The Rooms Used in Magnetic Resonance - Report Example

ial 1. What are the s of the rooms used in magnetic resonance imaging? The main rooms in magnetic resonance imaging are magnetic, technical, and operator.   2. List the major components inside the magnet and briefly describe their function. One component is the shims located at the bore which act to adjust the distributions of a magnetic field. There are two types of shims, passive and active. Passive shims are arranged along the bores as thin metal plates in trays and their function is to remove Bz inhomogeneities in the dsv. Active shims are a matrix of electromagnetic coils, each carrying a different current and act to cancel out impurities in the magnetic field.   Another component is the gradient coil which is pulsed rapidly when scanning and is used to vary the strength of the magnetic field. There are 3 sets of coils for different directions, known as X, Y and Z. This variation in the gradients provides localized images and helps with phase and frequency encoding.   Located close to the patient is the radiofrequency coil(s) used to produce MR signals and is really important in getting maximum signal-to-noise ratio. This coil is composed of two electromagnetic coils (transmitter and receiver) used to generate and receive electromagnetic fields. A transmitter coil lays just inside the gradient set generating magnetic fields orthogonal to the B0 filed in the X-Y plane. It pulses to produce radiofrequency energy at the Larmor precessional frequency. A receiver coil is used to receive the MR signal and functions to adjust and optimize the signal-to-noise ratio of the experiment. (more about this in module 5).   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 shape and timing of the RF pulses. The sequence timing controls the pulses issued which are 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 in order to generate an electromagnetic field.   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 filter plate of the Faraday cage. From here 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.   5. Why does a magnet need to be shimmed? Since the construction of magnets is prone to human error and can’t be absolutely perfect, a minor deviation of coil positioning or permanent magnet faces is enough to generate impurities which have to remove by shimming. For example, when magnets are energized, the impact of the internal forces can create impurities in the magnetic field which then have to be removed by passive or superconducting shimming processes. 6. 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 the surface of a sphere of diameter 45cm. What is this variation in parts per million (ppm)? Is such a variation 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 on 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.)   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.   8. For a Niobium Titanium (NbTi) superconducting wire, what are the values of the critical field strength and critical 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 critical field strength is about 10T.     9. As the magnetic field in which a superconducting wire resides reduces, does its consequent current carrying capability increases or deceases? What would happen if the temperature is reduced simultaneously? Reducing the magnetic field increases the current carrying capability. If the temperature is also reduced, then the current capability still increases.   10. What is the optimal separation for 2 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 2 current carrying circular loops is 20cm (i.e. 2Z1=a). Tutorial 2 1.  What are the desirable properties of a pulsed gradient coil system? They should be: •     Very linear gradients, less than 5%, over the DSV (i.e. all of the sample) •     able to produce rapid gradient pulses with a rise-time from 100-200µs. •     have a low resistance and low inductance to minimize power dissipation and impact on the rise time. •     shielded when the gradient is pulsed so that eddy currents don’t become incorporated in the magnet structure. •     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. 2. In designing a gradient coil system, how would you maximize the switching speed of the system, while minimizing the interference with other components in an MR system? Which of the other components in an MR system are particularly important in this regard?            Active shielding helps to maximize the switching speed while minimizing interference. Also, the cold structure in the magnet is important to control the flux leakage of eddy currents which can occur.   3. What effect does a gradient G(r) have on the Larmor precessional frequency at a position r?           The effect of the gradient G(r) vector adjusts and modulates the precessional frequency with spatial position.   4. What is the optimal separation of two current-carrying loops to produce as linear a magnetic field gradient as is possible at the centre of the 2 loops? Why is this separation different to that necessary to produce a homogenous field?            The optimal separation involves root 3 multiplied by the radius with the currents running in opposite directions.  This acts to cancel out the even order harmonics. With the currents running in the same direction, these act to cancel out the odd order harmonics and therefore, are best suited for designing constant or homogeneous fields.   5. What does "slew rate" mean and is it better for its value to be large or small? Why?            Slew rate is the maximum gradient strength / the rise-time. Usually it is preferred to have large slew rates in order to achieve strong, rapidly changing gradients. However, peripheral nerve stimulation also has to be taken into account when measuring these rates.   6. Why is it necessary to shield gradient coils? Shielding is important because the repeated on and off switching of the gradient coils results in pulses of the magnetic field. At a certain distance, these pulses induce currents on conducting materials and because of the magnetic field are called eddy currents. These undesirable eddy currents in turn induce their own magnetic field which can result in opposing gradient fields and therefore a distortion of the final image.   7.  What type of effect is caused by gradient coil shielding? Gradient coil shielding shifts the main magnet field (i.e. the B0 field). This in turn can cause the slices to shift resulting in a phase encoding noise.   8. What causes the undesirable sounds produced by magnetic resonance imaging at runtime? How could the noises be avoided?          The undesirable sounds heard by patients at run time are caused by different directional magnetic fields acting on each other as they align. The noise produced is because of the cavity encountering forces (shear, torque etc.) on the fixed apparatus. Also, the different gradient field sources are fixed to the apparatus and as the cavity moves, the acting forces result in acoustic noise. The noises can be reduced by commonly using ear plugs. Another method which is very difficult is to modify the design of magnetic resonance imaging devices so that the total amount of torque due to magnetic fields is reduced. Also, a chamber can be used which has been tuned to cut out acoustic frequencies of the forces. This is also difficult because usually chambers are built after designing the devices, limiting the change in the final dimensions.   9. What is the main cause of nerve stimulation due to magnetic resonance imaging and how is this observed in patients? Electromagnetic fields with change in time induce current in conductors which carry electrical current and have certain resistivity. The resistivity of any material is its ability to carry current. With time varying magnetic field, a conductor will be induced by current where their flow will be dependent on the surrounding and its resistivity. Since humans are made up of different types of muscles and tissue all having a variety of resistivity, their ability to carry current ranges from good to poor. Therefore, human tissue within a MRI apparatus in the presence of a time varying magnetic field will induce different currents in different tissues. Since, the brain stimulates signals in the form of currents; the induced currents within the nerves of the tissue will result in movement and nerve stimulation felt as tingling sensations, cramping, twitching, etc. 10. What are the causes of impurities in the static magnetic field (B0)? Impurities can be caused by limitations when manufacturing due to engineering imperfections. Also, conducting structures when large enough can affect the images produced by the device. The different tissue resistivity present in humans can act to alter the images. Commonly, the RF magnetic field is influenced by the variety of patients causing major image impurities. 11. State the two types of image quality reductions caused by zonal impurities in the static magnetic field? The two types are pixel broadening/ blurring and decreased pixel intensity.   TUT 3 1-What other types of magnets could be used in MRI to provide more access to patients than the conventional cylindrical system? The other types are open, short bore cylindrical, or extremity magnets.   2. Discuss the salient features of a birdcage coil for use as a transmitter RF coil in MRI. The main feature of a birdcage coil is its design where its many parallel rungs are joined together at the top and bottom by 2 circular loops. The loops excite the rungs allowing uniformity within the RF (B1) field. Also, by allowing varying currents to flow in the different rungs, the resultant magnetic fields superimpose to form a high uniformity B1 magnetic field. This type of transmitter coil, excited by a 90 degree out of phase voltage, produces a circularly polarized wave generating an efficient homogenous B1 field requiring less power and maintaining lower temperatures.       3.  What are primary differences between RF surface coils and body coils?   There are two main differences between RF surface and body coils. First, body coils are usually cylindrical and are comparatively far from the imaging volume resulting in very uniform B1 fields. The surface coils on the other hand, are planar and are located closer to the body with less uniform B1 fields. Second, the signal to noise ratio of surface coils due its nearness to the volume or load is superior to that of the body coil.   4. State the particular advantages and disadvantages of using a separate surface RF coil for imaging, for example, the heart, as opposed to just using the body coil. The advantages of a surface RF coil are that it generates higher signal to noise ratio, better image resolution and better image contrast. The disadvantages are that it doesn’t provide the same B1 uniformity as the body coil, its penetration depth can be limiting, and there is also a need for additional coils to be plugged in.  5. Why is it that in clinical applications surface coils are generally receive only and transmit is achieved through the body coil?  Due to the strict electromagnetic field exposure limitations, transmit surface coils are less likely to be used in clinical applications, because they lie closer to the body and deposit more energy. Receive coils, on the other hand, do not deposit energy within the body and therefore don’t have to be tested under the same strict requirements. With an existing body coil used for transmitting, performance can be improved by adding receive surface coils.   6. Compare the resultant fields in the rotating frame generated by linearly polarized RF fields to those produced by circularly polarized RF fields.   Linearly polarized magnetic RF fields are produced in one transverse direction. When generating it, a linearly alternating transverse field is decoupled mathematically into two consecutive rotating components opposite one another. These components oppose each other producing half the amplitude, with only one component contributing to the final magnetic field (B1). Circularly polarized RF coils have 2 superimposed coils, where one is rotated by 90 degrees so that circular polarization can occur. This way all the parts are active, producing a more efficient resultant magnetic field (B1). 7. What are the advantages of circular polarization in RF coils?  Circular polarization are more efficient, need less power to operate, produce less heat, and have a better signal-to-noise ratio which increases the homogeneity of the RF magnetic field.   8. What is the Q factor and what are the associated desirable properties for RF coil design?  ‘Q’ is the quality factor and for the RF coil design it measures the coil’s performance taking into account many variables. Q is proportional to the inductance over resistance (L/R) where the proportionality constant is the angular frequency (Q=omega L/R). When no load is present in the chamber, RF coils should have low resistance, resulting in a higher Q value, meaning increased quality. When there is a load interacting with the B1 field, the Q factor decreases and is directly correlated to the signal-to-noise-ratio. The larger the change in the value of Q, the better the signal-to-noise-ratio of the RF coils.   9. The conductivity of a material is defined as one over the resistivity of that material. Is the conductivity of human tissue frequency dependent? If so, then provide at least one supporting condition why this is the case.  Yes the conductivity of human tissue is frequency dependent. For an example see Exercise 7.1 on your MRT-CD.     TUT 4   1. What is the Nyquist criterion for sampling? What can happen if this criterion is not met?  The Nyquist criterion for sampling is the ability to take Fourier transforms of data and accurately reconstruct it. It states that the associated Nyquist frequency is given by   fc=1/(2dt)   with fc being the critical Nyquist frequency and dt being the time interval between consecutive samples. Frequencies below fc can be captured when reconstructing the data but not necessarily when frequencies are above fc. So by changing the time between samples, the band of frequencies can be increased or decreased accordingly. If frequency above the threshold is used then accuracy can’t be guaranteed. Therefore, prior knowledge of reconstruction frequency is required in order to adjust the sampling rate and to make sure that correct images are achieved.   2. Assume that the Dwell time in a quadrature magnetic resonance receiver is 9 micro seconds, then what is the associated sweep width? From question 1 it can be inferred that dt=9x10^-6 seconds. This means that fc is given by:   fc=1/(18x10^-6)=55.55555x10^3.   So the sweep width is 55.55KHz. Therefore by decreasing dt, fc is increased.   3.  State what is meant by the Specific Absorption Rate (SAR). Why is SAR important for patient safety?   SAR=(total energy dissipated in tissue)/(exposure time x weight of sample).   Energy is measured in Joules (J), time in seconds (s) and weight in kilograms (kg), so SAR is measured in J/s/kg or Watts/kg (W/kg), where W=J/s. The total energy in MRI applications is caused by the RF excitation. Also, watts measure power created by the RF coil in tissue. Since, the power from RF is spread in the tissue as heat; there is only so much heat that can be dissipated by tissue in a given amount of time. Therefore, the rates of dissipation are limited by the SAR measure which is an important factor to consider in order to minimize tissue damage.     4. Given two systems, one 1.5T and one 3T with the same gradients, and matched RF coils:   a. What is the RF wavelength for the 1.5T and 3T systems?   Wavelength = 300,000,000/frequency 1.5T frequency = 42,580,000 x 1.5 and 3T frequency = 42,500,000 x 3. Therefore, wavelength at 1.5T is 4.697m, and at 3T it is 2.348 meters. So the wavelength at 3T is half that at 1.5T, because of the doubled frequency.   b.      How does a decreased wavelength impact imaging?   Decreased wavelength with regards to imaging means increased non-linearity in the RF field. So it is expected that at 1.5T the RF intensity map is more uniform when compared to the 3T intensity map.    c. Would you expect the observed SAR in a human head to be different? State your reasons.    If the magnets are different then the frequency of the matched RF coil must also be different. For example, the resonant frequency of the RF coil at 1.5T is around 64MHz and at 3T is about 128MHz. Since tissue has certain conductivity and frequency dependence, there would be different SAR plots of the human head in the two different systems. Also, as the wavelength is decreased, or the frequency is increased, more hot spots are formed resulting in more drastic SAR impact. There are many different types of tissue in the human head, making it hard to precisely predict what happens at varying frequencies. For this reason an energy balance approach is usually adopted to say what the maximum SAR may be. Meaning whatever put in as power can be absorbed by the head making absolute bound and not taking into account anything that may be absorbed in other structures, reflected and lost.       5. In high field magnetic resonance imaging, what effects can be caused by tissue conductivity at different frequencies?   The conductivity of materials is mostly frequency and temperature dependent. For example, the conductivity of ceramics is highly temperature dependent. In ceramics the conductivity is very small to a certain temperature (like 400 K) and then greatly increases as the temperatures passes 400 K resulting in a highly non-linear behavior. Usually, the behavior tends to be more linear and the conductivity increases with temperature. The same is true for frequency because as the frequency at a given delivered energy changes, the conductivity (or the materials ability to absorb energy) also changes. In high field MRI this type of behavior can cause local hot spots in tissue and decrease the B1 homogeneity. Read More