The Significance Of Larmor Frequency In Mri English Language Essay

The Signifi rotterce Of Larmor Frequency In Mri side Language EssayWhen a charismatic topic is utilize to an atom, the thermonuclear spin testament be orientated with the theme. The spin all(prenominal)ows absorption of a photon of frequence L, which is dependent on the magnetic field utilise.L = * BIn the equation above, B is the magnetic field, is the gyromagnetic ratio and L is the larmor frequence. When atoms atomic number 18 placed in a non-uniform magnetic field, the nuclei of the atom have versatile larmor frequencies collect to the equation.(b) Why does the RF field have to be applied exactly at the Larmor frequency to tip the net nuclear magnetic intensity?Some nuclei are said to be duplicate to the magnetic field with somewhat nuclei orientated anitparallel which are at a higher efficiency say. The expertness divagation between the two states is overturn by E. When this goose egg passing game is applied there is a transition from the dismount energy level to the higher energy level. There is now more than nuclei in an anitparallel orientation. When using the RF pulse it has to be ring with the Larmor frequency. As the RF energy is applied the nuclei will orient themselves in the anitparallel position, this offsprings in more nuclei being in a higher energy state.(c)Classically the spin race states are divided into parallel and anitparallel what is the approximate population remainder between these two states at 1.5T?In order to calculate the population difference use the larmor equation to find the frequency at 1.5 Tf = BWhere = 42.6 megacycle per second/TB= 1.5 Tf = (42.6 x 10 6) (1.5)f= 63.9 x 106 unsweet flat use the pursuance equation E = hf to find the difference in population whereh is plancks constant which is 4.14 x 10- 15 eVsecf is 63.9 x 106 secE = hfE =4.14 x 10-15 )(63.9 x 10-6 sec)E = 2.65 x 10-7 eV(d) Describe what is meant by the Free Induction Decay (FID) of the MR communicateIn the rotating fra me, the net magnetic field strength vector decays as spins lose stagecoach coherence and begin to blow up apiece other out. They do this because they experience slightly variant magnetic field strengths due to interaction between spins (via their own oscillating magnetic handle). Slightly different magnetic fields means slightly different precession (Larmor) frequencies. This causes some spins to lag behind the average, and some get forrader of the average. Eventually the spins point in arbitrary oversights and the Mxy component of the net magnetisation vector is lost. This decaying, oscillating signal is the free induction decay.1(e) Describe what is meant by spin-fretwork relaxation behavior. How is it heard mathematically?One relaxation process involves a pass on of the protons to their original alignment with the smooth magnetic field. This process, called eagle-eyeditudinal or spin-lattice relaxation, is characterized by a age constant T1. The term spin-lattice re fers to the interaction of the protons (spins) with their surroundings (the lattice or network of other spins). This interaction causes a net unloose of energy to the surroundings as the protons return to the lower energy state of alignment.2The spin- lattice relaxation is given by M_z=M_0 1-exp (-t/T_1) where M_z and M_0 components of the magnetisation vector and t is the time.(f) Describe what is meant by spin-spin relaxation. How is it described mathematically?Before a radio wave is applied, the precessional orientation of the protons isRandom. The application of a radio wave brings the protons into coincidental precession, or in phase angle. When the radio wave is switched off, the protons begin to interact with their neighbours and give up energy in random collisions. In so doing, they run off to a state of random phase. As the protons revert to random orientation, the mountain signal decreases because the magnetic moments tend to supportcel each other. This process is called transversal or spin-spin relaxation and is characterized by a time constant T2.3The spin-spin relaxation is given by M_x, y=M_0 exp (-t/T2).(g) Using the spin rejoinder sequence, describe how the timing parameters back end be adjusted to reflect T1, T2 and proton density in the theatrical role.Question 2Figure 2.1 Image given in designation(i)What are the two user variable parameters marked as a and b? justify briefly in general terms how a and b might be chosen to achieve different image contrast weightings.The user variable a is the echo time and b is the repetition time. When there is a minuscule repetition time and short echo time the result will be a T1-weighted image. A long repetition time and short echo time will give a proton density image, and long repetition time and long echo time will give a T2-weighted image.(ii) Identify the slopes markedd. Explain what is illustrated by the dotted horizontal strings in this part of the diagram. What user variable para meter directly influences what is happening ind? Briefly relate this to total regard time.The gradients markedd represents the phase encoding gradient. The user variable parameter that directly influences the phase encoding gradient is the frequency encoding gradient. The total s privy time for a standard spin echo or gradient echo sequence is number of repetitions x the s fag end time per repetition (means the increase of repetition time (TR), number of phase encoding steps, and NSA).4(iii)Identify the gradient e. If all other variables are considered fixed what user variable parameter would wobble the strength of this applied gradient?The gradient is e is the slice pick gradient. The user variable parameter that would change the strength of this applied gradient is the slice onerousness parameter. The slice thickness is governed by the pastime equationthk = BWtrans / (0GS)Where thk is the slice thickness, BWtrans is the transmitted RF drawwidth (the range of frequencies it covers), 0 is the gyromagnetic ratio and GS is the magnitude of the slice selection magnetic field gradient. So, slice thickness is inversely proportional to GS increasing GS will yield a slender imaging slice.5(iv) What is the waveform show in g? Give a brief description of how the signal from g is captured by the MR computer brass and subsequently processed to form an image.The waveform g represents the echo signal get wordd.Question 3(A) Describe briefly the physical basis of the following magnetic resonance imaging artifactsTruncation or GibbsGibbs or truncation artefacts arise from going from a bright are to a dark area resulting in bright or dark lines which are present at parallel and adjacent borders where there is a sudden change in intensity. This instance of artefact is associated with the number of steps used in the fourier transform to trace the image . To minimize the effect of Gibbs or truncation artefacts more encoding steps are performed in order to lessen th e intensity and sign the artefacts.Figure 3.1 Diagram shows the Gibbs effect resulting from a Fourier transformation of a incisive change in image intensityZipperThere are various causes for zipper artefacts in images. Most of them are related to computer hardware or software problems beyond the radiologist immediate control. The zipper artefacts that can be controlled easily are those due to RF entering the see room when the entry is open during acquisition of images. RF from some radio transmitters will cause zipper artefacts that are oriented perpendicular to the frequency axis of your image. Frequently there is more than one artefact line on an image from this cause. Other equipment and software problems can cause zippers in either axis.6Figure 3.2 When this image was taken the scanner room door was left open during the acquisition causing the zipper artefacts shown.Chemical mongerChemical shift arises from a variation in the resonance frequency due to the nuclear spin of protons in different environments like round or pissing. Due to the magnetic shielding of different protons , will result in different resonance frequency and hence lead to discharge alteration of protons in the same slice during the fourier transform. The chemical shift artefact will appear as bright or dark band at the edge of the anatomy. High field strength increases the miss registration of the protons.Figure 3.3 In this image the chemical shift artefact is seen as a small bright line in front of the femoral bone.(b) In a magnetic field, fat precesses at a lower frequency that water giving rise to a Chemical shift. At 1.5T, the frequency difference is 210Hz. For an image matrix of 256 x 256 and a receive coil bandwidth of 16 kHz, calculate the size of the shift between water and fat in pixels.To find the chemical shift use the following equationsd = (n nref) x 106 / nrefBut know thatnref = gBoThe equation now becomesd = (n nref) x 106 / gBowherenref is resonance frequen cyg is gymagnetic ratio where g is 42.58 MHz/TBo is magnetic field strength where Bo is 1.5 Tn is resonance frequency of second componentd is chemical shift differenceCan now put these set into the above equations. Given that the frequency difference is 220 Hzd = (n nref) x 106 / gBod= (220) x 106 / (42.58 x 106) (1.5)d= 3.44 ppmQuestion 4Using diagrams where appropriate, briefly describe k-space to a lower place the following headingsWhat does k-space representK space corresponds to a matrix of the MR entropy and represents the image before processing like fourier transforms are performed. Within k-space each line represents a measurement, with a separate line for varying phase gradients. A line of height 0 represents a line with no phase gradient.Figure 4.1 In this diagram, Kx represents frequency, Ky represents phase directions. Each measurement is positioned at a different Ky coordinate (height)How is k-space normally strikeedThe polarity and bounty of the frequency and pha se encoding gradients directly affect how k-space is filled. The amplitude of the frequency encoding gradients establish how far the k-space goes to the left or right and and then gives the field of view of the image in the frequency direction. Positive values go from left to right while negative values go from right to left.The amplitude of phase encoding gradient estimates how far up and down k-space is filled up and down in the phase direction and hence determines the field of view in this direction. Positive values fill the top half while the negative values fill the rat half.What determines k-space co-ordinatesThe k-space location (kx and ky coordinates) of data is governed by the accumulated effect of gradient events and upheaval pulses.7How does an absence of data in k-space affect the imageIf there is some date missing in k-space this will result in a loss of resolution of the image.Question 5(a) Describe four all-important(a) safety hazards in MRI.Static magnetic fields Depolarization can be caused by voltages bring on in flowing blood and the drive of muscles in the heart which can be detected by an electrocardiogram. In order to minimize this effect the magnetic field strength unploughed withing the following specialises. 2.5 T for body of patients 0.2 T for arms and hands of cater and 0.02 T for whole body of staff.Switching of the gradient magnetic fieldsInvoluntary muscular contraction, breathing difficulties and ventricular fibrillation arise from eddy currents induced in the body. MRI should not be performed on patients with implants, as it can cause harm to the patient. In the case of strong fields, taste sensations can be experienced by the patient while flashes of light can be present on the patients retina. Magnetic field build up is usually 1-5 Ts to avoid symptoms. radio setfrequency fieldsIn strong static fields, at high frequencies, heating can occur. The temperature should not rise above 1 degree Celsius on the patients skin. In order to minimize the heating affects the unique(predicate) absorption ratio should not surpass 0.4 W/Kg and pulsed RF field should not go beyond 70 W machinelike attraction of ferromagnetic objectsThis changes with the square of the magnetic field and inverse of the distance. coat objects are made into projectiles if they come into the fringe field. For this reason non-magnetic material should be used.(b)What is meant by SAR? Define the units used to measure this parameter.The Specific Absorption charge per unit is defined as the RF power absorbed per unit of kettle of fish of an object, and is measured in watts per kilogram (W/kg). The SAR describes the potential for heating of the patients tissue due to the application of the RF energy necessary to arise the MR signal. Inhomogeneity of the RF field leads to a local exposure where most of the absorbed energy is applied to one body region rather than the entire person, leading to the idea of a local SAR.8What is the whole body limit?4 W/kg averaged over the whole body for any 15-minute period.9Which sequences are a particular concern and may give rise to radio-frequency effects?Radio frequency effects occur when a patient is exposed to static magnetic fields in MRI. The Radio frequency pulses mainly produce heat, which is absorbed by the body tissue. If the power of the RF radiation is precise high, the patient may be heated too much. To avoid this heating, the limit of RF exposure in MRI is up to the maximum precise absorption rate (SAR) of 4 W/kg whole body weight (can be different from country to country). For MRI safety reasons, the MRI machine starts no sequence, if the SAR limit is exceeded.10