is removed. Ferromagnetic - answer-the property of substances with large positive magnetic susceptibility that become magnetized within a magnetic field and remain magnetized agter being removed from the magnetic field. Ferromagnetic substances include a number of iron an cobalt-containing metals. Magnetic field - answer-a vector quantitiy consisting of both a north and south pole; it exerts an induction force on ferromagnetic and paramagnetic substances. When symbolizing the flow and area of a magnetic field, special lines are used; they are field or flux lines Field or flux lines - answer-measure the flow and area of a magnetic field. The strongest area of the magnetic field is where the field or flux lines converge and is called the isocenter. Away from the isocenter the field or flix lines represent the fringe fields. Isocenter - answer-area of the magnetic field where the field strength is the greatest, in mri the center of the scan area Fringe fields - answer-weaker magnetic fields that are away from the isocenter, in mri outside of the scanning area. Flux - answer-flow of energy; magnetic field flowing from north to south poles represented by magnetic lines of force. Force - answer-the ability to create work or change (measured in horsepower). When describing the strength of an mri main magnetic field, the si unit most commonly used is tesla. A symbol to describe the direction and strength of a magnetic field is called vector.
spin exists in a distinct energy state and has an identifiable spin quantum number. Due to its spin the h nucleis proton has the same properties as a bar (dipole) magnet. This spinning motion gives the h proton a magnetic moment. Magnetic moment - answer-the measure of th magnetic properties of an object or proton that causes them to align with the static (main) magnetic field, it is the magnetic field of a proton without any rf pulse applied Angular momentum - answer-the angle formed between a precessing object (proton) and its imaginary axis. The vector quantity given by the product of the momentum of a proton and its position. In the absence of external forces, the angular momentum remains constant, with the result that any rotating body tends to maintain the same axis of rotation. When a magnetic field is applied to a rotating body the resulting change results in precession. Precession - answer-the phenomenon of a magnetic field spinning r gyrating around the axis of its own creation Antenna - answer-a device enabling the sending and / or receiving of electromagnetic waves Radiofrequency (rf) - answer-electromagnetic radiation lower in energy than infrared. The rf used in mri is in the form of a burst of rf energy (pulse) i the 10 mhz to 100mhz range rf puls, delivered to the patient by rf transmitter. The rf frequency, at the larmor frequency, results in rotation of the net magnetization vectro and phase coherence of the nuclear spins. An rf coil is used for transmitting and receiving mri signals (rf). Three basic components of magnetic resonance - answer-1. Hydrogen proton
- Large static homogeneous main magnetic field.
- Radio frequency signal Hydrogen - answer-symbolized as h or h1. Also known as the proton, spin or nuclei. As the information uinit (iu) for magnetic resonance, hydrogen's proton, sub-atomic nuclei is useful because:
- Abundance; it is the most abundant substance in the human body.
- Magnetic moment; is has the strongest magnetic moment of any substance. Main magnetic field/three criteria for bo - answer-symbolized as bo and measured in tesla, it is the environment for magnetic resonance.
- Stable; must have a field strength that is static and does not fluctuate.
- Homogeneous; the field must be uniform from the isocenter (scan area) to fringe fields (outside the scan area), in all directions.
- Large; bo must be large enough to establish a sample of protons for diagnosis.
Radiofrequency/ criteria for rf signal - answer-symbolized by a sine wave and known as b1, the rf or rf pulse is the signal which is transmitted to and received from the protons.
- Larmor frequency; also known as resonance frequency or precessional frequency. The rf pulse must be introduced at the same frequency as the hydrogen protons are precessing.
- The rf pulse must be introduced perpendicular to the magnetic field of bo Overview: radio-frequency pulses are low energy electromagnetic waves, which are commonly employed in broadcasting 9i.e radio, tv). Directing rf pulses at the larmor frequency of the h protons within a specific strength bo will allow the generation of a detectable mr signal. Hydrogen protons respond to rf pulses, which are a form of electromagnetic energy, in a large static homogeneous main magnetic field because they have magnetic properties Three types of mri magnets - answer-1. Permanent
- Resistive
- Superconductive Permanent magnets - answer-made up of naturally ferromagnetic material. Materials employed in permanent magnets may include; lodestone (iron oxide), neodymium, or brontium ferrite. Weight range of permanent magnets: 18,000 - 200,000 lbs. Field strength range: 0 - 0.3 t Field homogeneity: good Advantage:
- Relatively no fringe field because of design and low field strength range.
- No electrical power requirement to maintain bo
- No cryogens.
- No cryogen hazards
- More variable patient bore size. Disadvantage:
- Weight range.
- Low attainable field strength. Resistive magnets - answer-require a constant current of electricity in electromagnetic coils to generate and maintain a bo and operate at temperatures higher than superconductive. Due to heat produced in the main magnetic coils a cooling system (water or polyethylene glycol) is required to dissipate heat. The standard resistive magnet is an iar-core magnet by iron-core systems have been produced. Field strength range: 0 - 0.6t Field homogeneity: fair
superimposed within the patient's body to perform spatial localization (slice selection). Gmf: small magnetic fields that vary from stronger to weaker predictably within the scan area of the patient bore. Radiofrequency coils - answer-smaller devices that are placed close to the patients body to transmit, receive or both. The radio signal (mr signal). Surface coils are rf coils for superficial body structures and are receive only coils Crygogens - answer-defined as substances that produce low temperatures Types:
- liquid helium - cools the main coils (niobium titanium) to 4 degrees kelvin.
- Liquid nitrogen - acts as an insulator for the liquid helium from the room temperature. Today, because of helium recycling systems (chillers) there is less or no need for liquid nitrogen. Dewar - answer-containers for the cryogens for storage or transportation Scan room shielding - answer-special designed and shielded for two reasons Magnetic shielding - answer-large magnetic fields (bo) produce fringe fields that can interfere with nearby patients and personnel with implants and electronic equipment such as, pacemakers, aneurysm clips, computers and radio reception. Purpose: to protect personnel with implants and equipment outside of the mri environment from the detrimental effects of bo. Types: A. Active - electromagnetic coils placed strategically to contain the fringe fields B. Passive - structural steel around the main cryostat or within the walls of the mri scan room to contain the fringe fields. Radio-frequency (rf) shielding - answer-consisting of sheet-metal copper or galvanized steel. Companies as lindgren (copper) and universal (galvanized steel) Purpose: to protct the mri environment from the effects of extraneous electromagnetic energy from equipment such as televisions, passing automobiles, elevators, or trains that might interfere with the production and detection of signal from the mri environment and the patients body. Operator's console - answer-consisting of a monitor (crt), central processing unit (cpu), keyboard, mouse, function keys, trackball, joystick and dials.
Purpose: controls the mri scanning process from outside of the mri environment to protect sensitive hardware and software from the harmful effects of the main magnetic field. Filming/types - answer-transferring images from the mr monitor to laser film for review and diagnosis.
- Matrix camera - a traditional style camera on a sophisticated track system that would expose the film (usually 14" x 17") over a designated area contained in mainframe housing. The film cassette held only two films at a time.
- Laster camera - a camera that more accurately portrays the mr image from the mr monitor. Employing specially designed laster film in a cassette that can hold up 100 films Film processor/types - answer-develops the mri film for review and diagnosis
- Wet processor - requires fixer and developer, which are photographic chemicals that must be replenished regularly. The wet processor may be separate from the camera and in a dark room or may be attached to the camera by a docking unit.
- Dry imager - a laster camera that does not require a separate processor because the developing chemicals are already on the films. The films pass through a specialized heating area at a specific temperature which activates the chemicals on the film Archiving/types - answer-saves all films as a medical record of the patient's mri procedure. Today, mri facilities are required to maintain a record of mri studies for the life of the patient.
- Magnetic tapes - reel to reel tapes that are limited to 3 to 5 mri studies.
- Optical disks - 5 inch or 12 inch disks that look much like compact disks archive up to 300 studies. Pacs - answer-picture archiving communications systems. Also known as "teleradiology". A system that can film, archive and transmit mri images form the operator's console to other monitors such as the reading physician, the referring physician, the emergency or operating rooms. The goal here is to make mri filminess. Patient table - answer-a mechanical and electrical device that a patient is positioned on and then the body structure interest if accurately place in the isocenter of the main magnetic field. There is usually a weight limit anywhere from 275-500 lbs. The mri technologist is responsible to know the weight limit of their particular patient table and how it functions. Basic mr imaging technique - answer--a patient is paced int eh isocenter of a large static homogeneous main magnetic field.
Rf pulse - answer-a second smaller magnetic field in the form of radio-frequency (rf), symbolized as b1 that is introduced to the h protons in order to cause them to move from the low energy state to the high energy state Purpose:
- To shift m in size and direction
- To cause h protons to precess in phase, have phase coherence. B1 - answer-symbolizes the rif signal that is introduced to the patient's body in order to generate a mr signal B1 vs bo - answer-bo - a magnetic field that is constant and either horizontal or vertical depending on the manufacturer's design. B1 - a magnetic field that is rapidy rotating (oscillating) and always oriented perpendicular to bo. Signal generation - answer-b1 is turned and signal decay begins and the protons dephase. Mz and mxy - answer-the net magnetization vecor m breaks down into two measurable components: Mz - measurement of the h protons' magnetization in the longitudinal direction. Mxy - measurement of the h protons' magnetization in the transverse direction. Longitudinal magnetization - answer-describes m's magnitude in the direction of bo Transverse magnetization - answer-describe's m's magnitude in the transverse direction or perpendicular to bo. Longitudinal relaxation - answer-also known as spin-lattice relaxation or t1 relaxation. It is the measurement of:
- A loss of energy
- The regrowth of mz Transverse relaxation - answer-also known as spin-spin relaxation or t2 relaxation. It is a measurement of:
- Not a loss of energy
- The decay of mxy
- T2 interactions. Developed to delay the loss of transverse magnetization, such as: - answer-1. Spin echo - 90 degree rf pulse followed by a 180 degree rf pulse.
- Gradient echo - usally less than 90 degree rf pulse followed by a gmf
- Inversion recovery - 10 degree rf pulse followed by a 90 degree rf pulse.
Spin echo (se) pulse sequence - answer-"the gold standard", the most commonly employed pulse sequence. Se employs an initial 90-degree rf stimulating pulse, which is then followed by a 180 egree rf rephrasing pulse. This is followed by the recording of the echo signal. Employs a 90 degree rf pulse followed by a 180 degree rephrasing rf pulse to produce an echo. Spin echo principle - answer-because of b0 inhomogeneities, t2* effects, the fid signal decays too quickly. A 180 degree rf pulse is employed to correct for t2* effects by rephrasing the protons. This delays the loss of the fid signal, therefore t2 decay. Simply by adjusting the parameters of tr and te, spin echo brings out the physical characteristics of the tissue being imaged. Purpose of the 180 degree rf pulse - answer-to correct for t2 * effects and cause the protons to come back into phase. Even though a 180-degree rf pulse is applied, fid (t decays) goes unabated. Te - answer-known as time of echo or "echo time", it is the time delay between the original 90 degree rf pulse and the echo signal. It is a parameter used to specify a spin- echo pulse sequence. If more than one echo signal is recorded, the time delay between the 90 degree rf pulse and each echo signal is called te1, te2, te3, etc. In a spin echo pulse sequencee, a parameter that represents the time interval between application of the 90 degree rf pulse and the echo signal (when the signal is detected). If more than one echo signal is detected, then the time intervals between the 90 degree pulse and each echo signal are designated; te1, te2, te3, etc... Measured in ms Tr - answer-known as the time of repetition or "repetition time", it is the time interval between 90 degree rf pulses or the time between the beginning of a pulse sequence and the beginning of the succeeding pulse sequence. The time interval between the beginning of a pulse sequence and the succeeding pulse sequence, usually chosen in the range between 300 ms and 2000 ms. Measured in ms Process of mr signal generation - answer-1. Stimulation to resonance
- Relaxation from resonance Stimulation - answer-once equilibrium has been established, a second, small magnetic field in the form of an rf pulse is introduced to m (net magnetization vector) at the larmor frequency. This rf pulse is symbolized as b1 and is always introduced perpendicular to bo. Introduction of b1 causes m to shift in size and direction away from bo (equilibrium).
T2: tr 1600-3000 te: 80- Examples: T1w - tr 500, te 20 Pdw - tr 2500, te 20 T2w - tr 2500, te 110 Ti - answer-(time of inversion) - a parameter in an inversion recovery pulse sequence. It is the time interval beween the initial 180 degree inverting rf pulse and the following 90 deree rf pulse. Ti ranges from 80-800. Tr shoud be 3 ties ti Fa - answer-(flip angle) in a gradient echo pulse sequence only one rf pulse is utilized, at less than 90 degrees, usually. This is followed by a gradient magnetic field to rephrase ma nd produce an echo. Examples: Large fa (less than 45 degrees) t1w Small fa (less than 20 degrees) pdw Medium fa (between 24 and 45 degrees) t2w Four tissue characteristics that determine the signal strength emitted - answer-1. Hydrogen concentration
- Velocity effects
- Ti effects
- T2 effects Hydrogen concentration - answer-the signal intensity recorded from a tissue is directly proportional to how many h protons are contained in that particular tissue. So, tissues with more hydrogen protons will have a righter, more hyperintense signal than tissue with less hydrogen. A) hydrogen density in body tissues have been measured and recorded B) in most tissues, hydrogen concentration only varies 10-15 % (ie brain tissue and csf varies only 3%) C) prton density alone does not demonstrate much contrast resolution. Two types of tissues that benefit from the pd image - answer-both will have very hypointense signals.
- are and air containing structures have very low concentrations of h (lung, bowel, nasal sinuses).
- Cortical bone (cortex) - contain calcium and phosphate but very little hydrogen Factors that decide how flowing blood will appear o the mr image - answer-1. Velocity - a vector quantity which refers to the rate at which an object changes its position
- Imaging plane.
- Pulse imaging sequence
- Methode of slice acquisition (multi slce or volume imaging)
- Flow turbulence
- Vessel's angle of intersection with the slice plane. Two generalizations of flow - answer-1. Fast blood flow - will have very little signal and appear dark, known as flow void
- Slow blood flow - will ahve hyperintense signal and appear bright Theories that explain the fast blood flow mr signal are - answer-1. Flow void phenomenon (fast transition phenomenon): magnetized flowing blood enters a slice area and is exposed to an rf pulse. Before a signal can be recorded, some of the nuclei have moved passed the slice acquisition area, producing a flow void.
- Turbulence: fluids that begin to have random motion (other than laminar flow); will more rapidly lose phase coherence and therefore signal. Theories thatexplain the slow blood flow mr signal are: - answer-1. Entry phenomenon (paradoxical enhancement): blood that is entering the slice acquisition area is magnetized and unsaturated with rf. The blood and tissue already in the slice acquisition area are partially saturated by not completely back to equilibrium. The result is the unsaturated blood will paradoxically produce a higher signal than the partially saturated blood and stationary tissue.
- Even echo rephasing: will occur in multi-echo pulse sequences only (more than one 180 degree rf pulse) Result: mr signal from slow blood flow is increased and appears more hyperintense after an even # of echoes than an odd # of echoed. T1 effects - answer-1. The signal strength from all images, acquired from all pulse sequences, is dependent in some form on t1 relaxation rates of the tissue.
- T1 relaxation time is not exact for a specific tissue. Many different tissues may have t1 relaxation values that overlap.
- T1 is dependent on choice of parameters, methodology (ie tr, te etc.) And absolute values are not a priority.
- T1 relaxation values are dependent on the field strength of bo (main magnet) as well as temperature and other factors (equipment). Example: t1 relaxation time values in a 1.5t mr system are approximately twice as long as t1 relaxation time values in a .2t mr system
- The t1 relaxation time value for any tissue is dependent very strongly on the relative content of fat and free water in that particular tissue.
- Tissues that have "short" t1 relaxation time values are efficient at spin-lattice and will emit a high intensity (hyper-intense) singal, bright. 7.tissues that have "long" t1 relaxation time values are in-efficient at spin-lattice and will emit low intensity (hypo-intense) signal, dark. T1 and molecular motion: - answer-t1 relaxation involves the transfer of energy between the hydrogen protons and the environment (spin-lattice). Therefore, t1 relaxation is sensitive to any changes in the environment.
Nex - answer-number of excitations, also known as number of signal averages (nsa) or averages (avgs) or data buffers. The number of times the signal will be sampled for each slice volume within a tr. For se t1 images usually 3 to 4 nex, for se t2 images usually 1-2 nex Matrix - answer-also known as projections, levels, "steps", specifically the number of phase encoding steps. It is actually the number of frequency encoding steps by the number of phase encoding steps. Usally 256 x 256 but can be 512 x 512. The only nuber that can be adjusted is the number of phase encoding steps (the number on the right) Spatial localization/slice selection - answer-to perform spatial localization the mr signal must be spatially encoded in three dimensions by three gradient magnetic fields (gmf). These dimensions are encoded in the directions of the three orthogonal axes, z, y, and x. The order in which the gradients are activated are as follows: the first gmf performs slice selection, the second gmf performs phase encoding and the third m performs frequency encoding and is known as the read out gradient. Gs= slice select gradient Gp= phase encoding gradient Gf = frequency encoding gradient Logical vs. Physical gradients - answer-anyone of the gradients can do slice selection, phase encoding or frequency encoding. To explain their purpose educationally each gmf is assigned one orthogonal direction and one function of spatial encoding. Logical gradients: Slice selection = gz Phase encoding = gy Frequency encoding = gx (readout gradient) When discussing spatial localization it is also standard to simplify by assuming a horizontal bo. The z-axis is always parallel to bo, the y-axis is from ceiling to floor, and the x -axis is from right to left. Physical gradients The actual gradients employed in spatial localization are determined by the slice plane (axial, sagittal and coronal). And the particular anatomical structures being imaged. The gs will always be per
