&nbsp. 3. Describe the path of radiofrequency pulses through the units of the MRI instrument.

A proper sequence is selected from the central computer and used to determine the RF pulses’ shape and timing. The sequence timing controls the vibrations issued, picked up by the transmitter unit to be modulated and translated to the Larmor frequency. A high-power amplifier with a maximum of 15kW carries the pulses through the filter plate of the Faraday cage. These then pass through the circulators and its associated equipment to be received by the RF transmitter coil to generate an electromagnetic field.&nbsp.

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4. What basic units does the MR signal pass through before arriving at the central computer?

The RF receiver coil picks up the MR signal and passes it through the preamplifier, circulator, and the Faraday cage’s filter plate. It goes through another amplifier and enters the receiver unit to be translated into a lower baseband frequency. It is then filtered and digitized to become part of the central computer’s memory.

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5. Why does a magnet need to be shimmed?

Since magnets’ construction is prone to human error and can’t be perfect, a minor deviation of coil positioning or permanent magnet faces is enough to generate impurities that have to remove by shimming. For example, when appeals are energized, the internal forces’ impact can create pollutants in the magnetic field that have to be removed by passive or superconducting shimming processes.

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In a 3T MRI system that has just been passively shimmed, the variation of the resonance frequency of a small spherical sample filled with water varies from 150 Hz to 300 Hz over a sphere’s surface of diameter 45cm. What is this variation in parts per million (ppm)? Is such an interpretation reasonable?

Since 1ppm in a 1T MRI system is approximately 42.57 MHz/T

3T system = 127.7 Hz

Variation at 150 Hz is 150/127.7= 1.17ppm

Variation at 300 Hz is 300/127.7= 2.35ppm

Therefore, the variation is 1.17-2.35 ppm, which is very reasonable for a magnet passively shimmed, assuming that temperature shimming hasn’t been done.

(Note that these values are on the surface of the DSV – a volume RMS measurement would be slightly lower than these.)

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7. Above what field strength is it necessary to construct MR magnets from superconductors? A best approximate is sufficient for an answer.

Field strength above approximately 0.3T requires the use of MR magnets from superconductors because the excessive power dissipated in resistive systems becomes unstable for power supplies.

&nbsp. 8. For a Niobium Titanium (NbTi) superconducting wire, what are the values of the critical field strength and crucial temperature above which the wire ceases to act as a superconductor?

As seen from the graphs in module 2, the critical temp for NbTi is about 4.2K, and the necessary field strength is about 10T. &nbsp.

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9. As the magnetic field in which a superconducting wire resides reduces, does its consequent current carrying capability increases or decreases? What would happen if the temperature is decreased simultaneously?

Reducing the magnetic field increases the current-carrying capability. If the temperature is also reduced, then the current ability still increases.

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10. What is the optimal separation for two current-carrying circular loops of radius 20cm and each carrying 20amps, to ensure that the total magnetic field produced in a region enclosed by the two coils is as uniform as possible?

As seen from the interaction in module 2, the optimal separation for the two current-carrying circular loops is 20cm (i.e., 2Z1=a).

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Tutorial 2

1. &nbsp.What are the desirable properties of a pulsed gradient coil system?

They should be:

•&nbsp.&nbsp.&nbsp.&nbsp. Very linear gradients, less than 5%, over the DSV (i.e., all of the sample)

•&nbsp.&nbsp.&nbsp.&nbsp. able to produce rapid gradient pulses with a rise-time from 100-200µs.

•&nbsp.&nbsp.&nbsp.&nbsp.&nbsp. have low resistance and low inductance to minimize power dissipation and impact on the rise time.

•&nbsp.&nbsp.&nbsp.&nbsp. shielded when the gradient is pulsed so that eddy currents don’t become incorporated in the magnet structure.

• &nbsp.&nbsp.&nbsp. the torques and forces, as well as acoustic output and vibration, should be minimal. Also, their interaction with the patient should be as low as possible to avoid peripheral nerve stimulation.