Chapter 9 Instrumentation Flashcards
MRI Scanners must all incorporate the following.
- A powerful magnet /magnetic field of over 40-50 cm spherical volume
- shim system
- Gradient system
- RF transmission system
- RF receiver coils
- Computer system
- Computer subsystem
What MRI system improves homogeneity of the magnetic field?
Shim System
What system in MRI creates linear slopes in field strength in any direction?
Gradient System
What MRI system generates and transmits pulses of electromagnetic radiation?
RF Transmission System
What detects signal from the patient?
RF Receiver Coils
What system allows input of parameters and displaying of images?
Computer System
System capable of coordinating the application of RF pulses and gradients and reconstructing the acquired data into images and storing them?
Computer Sub-System
What is the magnetic field in teslas (T)?
Bo
The magnetic intensity in amperes/m
Ho
Exhibit a weak repulsion to an external magnetic field Slightly oppose (decease) the applied magnetic field
Diamagnetic Compounds
Exhibit a weak attraction to an external magnetic field Add to (increase) the applied magnetic field
Paramagnetic Compounds
Have a large positive magnetic susceptibility and are powerfully attracted to an external magnetic field?
Are permanently magnetized even when removed from the magnetic field
Ferromagnetic Compounds
4 naturally occurring elements that are Ferromagnetic at a normal temperature?
Iron
Nickel
Cobalt
Gadolinium
Moving a conductor through a magnetic field induces an electrical charge in it
Moving electrical charge in a conductor induces a magnetic field around it
Three main types of MRI scanner configurations?
Closed-bore System
Open Systems
Extremity Systems
Most popular MRI system, have a tunnel shaped magnet bore almost look like a larger CT unit. Has longitudinal table movement and can generate very high magnetic field strengths over 8 T ?
Closed-bore System
Closed-bore Systems use what field strengths for clinical use and research studies?
Clinical Use: 1 - 3 T
Research: 8 T and above
Closed-Bore MRI Sytems use what type of magnet?
Solenoid Electromagnets that are infersuperiorly placed
This system has the following features Wider Imaging Table Magnetic poles above and below Great for large people or animals Lateral access great for biopsies Magnets are horizontal above and below the patient
Open Systems
What type of magnet is used in Open MRI systems?
Large Permanent Magnets
What is the field strength for Open MRI systems?
1.2 T
Designed to scan limbs a much smaller unit used permanent magnets and has a field strength restricted below 1T?
Extremity Systems
MRI field strength (flux density) must be high and between?
1.0 - 8.0 T
The fringe field having a strength of 0.5 mT (5G) or greater must not extend outside the safety of what zones?
Zones III and IV and should be contained within the magnet room.
Ferromagnetic discs are known as ___________ and are mounted on a yoke that positions them directly above and below the imaging volume
Pole Shoes
The area inside the cylinder of the cryostat is known as ________ contains the patient bore and components of the MRI system that operate at room temperature.
WARM BORE
Substances used to supercool the coils of wire in superconducting magnets.
CYROGENS
Solenoid electromagnets that use supercooled coils of a wire so that there is no inherent resistance in the system. The current flows and therefore, the magnetism is generated without a driving voltage
Superconducting Magnets
Cyrogen of choice for superconducting magnets because of its extremely low temperature
Liquid helium
Main magnetic field is induced using ________
Solenoid Magnet
The process of energizing an MRI solenoid
RAMPING
Bringing an MRI magnet up to the required field strength involves a simple parallel circuit known as
Persistent Switch
Magnetic field strength is measured in two main ways?
Measured in Amperes (A) per m and is given the symbol H
Flux Density is measured in Tesla’s and given the symbol of B
The number of flux lines passing through a given area?
Flux Density
The earth’s magnetic field is usually around 0.5 G
The field strength (flux density) of and MRI magnet varies by scanner design and typically has a range of?
0.15 - 8T
In SI units the Tesla is used to quantify flux density.
In older CGS systems the gauss is used
(Where 1 T equals 10 000 G)
Magnetic Shielding occurs in 2 ways:
- Passive Shielding, requires scanner to surrounded by large steel plates
- Active Shielding, uses additional solenoid magnets
Used in active shielding to constrain 5G footprint of the fringe field to within a short distance from the scanner
Bucking Coils
The resistivity of electromagnetic coils is zero because they are immersed in a cryogen that reduces their temperature to almost absolute zero
Law of electromagnetic induction that states charge, movement and magnetism are inextricably linked
Faraday’s Law
The force exerted on a charge particle moving with velocity through an electric magnetic field
Lorentz Force
1st system to be encountered within the warm bore of the inner wall of the cryostat that operates at room temperature. It ensures homogenicity of the magnetic field within the imaging volume
Shim System
Process by which the evenness of the magnetic field is optimized
Shimming
Uses shims to adjust for large changes in field homogeneity
Passive Shimming
Uses electromagnets, additional solenoid magnets to adjust the field homogeneity
Active Shimming
Another way of shimming in which another electromagnet is designed to manipulate the magnetic field during image acquisition
Gradient Offset (dynamic) Shimming
Center of the warm bore, this system must localize origin of the MRI signal ?
Gradient System
Closed-bore system, the y-axis lies vertically (posteroanteriorly) & x-axis horizontally from left to right
Z axis is horizontal aling the length of the magnet
Open System flux lines of Bo run vertically, so y-axis is horizontal, and z-axis is vertical
Supply power to the gradient coils
Gradient Amplifiers
These amplifiers help reduce heating by applying voltage to the coil in short closely spaced discrete bursts
Pulse Width Modulated (PWM) Amplifiers
The percentage of time that a gradient spends at maximum amplitude
Duty Cycle
Defines the power of the gradient specifically how steep a gradient slope can be achieved when the gradient coil element is activated
Gradient Amplitude
The ideal gradient should be the following:
Powerful
Capable of high amplitude gradient slopes
Rapid to reduce scan times
(rise time, slew rate, duty cycle)
The unit used to measure Gradient Amplitude are:
mT/m or G/cm
remember the unit of gauss are 10 000 times smaller than teslas
On modern day scanners the typical values of gradient amplitude are below __________
40 mT/m
The time taken to for the magnet field gradient to reach the required amplitude.
Is measured in microseconds (us)
Values are typically below 1000 us
Gradient Rise Time
How quickly the gradient magnetic field can be applied at a given amplitude and over a given distance measured in meters.
Units are T/m/s
Gradient Slew Rate
Strength of gradient over distance
Slew Rate
Slew rate is determined by dividing the amplitude of the gradient by the gradient rise of time. Example
Amplitude: 30 mT/m divided by Rise Time 0.3 ms
= 100 mT/m/ms or 100 T/ms
The time that the gradient coil is operating at required maximum amplitude and is expressed as a percentage % of the total acquisition time____________
Gradient Power Duty Cycle
Acoustic Noise has been reported of intensities of up to
115 dB or higher
What ate the three (3) gradients that perform several functions during a sequence ______
x, y, z
The amplitude of the gradient is determined by the amount of ________________ passing through the coil
Current
The _____________ of the gradient is determined by the direction of the current flowing through the coil
Polarity
The amplitude of the gradient determines the ___________
Spatial Resolution
Determines how fast data can be acquired_________
Slew Rate
Whay system determines the scan time, slice location, spatial encoding and (indirectly) image contrast in studies using gradient echo pulse sequences (GES), perfusion, and diffusion imaging?
Gradient System
This system purpose is to transmit and receive electromagnetic radiation during image acquisition and in MRI is often referred to as B1 (secondary magnetic field)
RF System
Equation that states the precessional frequency of the magnetic moment of an atomic nucleus is proportional to the external magnetic field
Larmor Equation
The purpose of the transmitted RF is to target a volume (slice) of tissue within the region to be examined and increase the energy in proportion of the hydrogen nuclei within that slice
Hydrogen has a geometric ratio of 42.58 MHz/T
Supplies power to the RF transmitter coils_____
RF Amplifier
RF Amplifier typically operates at
18 kW or greater in 1.5 T systems
Up to 40 kW in 3T systems
The RF pulses are described based on their function name them__________ and are timed to occur at certain time periods
Excitation Pulse
Rephasing Pulse
Inversion Pulse
Known as resonator coils, goal is to transfer energy of the hydrogen nuclei.
Is circularly polarized (quadrature design) which means the coil is connected to 2 power supplies coming from the RF amplifier
RF Transmit Coil
Coils that transmit RF at the resonant frequency of hydrogen to excite the nuclei and move them into high energy state___________
RF Transmit Coil
Coil that both transmits RF and receives the MR signal _____
Transceiver
Symmetrical birdcage arrangement of several electrically conductive elements in a transceiver coil __________
Birdcage Design
Signal quality is determined by this, and it determines how well the shape and size of the coil are matched to the anatomical region under investigation. Needed to ensure good signal quality from a coil __________
Filling Factor
Receives the signal returning from the patient at time TE and can also detect a certain amount of undesirable noise
RF Receive System
Is caused by thermodynamic motion of electrons both in the coil and inside the body tissues_________
Noise
Coil designed to receive signal from any of the anatomical areas that may be examined using the MRI scanner
Receiver Coils
Receive only coils used to image surface anatomy
Surface Coils
Coil that transmits and receives signal over a large volume or tissue are of the patient _________
Volume Coils
Large coils provide large coverage, but poor SNR and filling factor is low
Small coils provide small coverage, but good SNR due to high filling factor and aliasing is more likely
Best type of coils___ because they combine the benefits of using small coils with those using large coils
Phased Array Linear & Volume type
Permit parallel data acquisition __________
Phased Array Coils
Offer homogenous RF field over a large imaging volume and can image anatomy at any location and at any depth _____
Volume Coil
Disadvantages of Volume Coils _______
Lower Filling Factor and can reduce SNR
The large nonferromagnetic patient couch (table) that is raised and lowered to facilitate patient access and driven horizontally into the magnet bore
Patient Transport System
Evenness of the magnetic field _________
Magnetic Homogeneity
Center of the bore of the magnet in all planes _______
Magnetic Isocenter
Entire process of MRI acquisition is completed by ________
Host Computer / Computer System
Is used to identify the patient usually via a network link to the radiology information system ________
Graphical User Interface (GUI)
An independent computer that is responsible for sending instructions relating to timing, amplitude, duration gradients and shape to the transmitted RF pulses and the time duration of the sampling window __________________
Pulse Generator Module (PGM)
Independent drives /Used to protect form data loss _______-
Redundant Array
