MRI PULSE SEQUENCE.pptx

MRI PULSE SEQUANCE ARE DESCRIBED IN DETAILIN THIS PPT. CONTENT TAKEN FROM MUTIPLE BOOKS AND GENERALS.

1. MRI PULSE SEQUENCE ROHIT BANSAL Assistant Professor Department of Radio-Diagnosis MAMC, Agroha

2. An MRI pulse sequence is a  programmed set of changing magnetic gradients . Each sequence will have a number of parameters, and multiple sequences grouped together into an MRI protocol. A pulse sequence is generally defined by multiple parameters, including: Different combinations of these parameters affect tissue contrast and spatial resolution. The different steps that make up an MR pulse sequence . Excitation of the target area Switching on the slice-selection gradient, Delivering the excitation pulse (RF pulse), Switching off the slice-selection gradient. Phase encoding Switching on the phase-encoding gradient repeatedly, each time with a different strength, to create the desired number of phase shifts across the image.   Formation of the echo or MR signal Generating an echo, this can be done in two ways (discussed below). Collection of the signal Switching on the frequency-encoding or readout gradient, Recording the echo.

3. These steps are repeated many times, depending on the desired image quality. A wide variety of sequences are used in medical MR imaging. The most important ones are the spin echo (SE) sequence, the inversion recovery (IR) sequence, and the gradient echo (GRE) sequence, which are the basic MR pulse sequences. We have already briefly mentioned echoes and said that some time must elapse before an MR signal form after the hydrogen protons have been excited. Now we can explain why this is so: Before an MR signal can be collected, the phase-encoding gradient must be switched on for spatial encoding of the signal. Some time is also needed to switch off the slice-selection gradient and switch on the frequency-encoding gradient. Finally, formation of the echo itself also takes time, which varies with the pulse sequence used.

4. Conventional spin echo Dual Spin echo Fast or turbo spin echo Inversion Recovery Spin echo pulse sequences

5. Spin echo uses a 90° excitation pulse followed by one or more 180° rephasing pulses to generate a spin echo. This pulse sequence can be used to produce T1 weighted images if a short TR and TE are used. One 180° RF pulse is applied after the 90° excitation pulse. The single 180° RF pulse generates a single spin echo. Conventional Spin Echo

6. This can be used to produce both a proton density and a T2 weighted image in the TR time. Dual Spin echo uses a 90° excitation pulse followed by two 180° rephasing pulses to generate a spin echo. Dual Spin Echo

7. The first echo has a short TE (TE1) and a long TR and results in a set of proton density weighted images. The second echo has a long TE (TE2) and a long TR and results in a T2 weighted set of images. This echo has less amplitude than the first echo because more T2 decay has occurred by this point. Typical parameters Single echo (for T1 weighting) TR 300–500ms TE 10–30 ms Dual echo (for PD/T2 weighting) TR 2000+ms TE1 20ms TE2 80 ms

8. FSE employs a train of 180° rephasing pulses, each one producing a spin echo. This train of spin echoes is called an echo train. The number of 180° RF pulses and resultant echoes is called the echo train length ( ETL) or turbo factor. The spacing between each echo is called the echo spacing. After each rephasing, a phase encoding step is performed and data from the resultant echo are stored in K space. Therefore several lines of K space are filled every TR instead of one line as in conventional spin echo. As K space is filled more rapidly, the scan time decreases. Typically 2, 4, 8 or 16, 180° RF pulses are applied during every TR. As 2, 4, 8 or 16 phase encodings are also performed during each TR, the scan time is reduced to 1/2, 1/4, 1/8 or 1/16 of the original scan time. The higher the turbo factor the shorter the scan time. Fast or Turbo Spin Echo

9. Typical parameters Dual echo TR 2500–4500 ms (for weighting and slice number) effective TE1 17ms effective TE2 102ms ETL 8 – This may be split so that the PD image is acquired with the first four echoes and the T2 with the second four. Single echo T2 weighting TR 4000–8000ms TE 102ms ETL 16 Single echo T1 weighting TR 600ms TE 17ms ETL 4

10. Uses FSE produces T1, T2 or proton density scans in a fraction of the time of Conventional Spin Echo. Because the scan times are reduced, matrix size can be increased to improve spatial resolution. FSE is usually used for brains, spines, joints, extremities and the pelvis. As FSE is incompatible with respiratory compensation techniques, it can only be used in the chest and abdomen with respiratory triggering or multiple NEX.

11. 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) Signal intensities seen in T1 weighted images. High signal fat Haemangioma Intra-osseous lipoma Radiation change Degeneration fatty deposition Methaemoglobin Cysts with proteinaceous fluid Paramagnetic contrast agents Slow flowing blood T1 Weighted Image

12. Low signal Cortical bone Avascular necrosis Infarction Infection Tumors Sclerosis Cysts Calcification No signal air Fast flowing blood Tendons Cortical bone Scar tissue Calcification

13. T2 weighted images best demonstrate pathology as most pathology has an increased water content and is therefore bright on T2 weighted images. Typical parameters TR 2000 ms + TE 70 ms + Signal intensities seen in T2 weighted images. High signal CSF Synovial fluid Haemangioma Infection Inflammation Oedema Some tumors Hemorrhage Slow-flowing blood Cysts T2 Weighted Image

14. Low signal Cortical bone Bone islands De- oxyhaemoglobin Haemosiderin Calcification T2 paramagnetic agents No signal air Fast flowing blood Tendons Cortical bone Scar tissue Calcification

15. Cortical bone and air are always dark on MR images regardless of the weighting as they have a low proton density and therefore return little signal. Proton density weighted images show anatomy and some pathology. Typical values TR 2000ms+ TE 10–30ms Proton Density Images

16. Inversion recovery (IR) is a spin-echo pulse sequence that uses an RF inverting pulse to suppress signal from certain tissues, although it is also sometimes used to generate heavy T1 contrast . The IR pulse sequence begins with a 180° RF pulse . This is applied at the beginning of the TR period when the NMV is aligned in the same direction as B0 in the longitudinal plane (termed +z). The RF pulse inverts the NMV through 180°, which means that after the pulse, the NMV still lies in the longitudinal plane but in the opposite direction to B0 (termed−z). When the RF inverting pulse is removed, the NMV relaxes back to B0 because of T1 recovery processes. At a certain time-point during this recovery, a 90° RF excitation pulse is applied and then switched off. The resultant FID is then rephased by another 180° RF rephasing pulse to produce a spin echo at time TE. Inversion Recovery

17. The time from the 180° RF inverting pulse to the 90° RF excitation pulse is known as the TI (time from inversion) . Image contrast depends primarily on the TI. If the 90° RF excitation pulse is applied after the NMV has relaxed back through the transverse plane, image contrast depends on the amount of longitudinal recovery of each vector (as in spin-echo). The resultant image is T1-weighted. If the 90° RF excitation pulse is not applied until the NMV has reached full recovery, a PD-weighted image is produced, as both fat and water are fully relaxed.

18. T1 weighting: • Medium TI 400–800 ms (varies at different field strengths) • Short TE 10–20 ms • Long TR 3000 ms+. Proton density weighting: • Long TI 1800 ms • Short TE 10–20 ms • Long TR 3000 ms+. Pathology weighting: • Medium TI 400–800 ms • Long TE 70 ms+ • Long TR 3000 ms+. Suggested Parameters

19. In this sequence, the 180° RF inverting pulse is followed at time TI by the 90° RF excitation pulse and a train of 180° RF rephasing pulses to fill out multiple lines of k-space as in TSE. This reduces the scan time compared to conventional IR. However, instead of T1-weighted images, fast IR is usually used to suppress signal from certain tissues in conjunction with T2 weighting so that water and pathology return a high signal. The two main sequences in this category are STIR and FLAIR. Fast Inversion Recovery

20. Mechanism STIR is an IR pulse sequence that uses a TI that corresponds to the time it takes the fat vector to recover from full inversion to the transverse plane so that there is no longitudinal magnetization corresponding to fat. This is called the null point . As there is no longitudinal component of fat when the 90° RF excitation pulse is applied, there is no transverse component after excitation, and signal from fat is nulled. A TI of 100–175 ms usually achieves fat suppression, although this value varies slightly at different field strengths. Uses STIR is an extremely important sequence in musculoskeletal imaging because normal bone, which contains fatty marrow, is suppressed, and lesions within bone such as bone bruising and tumours are seen more clearly. It is also a very useful sequence for suppressing fat in general MR imaging. STIR (Short Tau Inversion Recovery)

21. Suggested parameters • Short TI (tau) 150–175 ms (to suppress fat depending on field strength) • Long TE 50 ms+ (to enhance signal from pathology) • Long TR 4000 ms+ (to allow full longitudinal recovery) • Long turbo factor 16–20 (to enhance signal from pathology). Scan tip: When not to use STIR STIR should not be used in conjunction with contrast enhancement, which shortens the T1 recovery times of enhancing tissues, making them relatively hyperintense . The T1 recovery times of these structures are shortened by the contrast agent so that they approach the T1 recovery time of fat. In a STIR sequence, therefore, enhancing tissue may also be nulled.

22. Mechanism FLAIR is another variation of the IR sequence. In FLAIR, a TI corresponding to the recovery of the vector in CSF from full inversion to the transverse plane is selected. This TI nulls signal from CSF because there is no longitudinal magnetization present in CSF. As there is no longitudinal component of CSF when the 90° RF excitation pulse is applied, there is no transverse component after excitation, and signal from CSF is nulled. FLAIR is used to suppress high CSF signal in T2 weighted images so that the pathology adjacent to CSF is seen more clearly. A TI of 1700–2200 ms usually achieves CSF suppression (although this varies slightly at different field strengths). FLAIR (Fluid Attenuated Inversion Recovery)

23. Uses FLAIR is used in brain and spine imaging to see periventricular and cord lesions more clearly because high signal from CSF that lies adjacent is nulled. It is especially useful in visualizing multiple sclerosis plaques, acute subarachnoid haemorrhage, and meningitis. Another modification of this sequence in brain imaging is selecting a TI that corresponds to the null point of white matter. This TI value nulls signal from normal white matter so that lesions within it appear much brighter by comparison. This sequence (which requires a TI of about 300 ms) is very useful for white matter lesions such as periventricular leukomalacia and for congenital grey/white matter abnormalities.

24. Suggested parameters • Long TI 1700–2200 ms (to suppress CSF depending on field strength) • Long TE 70 ms+ (to enhance signal from pathology) • Long TR 6000 ms+ (to allow full longitudinal recovery) • Long turbo factor 16–20 (to enhance signal from pathology). Learning tip: FLAIR and gadolinium Sometimes gadolinium is given to enhance pathology in the FLAIR sequence. This oddity (gadolinium enhancement in T2-weighted images) may be due to the long echo trains used in FLAIR sequences that cause fat to remain bright on T2-weighted images. As gadolinium reduces the T1 recovery time of enhancing tissue so that it is similar to fat, enhancing tissue may appear brighter than when gadolinium is not given.

25. Gradient-echo pulse sequences differ from spin-echo pulse sequences in two ways: • They use variable RF excitation pulse flip angles as opposed to 90° RF excitation pulse flip angles that are common in spin-echo pulse sequences. • They use gradients rather than RF pulses to rephase the magnetic moments of hydrogen nuclei to form an echo. The main purpose of these two mechanisms is to enable shorter TRs and therefore scan times than are common with spin-echo pulse sequences. Gradient Echo Pulse Sequence

26. Variable flip angle A gradient-echo pulse sequence uses an RF excitation pulse that is variable and therefore flips the NMV through any angle (not just 90°). Typically, a flip angle of less than 90° is used. This means that the NMV is flipped through a lower angle than it is in spin-echo sequences when a larger 90° flip angle is usually applied. As the NMV is moved through a smaller angle in the excitation phase of the pulse sequence, it does not take as long for the NMV to achieve full relaxation once the RF excitation pulse is removed. Therefore, full T1 recovery is achieved in a much shorter TR than in spin-echo pulse sequences. As the TR is a scan time parameter, this leads to shorter scan times.

27. Gradient rephasing After the RF excitation pulse is withdrawn, the FID immediately occurs due to inhomogeneities in the magnetic field and T2* decay. In spin-echo pulse sequences, the magnetic moments of hydrogen nuclei are rephased by an RF pulse. As a relatively large flip angle is used in spin-echo pulse a sequence, most of the magnetization is still in the transverse plane when the 180° RF rephasing pulse is applied. Consequently, this pulse rephases this transverse magnetization to create a spin-echo. In gradient-echo pulse sequences, an RF pulse cannot rephase transverse magnetization to create an echo. The low flip angles used in gradient-echo pulse sequences result in a large component of magnetization remaining in the longitudinal plane after the RF excitation pulse is switched off. The 180° RF pulse would therefore largely invert this magnetization into the−z direction (the direction that is opposite to B ) rather than rephase the transverse magnetization. Therefore, in gradient-echo pulse sequences, a gradient is used to rephase transverse magnetization instead.

28. How gradients diphase Look at Figure, with no gradient applied, all the magnetic moments of hydrogen nuclei precess at the same frequency, as they experience the same field strength (in reality they do not because of magnetic field inhomogeneities, but these changes are relatively small compared with those imposed by a gradient). A gradient is applied to coherent (in phase) magnetization (all the magnetic moments are in the same place at the same time). The gradient alters the magnetic field strength experienced by the coherent magnetization. Some of the magnetic moments speed up, and some slow down, depending on their position along the gradient axis. Thus, the magnetic moments fan out or dephase because their frequencies are changed by the gradient. The trailing edge of the fan (shown in purple) consists of nuclei whose magnetic moments slow down because they are situated on the gradient axis that has a lower magnetic field strength relative to isocenter . The leading edge of the fan (shown in red) consists of nuclei whose magnetic moments speed up because they are situated on the gradient axis that has a higher magnetic field strength relative to isocenter . The magnetic moments of nuclei are therefore no longer in the same place at the same time, and so magnetization is dephased by the gradient. Gradients that dephase in this way are called spoilers , and the process of dephasing magnetic moments with gradients is called gradient spoiling .

29. How gradients rephase Look at Figure, A gradient is applied to incoherent (out of phase) magnetization to rephase it. The magnetic moments initially fan out due to T2* decay, and the fan has a trailing edge consisting of nuclei with slowly precessing magnetic moments (shown in purple) and a leading edge consisting of nuclei with faster precessing magnetic moments (shown in red). A gradient is then applied so that the magnetic field strength is altered in a linear fashion along the axis of the gradient. The direction of this altered field strength is such that the slowly precessing magnetic moments in the trailing edge of the fan experience an increased magnetic and speed up. In figure, these are the purple spins that experience the red “high end” of the gradient. At the same time, the faster precessing magnetic moments in the leading edge of the fan experience a decreased magnetic field strength and slow down. In Figure, these are the red magnetic moments that experience the purple “low end” of the gradient. After a short period of time, the slow magnetic moments speed up sufficiently to meet the faster ones that are slowing down.

30. At this point, all the magnetic moments are in the same place at the same time and are therefore rephased by the gradient. A maximum signal is induced in the receiver coil, and this signal is called a gradient-echo . Gradients that rephase in this way are called rewinders . Whether a gradient field adds or subtracts from the main magnetic field depends on the direction of current that passes through the gradient coils. This is called the polarity of the gradient. Gradient-echoes are created by a bipolar gradient . This means that it consists of two lobes, one negative and one positive. The frequency-encoding gradient is used for this purpose. It is initially applied negatively, which increases dephasing and eliminates the FID. Its polarity is then reversed, which rephases only those magnetic moments that were dephased by the negative lobe. It is only these nuclei (those whose magnetic moments are dephased by the negative lobe of the gradient and are then rephased by the positive lobe) that create the gradient-echo at time TE. The area under the negative lobe of the gradient is half that of the area under the positive lobe.

31. For T1 weighting TR less than 50 ms (short) Flip angle 60–120° (large) TE 5–10 ms (short) For T2* weighting TR less than 500 ms (long) Flip angle less than 30° (small) TE 15–20 ms (relatively long) For proton density weighting TR 200–600 ms (long) Flip angle 5–20° (small) TE 5–15 ms (short)

32. It is a MRI sequence which uses steady state of magnetization. This is a situation when the TR is shorter than both the T1 and T2 relaxation times of all the tissues. Therefore there is no time for the transverse magnetization to decay before the pulse pattern is repeated again. The only process that has time to occur is T2*. Therefore the NMV does not move between repetition times. This is called steady state In general, SSFP MRI sequence are based on gradient echo with a short repetition time, it is also called FLASH MRI Technique. SSFP is beneficial for localizer sequence. This sequence also named as FFE (Fast Field Echo) and FISP (Fast Imaging with Steady State Precession) in many commercials. Useful for blood vessels and fluid field spaces in body. Fluid = Bright Fat = Intermediate Signal Steady State Free Precession Imaging (SSFP)

33. GRADIENT ECHO SHOWING EXCESSIVE IROM DEPOSITION

34. EPI is an MR acquisition method that collects all the data required to fill all the lines of K space from a single echo train. In order to achieve this, multiple echoes are generated and each is phase encoded by a different slope of gradient to fill all the required lines of K space. Echoes are generated either by 180° rephasing pulses (termed spin echo EPI) , or by gradients (termed gradient echo EPI). Gradient rephasing is much faster and involves no RF deposition to the patient but does require high speed gradients. In order to fill all of K space in one repetition, the readout and phase encode gradients must rapidly switch on and off. As data acquisition is so rapid in EPI, images may be acquired in 50 ms to 80 ms. Axial images of the whole brain are possible in 2s to 3s and whole body imaging in about 30 s. EPI sequences place exceptional strains on the gradients and therefore gradient modifications are required. Echo Planar Imaging (EPI)

35. Typical parameters Either proton density or T2 weighting is achieved by selecting either a short or long effective TE which corresponds to the time interval between the excitation pulse and when the centre of K space is filled. T1 weighting is possible by applying an inverting pulse prior to the excitation pulse to produce saturation. Uses Functional imaging Real time cardiac imaging Perfusion/diffusion

36. Diffusion weighted imaging measures the motion of spins (specifically in water). The signal is dependent on the  diffusion coefficient  within the material i.e. how freely the water can diffuse. The more a particle can move in a given amount of time, the higher the diffusion coefficient. Water diffuses randomly via Brownian motion. In pure water and gel, water can diffuse freely with no impediment or restriction. within soft tissues, water diffusion is impeded by cell membranes and intracellular organelles. A spin-echo sequence is typically used, specifically echo-planar imaging (EPI). EPI minimizes the effect of patient motion as it is a very quick sequence. This is important as DWI images the very small motion of water molecules which will be masked by any macroscopic body motion. Diffusion-Weighted Imaging (DWI)

37. Two diffusion gradients are added either side of the 180º RF pulse. The first diffusion gradient dephases the spins. The second diffusion gradient rephases and returns a signal only from the spins that have remained within the area i.e. those that are stationary. Any spins that have moved out of the area aren’t rephased and do not return a signal. The diffusion gradient is applied in multiple directions. The minimum number of directions is 3 run perpendicular to each other (e.g. x-, y-, and z-axes) but, usually, 6-20 directions are used. Each voxel’s signal is an average of the signal from all directions. Then, a standard sequence is run to generate echoes and create the signal. b-value The degree of diffusion weighting is represented as the b-value. The more sensitive the DWI sequence is to molecular motion, the higher the b-value. The strength, duration, and interval of the gradients (collectively known as the b factor/value expressed in units of s/mm2). This is one of the extrinsic contrast parameter.

38. Higher b-value: More sensitive to diffusion More noise Less signal Increase the b-value by: Larger diffusion gradient (increase the amplitude or the duration) Increased time between dephasing and rephasing diffusion gradients b0 – A DW pulse sequence is first run with the diffusion gradients switched off. This creates a  T2*-weighted  image that is used for the calculated maps later. b600-700 – Useful in neonatal brain imaging and body MRI. b1000 – Strong diffusion weighting. Used to look for cerebral infarcts.

39. Apparent diffusion coefficient As DWI images have T2 weighting. Therefore, a lesion that shows as bright on DWI may be bright because of restricted diffusion or because of inherent high T2 signal. The apparent diffusion coefficient (ADC) map is a calculated image that removes the effects of inherent T2 signal. The signal of a tissue decreases exponentially with increasing b-values. If we plot the log of the signal against the b-value, the slope will give us the diffusion characteristics without any T2 signal influence i.e. the ADC signal. Tissues with free diffusion will change signal over different b-values much more than those with restricted diffusion. More diffusion = greater change in signal = a steeper slope = a higher ADC value. This is why restricting lesions will appear dark on the ADC map. Clinical applications Diagnosis of stroke where areas of decreased diffusion, which represent infarction, are either dark or bright depending on the technique used.

40. Diffusion-weighted imaging has a major role in the following clinical situations:   early identification of  ischemic stroke differentiation of acute from chronic stroke differentiation of acute stroke from other stroke mimics differentiation of  epidermoid cyst  from an  arachnoid cyst differentiation of  abscess  from necrotic tumors assessment of cortical lesions in  Creutzfeldt-Jakob disease (CJD ) differentiation of  herpes encephalitis  from diffuse temporal gliomas assessment of the extent of  diffuse axonal injury grading of  diffuse gliomas  and  meningiomas assessment of active  demyelination   grading of prostate lesions (see  PIRADS ) differentiation between  cholesteatoma  and  otitis media  

41. B- value map calculation

42. If the probability of diffusion is the same in every direction, this is called  isotropic diffusion  e.g. in CSF.  Anisotropic diffusion  is when diffusion is not equal in every direction e.g. along nerve bundles and white matter tracts. In standard DWI we remove this effect by averaging out the signal obtained from multiple directions. However, we can use this asymmetry in diffusion tensor imaging. The three main techniques are the fractional anisotropy map, the principal diffusion direction map and fibre -tracking maps. Fractional anisotropy map Fractional anisotropy (FA) is a measure, from 0 to 1, of the amount of diffusion asymmetry within a voxel. A sphere, which is isotropic, has an FA of 0. The more asymmetric the diffusion becomes the closer it is to 1. The FA map is gray-scale. The brighter the voxel, the more anisotropic the diffusion. Principal diffusion direction map Colours and brightness are assigned to the voxels based on the degree of anisotropy (represented as brightness) and the direction (represented as colours). Fibre tracking map The direction of the asymmetry is used to compute fibre trajectories with automated software. A “seed voxel” is selected by the user and the software follows the direction of the adjacent voxels to create an image of the tracts. Diffusion Tensor Imaging

43. Perfusion is a measure of the quality of vascular supply to a tissue. Since vascular supply and metabolism are usually related, perfusion can also be used to measure tissue activity. Perfusion imaging utilizes a bolus injection of gadolinium administered intravenously during ultrafast T2 or T2* acquisitions. The contrast agent causes transient decreases in T2 and T2* in and around the microvasculature perfused with contrast. After data acquisition, a signal decay curve can be used to ascertain blood volume, transient time and measurement of perfusion. This curve is known as a time intensity curve. Time intensity curves for multiple images acquired during and after injection are combined to generate a cerebral blood volume (CBV) map. Mean transit times (MTT) of contrast through an organ or tissue can also be calculated. Clinical applications This is used for evaluation of ischaemic disease or metabolism. On the CBV map, areas of low perfusion appear dark (stroke) whereas areas of higher perfusion appear bright (malignancies). Perfusion Imaging

44. Functional MR imaging ( fMRI ) is a rapid MR imaging technique that acquires images of the brain during activity or stimulus and at rest. The two sets of images are then subtracted demonstrating functional brain activity as the result of increased blood flow to the activated cortex. The most important physiological effect that produces MR signal intensity changes between stimulus and rest is called blood oxygenation level dependent (BOLD). BOLD exploits differences in the magnetic susceptibility of oxyhaemoglobin and deoxyhaemoglobin. • Haemoglobin is a molecule that contains iron and transports oxygen in the vascular system as oxygen binds directly to iron. • Oxyhaemoglobin is a diamagnetic molecule in which the magnetic properties of iron are largely suppressed. • Deoxyhaemoglobin is a paramagnetic molecule that creates an inhomogeneous magnetic field in its immediate vicinity that increases T2*. Functional MRI ( fMRI )

45. At rest, tissue uses a substantial fraction of the blood flowing through the capillaries, so venous blood contains an almost equal mix of oxy and deoxyhaemoglobin. During exercise however when metabolism is increased, more oxygen is needed and hence more is extracted from the capillaries. The brain is very sensitive to low concentrations of oxyhaemoglobin and therefore the cerebral vascular system increases blood flow to the activated area. This causes a drop in deoxyhaemoglobin that result in a decrease in dephasing and a corresponding increase in signal intensity. Blood oxygenation increases during brain activity and specific locations of the cerebral cortex are activated during specific tasks. For example, seeing activates the visual cortex, hearing the auditory cortex, finger tapping the motor cortex.

46. More sophisticated tasks, including maze paradigms and other thought-provoking tasks, stimulate other brain cortices. BOLD effects are very short lived and therefore require extremely rapid sequences such as EPI or fast gradient echo. The images are usually acquired with long TEs (40–70 ms) while the task is modulated on and off. The ‘off’ images are then subtracted from the ‘on’ images and a more sophisticated statistical analysis is performed. Regions that were activated above some threshold levels are overlaid onto anatomic images. Clinical applications Primarily developments of the understanding of brain function including evaluation of stroke, epilepsy, pain and behavioural problems.

47. fMRI is becoming the diagnostic method of choice for learning how a normal, diseased or injured brain is working, as well as for assessing the potential risks of surgery or other  invasive  treatments of the brain. Physicians perform fMRI to: Examine the functional anatomy of the brain. Determine which part of the brain is handling critical functions such as thought, speech, movement and sensation, which is called  brain mapping . Help assess the effects of stroke, trauma, or degenerative disease (such as  Alzheimer's ) on brain function. Monitor the growth and function of  brain tumors . Guide the planning of surgery,  radiation therapy , or other invasive treatments for the brain.

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