Hardware QA-QC

The location of the coils installed in the MRI scanner, from the inner point closest to the patient to the outer most edge

RF radiofrequency coils, gradient coils, shim coils, main magnet

Gradient amplitude

strength of the gradient.

gradient rise time and what its measured in

time it takes for the gradient to reach its full amplitude, measured in microseconds

10mT/m = ?G/cm

1

trapezoidal gradient pulse used in conventional pulse sequences

gradient slew rate

speed rate of ascent or descent of a gradient from zero to its maximum amplitude, either positive or negative; strength of the gradient over a specific distance

how to calculate gradient slew rate

amplitude divided by the rise time in msec. Measured in mT/m/msec or T/m/sec.

shorter rise time does what

the faster the gradients and therefore echo spacing

Gradients with a shorter echo spacing will have

better resolution capabilities and more available slices per TR period.

convert vendor spatial gradient map into manufacturer conditions chart

1 Tesla/meter = 100 Gauss / cm

what is gauss/cm important to know for

metallic implants

duty cycle

time the gradients are on during a TR period, the “gradient working time.

parameter changes that affect pulse duty cycle

Increased # slices
Employment of fat suppression pulses (SPAIR requires increased TR compared to SPIR)
Utilization of presaturation slabs/bands
Increased ETL

what causes noise in mr room

rapid, successive switching of the gradient coils

what gradient is used for sagittal to enable magnetic field around pt’s body

x gradient

what gradient is used for coronal to enable magnetic field around pt’s body

Y gradient

what gradient is used for axial to enable magnetic field around pt’s body supine

Z gradient

what happens when you apply two gradients at the same time

oblique slice is created

RF coils are located where in the magnet

closest to the patient

utilization of multiple, powerful RF pulses (Fast Spin Echo sequences) can lead to

increased SAR induction in a patient.

180° RF pulse is _____ the power of the initial 90° RF pulse

4 times

RF heating is more of a concern in what sequences and why

fast spin echo sequences due to the multiple echo train lengths

what are major parts of the RF system

Radiofrequency coils (transmitter and reciever)

Radiofrequency coils consist of

two electromagnetic coils, the transmitter and receiver coils generating and receiving electromagnetic fields

radiofrequency absorption measurement

Specific Absorption Rate (SAR) Watts per kilogram (W/kg

what type of energy is RF

low energy non ionizing

SAR is _____ to _____ for the resonant frequency

proportional, power of 2

what happens to SAR when you double field strength

increases 4 times

primary biological effect of RF fields

tissue heating

larger pt does what to SAR

increases

how to protect pt when larger in scanner from RF heating/burns

place manufacturer pads on them to not let skin touch

why should you be careful with blankets in exams with high SAR/increased FSE usage

need a means for heat to escape

SAR/body temp correlates to

the amt of thermal induction minus escaped heat

room temp should be

65-75F

room humidity should be

50-70%

air flow in the scan room can help with what

make pt less vulnerable to RF heating effects

what does TR have to do with SAR

“shortest TR” as this has negative impact on pulse duty cycle (gradient ontime).

what do presaturation slabs and fat suppression pulses have to do with SAR

should minimize them

what do slices have to do with SAR

minimize them and use least number of slices in a given TR period

what does ETL have to do with SAR

Utilize shortest ETL possible while enabling flip angle modulation techniques, where applicable. Flip angle sweep methods – Philips: refocusing control; GE: VERSE, tailored RF; Siemens: Hyperecho, Restore; Toshiba: T2 Plus

siemens flip angle sweep method

Hyperecho, restore

a good way to reduce SAR with imaging techniques

parallel imaging to reduce workload of the phase encoding gradient

how does low SAR sequences work

longer and lower amplitude transmit RF pulses

what do low SAR sequences change

TE, so it requires a longer TR value which increases scan time

what is a coil that can help reduce SAR

Transmit/Receive coil (but significantly increased scan time)

coil not properly tuned to correct magnetic field strength

results in signal loss

For energy to be efficiently transferred between a transmitter and receiver

the two must be at the same frequency.

surface coils do what

(linear coils) yields a more localized, smaller FOV (field of view) capability, with increased SNR, thus providing opportunity for improved image quality on most systems.

SNR penetration depth on coils

½ the coil diameter

how should cables from local RF coils and gating leads be positioned

braided and positioned straight in the bore, NEVER in direct contact with the patient's skin. The cables should NEVER be coiled or looped, as this can lead to RF burns in the patient

CP coil

Circularly polarized transmission or receiver coil with two orthogonal transmission and/or receiver channels, also known as a quadrature coil. This yields better signal-to-noise than a linear coil.

which has better SNR, linear coil or CP coil

CP

When going from a linear coil to a quadrature coil, SNR is

increased by 40%

A phased array coil is comprised of

multiple coil elements combined with multiple receiver channels.

The local RF coil used to image a brain is typically located/kept

in the magnet room

Permanent magnets with a vertical magnetic field use what types of coils and why

surface coils that are solenoids because the secondary magnetic field (B1 )created by the RF coil must be perpendicular to the orientation of B0 .

multi-element phased array coils and why they are important

capable of acquiring multiple channels of data in parallel. This 'parallel imaging' technique uses unique acquisition schemes that allow for accelerated imaging.

Parallel imaging R factor does what

parallel imaging reduces scan time directly proportional to reduction -R- factor:

SENSE: P reduction

scan time reduction in phase dimension

SENSE: S reduction

scan time reduction in slice direction (3D acquisitions)

iPAT

parallel imaging phase direction.

iPAT²

parallel imaging phase and slice dimensions

patient image display is what and located where

with sequence parameter visualization is located in the scan control room on the operator console

CPU located where and for what

used to plot and adjust slices in an MR sequence is located in the scan control room at the technologist operator console.

array processor does what

responsible for reconstructing the collected MR images/data using the Fourier transform, and is located in the MR equipment room.

RF power amplifier is located

within an equipment room.

Radiofrequency (RF) shielding can be achieved by

lining the scanner room walls with copper

Magnetic field inhomogeneity is expressed in

parts per million (ppm)

Passive shielding and its importance

can be accomplished by lining the MR room with steel or other ferromagnetic plating, thus reducing the scope and distance of the fringe field.

Active shielding and its importance

requires the implementation of superconducting windings within the construction of the MR scanner to oppose a portion of the magnetic field

Field strength at magnet isocenter is measured in

Tesla

High field scanners typically have field strengths greater than or equal to

1.5T

Doubling the magnetic field strength will ____ the SNR (signal to noise ratio)

double

However, for field strength, SAR is proportional to _______ for the resonant frequency

the power of 2

doubling field strength results in a ______ in SAR potential.

4x

The unit of measurement of the magnetic field surrounding the periphery of the MR scanner (fringe field)

Gauss

fringe field

magnetic field surrounding the periphery of the MR scanner

intensity limit by FDA for clinical use for static magnetic field

4T in everyone, 8T in pts 1 month and older

most commonly used system in MR imaging

superconducting magnet

superconducting magnet field strength increased by doing what

the magnetic field strength is increased by increasing the turns of wire, current in the wires, or by reducing the spacing between the wires.

The orientation of the main magnetic field in a high field, superconducting, short bore magnet is

horizontal.

liquid cryogens are cooled to

4 kelvin, -270C, -452F

most common cryogen in MR

helium

shimming creates

additional magnetic fields

what do the additional magnetic fields created by shimming do

add to the overall magnetic field of the superconducting magnet in such a way that the total B0 field becomes more homogeneous.

safety issues in a quench situation

should be evacuated from the scan room to avoid asphyxiation, frostbite, and/or damage to tympanic membranes.

quench

refers to the sudden loss of superconductivity when its temperature is raised

how do quenches work

In the superconducting state, the resistance of the magnet coil windings is zero and hence no energy is required to maintain current flow. If the coil temperature rises above the superconductivity threshold (Tc), the windings suddenly develop a finite resistance. The several-dozen amperes of circulating current passing through this elevated coil resistance create heat. This heat causes a sudden, explosive boil-off of liquid helium

magnetic fields associated with MR

Static magnetic fields
Oscillating magnetic fields (RF)
Time varying magnetic fields (gradient)

Time varying magnetic fields (gradient) effects on pt

cause muscle contractions, cardiac arrhythmias, mild cutaneous sensations and visual light flashes (Magnetophosphenes)

Magnetophosphenes are caused by

time varying magnetic fields (gradient)

why is dB/dT limited?

so it doesnt cause peripheral nerve stimulation painfully

what sequences can have more of a detrimental effect on patients with regards to time varying magnetic fields.

EPI

Oscillating magnetic fields (radiofrequency) – primary biological effect

is tissue heating/deposition

body coil, what it does and its limitations

s an integrated part of the magnet design that acts as its own transceiver coil, with large FOV capabilities, no high SNR like localized coils

The MRI system component that produces the magnetization of proton spins (alignment)

main magnet

The MRI system component that provides the ability to perform spatial encoding

gradient system

goal of MRI Department Quality Assurance / Quality Control Program

To achieve and maintain a level of consistent, reproducible image quality in a Magnetic Resonance Imaging system, ensuring the safety of patients being scanned.

record keeping of data is important why

industry standard, producible upon request and accessible to technologists, physicists and radiology managers.

Quality Assurance

utilizes the goal of maintaining a desired level of consistency and image quality requirements within Radiology. “Are we operating the devices or equipment correctly?