MRI Lecture 3.pdf

Image weighting Contrast Image contrast  Contrast mechanisms  T1 recovery  T2 decay  T1 weighting  T2 weighting  Proton density weighting Contrast  Areas of high signal (large transverse component of magnetization): (white on the image)  Areas of low signal (small transverse component):...

Image weighting Contrast Image contrast  Contrast mechanisms  T1 recovery  T2 decay  T1 weighting  T2 weighting  Proton density weighting Contrast  Areas of high signal (large transverse component of magnetization): (white on the image)  Areas of low signal (small transverse component): (dark on the image).  Intermediate signal (medium transverse component) :shades of grey in-between white and black. Contrast in MR vs CT image Contrast In MR Image contrast is controlled by two types of parameters: 1. extrinsic contrast parameters (controlled by the system operator). 2. intrinsic contrast mechanisms (those that do not come under the operators control). Extrinsic contrast parameters 1. Repetition time (TR): The time from the application of one RF pulse to the application of the next (ms). 2. Echo time (TE): the time between an RF pulse and the collection of the signal (ms). 3. Flip angle: The angle through which the NMV is moved as a result of a RF pulse. 4. Turbo-factor or echo train length (ETL/TF) 5. Time from inversion (TI) Intrinsic contrast mechanisms T1 recovery T2 decay Proton density Flow Apparent diffusion coefficient (ADC). Molecular Motion  Molecules of all substances are constantly in motion.  Types of molecular motion:  rotational  transitional  Faster molecular motion means more difficult to release energy to surroundings The composition of fat  hydrogen atoms linked to carbon  large molecules  slow rate of molecular motion.  low inherent energy so they are able to absorb energy efficiently. The composition of water  hydrogen atoms linked to oxygen  small molecules  high rate of molecular motion  High inherent energy so they are not able to absorb energy efficiently.  The properties of fat and water  Different relaxation rates  Different image contrast  Represent the extremes in image contrast. Other tissues have contrast characteristics that fall between fat and water.  Relaxation processes  RF pulse applied– resonance formation– RF pulse removed– NMV in the transverse plane decreases – signal decay– low voltage in the receiver coil. free induction decay (FID) Causes of reduction of NMV in the transverse plane 1. Relaxation processes (each tissue relaxes at different rate form the other )– signal difference– different contrast 2. Field inhomogeneities. T1 recovery & T2 decay The withdrawal of the RF produces two effects:  Nuclei emit energy absorbed from the RF pulse (spin lattice energy transfer)– NMV recovers and realigns to B0 (T1 recovery).  Nuclei dephase– NMV decays in the transverse plane.(T2 decay)  T2*  lose of coherence happens in two ways: 1. Interactions of the intrinsic magnetic fields of adjacent nuclei (spin–spin energy transfer) 2. Inhomogeneities of the external magnetic field. T2* happens before T2  T1 recovery  T1 recovery : is the time the tissue takes, for 63% of the longitudinal magnetization to recover  is caused by spin lattice energy transfer.  The rate of T1 recovery is an exponential process and it occurs at different rates in different tissues.  The T1 time of a particular tissue is inherent to the tissue being imaged.  The TR determines how much T1 recovery occurs in a particular tissue  T1 recovery in fat & water  fat is able to absorb energy quickly– the T1 time of fat is very short.  Water is very inefficient at receiving energy from nuclei therefore the T1 time of water is quite long,  Short TRs do not permit full longitudinal recovery in either fat or water so that there are different longitudinal components in fat and water. T1 recovery in fat T1 recovery in water Magnitude of transverse magnetization Vs amplitude of the signal Saturation  As the NMV does not recover completely to the positive longitudinal axis, they are pushed beyond the transverse plane by the succeeding 90° RF pulse (saturation)  When saturation occurs there is a contrast difference between fat and water due to differences in T1 recovery using short TRs.  No contrast difference between fat and water due to differences in T1 recovery when using long TRs.  Any differences in contrast are due to differences in proton density Saturation No saturation T2 decay T2 decay: the time it takes for 63% of the transverse magnetization to be lost due to dephasing.  caused by spin–spin energy transfer.  produces a loss of phase coherence  It results in decay of the NMV in the transverse plane.  It is an exponential process and occurs at different rates in different tissues.  The T2 decay time is inherent to the tissue being imaged  The period of time over which T2 decay occurs is the time between the excitation pulse and the MR signal or the TE.  The TE determines how much T2 decay occurs T2 decay curve T2 decay in fat and water  Fat’s T2 time is very short compared with that of water.  Short TEs do not permit full dephasing in either fat or water so their transverse components are similar. There is little contrast difference between fat and water due to differences in T2 decay using short TEs.  Long TEs allow dephasing of the transverse components in fat and water. There is a contrast difference between fat and water due to differences in T2 decay times T2 decay in fat T2 decay in water T1 weighting  A T1 weighted image is an image whose contrast is predominantly due to the differences in T1 recovery times of tissues.  short TR is selected to ensure that the NMV in neither fat nor water has had time to relax back to B0 before the application of the next excitation pulse. T1 weighting  For T1 weighting: To diminish T2 effects the TE must also be short  Fat has short T1 (bright high signal).  Water has long T1 (dark, low signal).  T1 weighted images best demonstrate anatomy but also show pathology if used after contrast enhancement.  Typical parameters TR 300–600 ms (shorter in gradient echo sequences) TE 10–30 ms (shorter in gradient echo sequences) T1 weighted  Sagittal T1 weighted image of spine. Intraspinal lipoma is bright as it contains fat T1 contrast T1 differences between fat & water T2 weighting AT2 weighted image is an image whose contrast is predominantly due to the differences in the T2 decay times of tissues.  a long TE is selected  long TR  Tissues with a short T2 decay time such as fat are dark (low signal)  Tissues with a long T2 decay time such as water are bright (high signal)  T2 weighted images best demonstrate pathology  Typical parameters TR 2000 ms + TE 70 ms + T2 weighted  Sagittal T2 weighted image through the spine. The intraspinal lipoma is now dark. T2 contrast T2 differences between fat & water  Proton density weighting  In a PD weighted image differences in the number of hydrogen protons in the tissue must be demonstrated.  Achieved by reducing the effects of both T1 and T2 (a long TR and a short TE)  Tissues with a low proton density are dark (low signal)  Tissues with a high proton density are bright (high signal)  Cortical bone and air are always dark on MR images as they have a low proton density.  Proton density weighted images show anatomy and some pathology Typical values  TR 2000ms+  TE 10–30ms Proton density contrast Proton density weighting  Coronal FSE PD weighted image of the brain https://www.ole.bris.ac.uk/bbcswebdav/institution/Faculty%20of%20Health%20Sciences/MB%20ChB%20M edicine/Radiology/MRI%20e-tutorial/page_04.htm Summary Summary Thank you