Magnetic Resonance Imaging - Assignment Example

? Magnetic Resonance Imaging Perfusion Introduction Perfusion refers to the passage of fluid through the blood vessels or even lymphatic system to a tissue. Thus, perfusion scanning is the process of observing the perfusion activities, recording and quantifying the results. A wide range of medical imaging modalities has emerged that utilize perfusion application in a variety of medical fields. Perfusion applications are used to assess the distribution of blood to a vascular bed. Either endogenous or exogenous tracers can be utilized to regulate hemodynamic quantities, for instance blood movement, blood capacity, and the average time it takes for the tracer molecule to be passed through the tissue, or the average time of transit (Luypaert et al. 2001). MRI is used to measure tissue perfusion through the use of different techniques such as arterial sin labelling (ASL) and dynamic susceptibility contrasting imaging (DSC-MRI). DSC is based on injected contrast agent use that changes the blood’s magnetic susceptibility, thus, producing a MR signal continually measured throughout the bolus passage (Petrella & Provenzale 2000). ASL, on the other hand, is whereby before the arterial blood enters into the tissue to be assessed, it is magnetically tagged, and consequently, the labelling amount is measured and compared to a blank recording achieved without spin labelling. Currently, MRI is a powerful tool in a clinical setting for evaluation of brain anatomy, which is achieved via a number of metabolic or functional assessments. MRI perfusion is a technique used to measure cerebral perfusion non-invasively via several hemodynamic measurements assessments including cerebral blood flow, cerebral blood, volume, and meant transit time. This technique plays an important role in the diagnosis and treatment cerebrovascular disease patients, as well as patients with other brain disorders (Petrella & Provenzale 2000). As a result, its application is in assessment of tumours, evaluation of tissue that is at risk following acute stroke evaluation of neurodegenerative conditions, for example, Alzheimer's disease and even the assessment of various drugs administered for these conditions. The purpose of this report is to explore aspects of MRI including methods, perfusion information and application in a clinical setting. Exogenous Tracers Exogenous is a model of MR perfusion, which assumes that the tracer does not diffuse into the outer cellular space because it is constrained in the intravascular compartment. In this model imaging can either be performed dynamically or in a steady state. Dynamic imaging utilizes transient fluctuations in local magnetic field of the tissues in the surrounding that are induced by paramagnetic tracer bolus passing through the capillary network of the organ. The local magnetic field changes can be measured as signal fluctuations on MR imaging. Accurate measurements are enabled by Ultrafast imaging methods such as spiral MR and echoplanar imaging, which measures differing signal changes that occur rapidly (Ostergaard, et al. 1996). Data from the signal-time course is then changed to relative tracer tissue data from the concentration-time course. This results in tracer concentration-time curve that can be evaluated to establish different parameters of hemodynamic tissues such as blood flow, transit time, tissue blood volume and bolus arrival time. The hemodynamic parameters mentioned above are influenced by features of the bolus injection such as the injection rate, contrast agent paramagnetic properties, the amount of injected contrast material among others. Furthermore, these parameters rely on variables inside the subject under imaging, which are cardiac output and vascular volume of total-body (Buxton, et al. 1996). Therefore, it is not possible to compare the parameters between varied subjects, and at different times they may even cause variation on examination of the same subject. Nevertheless, there is an internal standard of reference, that is, normally appearing-white or grey matter, which can be used to obtain relative/semiquantitative values that create room for inter- and intra-subject comparisons. Semiquantitative value is used to categorize the processes of a disease after disease course, as well as for monitoring therapeutic interventions effects, given that it is only local alterations that occur and there is no fluctuation on the internal reference’s hemodynamic parameters. For instance, during stroke, relative hemodynamic measurements can be valuable in periinfarct ischemic tissue monitoring, which is at risk for infarction, and also for monitoring thrombolytic therapy effects (Pollock, et al. 2009). However, absolute quantitation is necessary for diffuse disease processes whereby the internal reference may be impacted on. The accuracy of the dynamic methods that measure the brain’s arterial input remains unproven, despite the fact that they have been tried for absolute quantitation of cerebral blood flow and cerebral blood volume. Nevertheless, absolute quantitation of these parameters may be possible with MR methods with improvements in techniques and signal-noise-capability when determining true arterial input. It is quite straightforward to determine the relative cerebral blood volume, and robust as well, from tracer concentration-time data, and this is simply achieved by integration of area under the curve of tracer concentration-time. The integration may be done on an analytic fit or the curve data points whereby the former has an advantage of faster imaging over time and high signal stability acquisition. In contrast, to determine relative cerebral blood flow calls for a more extensive data imaging processing, and is vulnerable to instability and image quality in the MR signal overtime. As a result, it requires deconvolution of function of an arterial input from tissue concentration-time data in order to determine the actual brain clearance, or even mean transit time via the cerebral capillary bed (Catherine, et al. 2005). Calculation of the cerebral blood flow is done by integration of the area under the curve of deconvolved tissue concentration-time, and then dividing this by the obtained mean transit time. Steady-state imaging is another method of imaging in MR used to approximate the complete cerebral blood volume across the brain that has high spatial resolution. This method proposes that the blood tracer is not diffusible from the intravascular space to an extravascular one. A baseline image in this technique is obtained prior to injection of the paramagnetic agent, which is then followed by an image acquired during the ‘steady state’ known as postinfusion image. A steady state is achieved after the contrast material has circulated throughout the body for a period of 30 minutes and reached a peak of relative concentration equilibrium. Map of absolute celebral blood volume can be obtained in percentage volume units by subtracting the image of the baseline from the postcontrast steady-state image, as well as making the pixel values normal to those of pixel containing blood only such as the saggital sinus (Anton 2000). This approach of imaging; however, it has several disadvantages, for instance, the resulting images record low-signal-noise ratio because of image subtraction performance. The second advantage is that the accuracy of the measurements might be affected by movement of patient between the pre- and postcontrast scans (Williams, et al. 1994). Finally, the third advantage is that it can give suspicious results in areas where there is disruption of blood-brain barrier and in addition, the cases whereby there is a violation of tracer non-diffusibility assumption. As a result, the technique proves to be invaluable in many cases of tumour and infarction. The figures below illustrates how circulation in the hemodynamic parameters occurs in a dynamic contrast-enhanced T2-weighted technique Source: Petrella, J. R. & Provenzale, J. M., 2000. MR Perfusion Imaging of the Brain: Techniques and Applications. American Journal of Roentgenology, 175(1), pp. 207-219. Measurements of CBV Cerebral blood volume abbreviated as CBV is one of the important hemodynamic parameters, which is the quantity of blood in voxel of the brain divided by the voxel mass. Measurement is achieved by establishing the sum between the cerebral red cell volume and cerebral plasma volume, but the measurements used in calculation involve positron emission tomography (Barbier, et al. 2001). The units commonly used in these measurements are millilitres per 100 gram. It is given by: CBV=100 (Volume of blood in a voxel / Volume of the voxel) Measurements of CBF CBF, cerebral blood flow is the net flow of blood through the voxel, and divided by the voxel mass. Measurement of this requires deployment of techniques sensitive to movement such as bolus tracking technique already in a state considerable steady. The units commonly used in these measurements are millilitres per 100 gram. It is expressed as: CBF = (Net blood flow through the voxel / Mass of the voxel). Measurements of MTT MTT is the average amount of time taken by any water molecule from the contrast agent to pass inside the voxel vasculature. MTT techniques are sensitive to movement such as bolus tracking methods, which are already in a state considered steady state. Its value is normally expressed in seconds, and the formula is illustrated by: MTT = (CBV / CBF) While CBT and MTT measurements rely on motion sensitive techniques, CBV does not and may be measured by other techniques. Endogenous tracers: This is a technique whereby protons of water contained in the blood act as a type of an endogenous tracer, protons which are labelled by applying a pulse of 180FR. This is called inversion pulse at large arteries level, a process that occurs before the protons flow into materials of interest. The protons are allowed time to travel to the capillaries and reach the materials/objects of interest. These protons are magnetically diverse because they are labelled unlike the normal images, which are not, and as a result, they can be separated in a magnetic field, which produces a signal of perfusion images or inflowing arterial blood. This approach is known as arterial spin labelling (Calamante et al. 2000). This method is recognized as arterial spin labelling. ASL’s major attribute is the use of water in the blood to act as an endogenous tracer. In order to achieve this, water protons in the blood are prepared by using an inversion pulse, at the level of large feeding vessels (Common carotid or Internal Carotids). The primary aspect of arterial spin labelling is that it utilizes water in the blood as an endogenous tracer. . In order to achieve this, water protons in the blood are prepared by using an inversion pulse, at the level of large feeding vessels (Common carotid or Internal Carotids) (Luypaert et al 2001). After a delay that depends upon the cardiac output, the age and the clinical history of the patient, these inversed protons reach the capillaries in the slice of interest and diffuse in the tissue water space. The prepared protons have a different magnetization to the surrounding spins in the interest region. By comparing the prepared and normal spin signals of inflowing arterial blood in the slices of interest, it is possible to characterize the signal that arose without the prepared spins and the image with the prepared spins (Calamante, et al. 2000). Hence, the generation of blood flow results can possibly be achieved by concentration of the labelled blood deduction from the normal and inverted spins. One of the schemes of accomplishing arterial spinning is Continuous Arterial Spin Labelling (CASL). This scheme utilizes a semi-continuous inverse pulse and applies pressure in detecting brain perfusion diversity. In addition, when it comes to artefacts it is less responsive, for example, in perfusion transfer times. However, it is advisable NOT to use the technique in patients because a significant amount of RF power gets deposited to the samples, which in turn overpasses the SAR limit (McRobbie et al 2006). The second scheme is the Pulsed Arterial Spin Labelling (PASL), which involves tagging a large volume of blood with a single pulse of RF at the level of the feeding arteries. The volume decays over the inversion time since it obtains an inversion time during which the tagged pins arrive at the brain tissue to be measured. This is so as to achieve signal diversity between perfused tissue and stationary pins. Unlike the first scheme, this one is safe for patients and can be used with only a single RF coil (Pollock et al 2009). The most preferred type of perfusion in a clinical setting is one whose perfusion results are derived from an exogenous tracer. However, these depend on several presumptions: The first presumption is that perfusion of the tissue essential to number is NOT controlled by the tracer. This is an imaginary presumption is an imaginary one because the tracer itself consists of a composition of chemicals and dissimilar biological dissemination linked to the patient’s usual body fluid (Buxton, et al. 1998). Hence, there is a possibility that it might influence perfusion of the tissue under assessment. Secondly, another presumption states that other than input, there are no other tracers. This is relatively close to an actual clinical dimension, but only if the tracer source is introduced by a unique injection. Thirdly, the supposition claims that the tracer is absolutely assorted with blood such that the perfusion of the tracer is the same as the perfusion of blood. However, it is unlikely to achieve a perfect intercourse between both materials because the tracer is similar to a different chemical compound associated to blood. Finally, the fourth presumption is a critical one, which might be affected by the clinical test (Williams, et al. 1992). It claims that except for the output, there should be NO other outflow of tracer; however, this cannot apply to patients with haemorrhage since they plunge blood in additional parts comparative to the output (Luypaert et al 2001). Differences between ASL and dynamic susceptibility contrast ASL is extremely dependent on the transit time because it decays with TI, which is considerable enough to be measured in relatively low blood flow areas. Furthermore, in ASL, the signal diversity for untagged and tagged blood is only a few percent different; therefore, the images have to record highest SNR possible with possible minimal artifacts in order to obtain reliable results (McRobbie et al 2006). In addition, ASL reduces the signal from the targeted area of the brain due to magnetization transfer by applying an inversion pulse on the vessels (Calamante et al 2000). Off-Eresonance impacts also influence ASL, which is achieved by the tagging RF pulse broad frequency spectra in the large vessels. Off-Eresonance excitation experienced the protons indulgence next to the tagging pulse, creates a serious loss in the signals believed to be in the brain, an impact referred to as magnetization transfer. In order to compensate this ASL artefact, one should use a similar RF pulse in opposed position with reference to the scanned volume (section of the brain cortex, scalp and hair) (Luypaert, et al. 2001). In addition, ASL is also susceptible to effects of emotion, which can cause large error in perfusion imaging interpretation. Hence, it is mandatory to acquire fast imaging techniques such as EPI. While ASL perfusion can only obtain rCBV, GD can obtain rCBV, MTT and rCBF. Furthermore, blood is the endogenous tracer in ASL while in GD gadolinium is used as exogenous tracer. While Dynamic susceptibility perfusion cannot be used for patients with abnormal kidney function, ASL is recommended for patients with conciliation kidney function (Moseley, et al. 1995). Finally, in ASL it is possible to assess the whole area of the brain because it is greatly reduced as more than the whole brain study offered by dynamic susceptible perfusion. The differences between dynamic susceptibility and ASL contrast are: 1- Decay of signal: The labelled water protons in arterial spin tagging methods have a decay rate of T1 (1 s at 1.5 T). This is usually sufficiently long to detect perfusion of the tissue and microvasculature. ASL is analogous to 15O positron emission tomography (PET). 2- Lack of signal: The signal strengths of the prepared and normal images generally vary by a given percent, for instance 2% change in equilibrium magnetisation. Therefore, the signal to noise ratios of the images is critical and is expected to be relatively high. 3- Motion effects: Motion between scans can cause huge errors because of the relatively small nature of signal change. Rapid methods, for instance, fast spin echo or EPI are usually employed to avoid this. This is because control and label images can be acquired and interleaved in seconds with these methods. 4- Off resonance effects: The reduction of transfer rate of magnetization in ASL is achieved by placing a similar slab after the piece to be assessed. This is based on the concept that e some spins outside the piece would be excited and contribute to the final signal. Therefore, it is advisable to be familiar with both spins that do not flow and the ones that actually flow. Significant Assumption in ASL analysis ASL follows the same theory of imaging similar to dynamic perfusion, which is the theory of gadolinium perfusion imaging. However, performing a study of ASL requires consideration of several imperative assumptions (Nelson 1995). The first assumption is that there has to be a complete exchange between the spins and tagged blood (blood whose magnetic movement has been inverted) of the target tissue (Pollock, et al. 2009). Second, fast blood flow in ASL might lead to underestimate perfusion measurement since it vastly relies on transit time. The third assumption is that local relaxation time might stay stable, as proved by varying properties of T1 and T2* properties from one tissue to another. Role in Clinical Application Perfusion weighted imaging has an important role in many clinical applications: One of the roles of perfusion in clinical applications is the assessment of breast pathology by MR. This employs the use of contrast enhancement to determine the washEin washEout kinetics, which in turn ascertain whether the region match up a benign of malignant mass. Perfusion also assists in characterization of the tumour lesion staging whereby the areas of greater perfusion match up the areas of improved malignancy. In addition, perfusion is used to give an explanation for varied areas within the tumour lesion, which is critical in knowing the real margins of malignancy. Perfusion is also valuable in expounding the difference between the necrosis and tumour recurrence, in which there is no increase of the rCBV shown by radioEnecrosis. Nevertheless, a high rCRV will also be recorded by a progression of the pathology in the tumour area. Furthermore, perfusion is also used for assessing cardiac viability. 1- Perfusion can be used to determine whether the structure correspond to a cyst or a tumour through imaging to differentiate various cysts. In the cyst, the water value and the ADC value with be similar since protons freely diffuse through it, but, on the other hand, DWI value will increase while ADC decrease due to lack of spin diffusion because of the restricted protons. 2- Perfusion carried out through CBV maps can demonstrate the oedema and tumour boundary. 3- The role of scaring PDI and Ischaemia is to evaluate the function of the kidney. This is possible because renal stenosis can be assessed by a normal angiography, though; the extent of function is well portrayed by the use of perfusion. However, this technique cannot be used for patients with impaired renal functions because of the technicalities involved, which can be treacherous. In addition, the theory that in exogenous perfusion that tracer that flows in must flow out cannot apply because in this case some of the tracer is filtered in glumerili, which renders the assumption invalid with biased measurements (Pollock, et al. 2009). Similarly, ASL also presents difficulties with regard to patients with kidney impairment because of low blood flow, which affects the accuracy of measurements using ASL. 4- PWI plays an important role in revealing ischemic region in the heart, particularly exogenous PWI, which evaluates the guarantee of myocardial perfusion. The ECG is used to synchronize the exam with the cardiac phases, and the gadolinium injection marks the dynamic exam performance. Consequently, before, during and after contrast pass the cardiac chambers, a set of images are taken for each instance. If the myocardial region is normal, it will record a rise in signal immediately after contrast injection. This implies that the cardiac muscle is feasible for perfusion. The heart is then scanned after several minutes to confirm that all the contrast has been washed-out to avoid compromise of the fibrous region. An infarct normally display different contrast pattern, for example, first, after the injection of gadolinium it will be hypo-perfused, and then, 5 minutes later, it will display a signal increase in the infrared area (Nelson 1995). This shows that the delayed enhancement area is affected by an infarct, thus, it is not viable. 5- The endogenous tracers are less commonly used in to determine the stage of a tumour as compared to exogenous tracers. Evidently, perfusion imaging is capable of differentiating high and low grade tumours with respect to their blood supply. The micro-vascular blood volume measurement is proportional to the tumour grade, for instance, the CBV can be utilized to measure the low-grade tumours that seem homogenous and intense. On the contrary, as compared to the normal low grade tumours, higher grade tumours consist of a higher perfusion signal. In addition, there are also contrast intake patterns, which might be of imperative help in performing accurate biopsy by highest tumour activity. 6- fMRI uses endogenous tracers instead of exogenous tracers, in which increase in local blood flow can be identified in increased activity area that relates to the resting state. The connection between neural activity and local blood flow is essential in sensory, visual and motor cortex. However, the signal mentioned earlier is stronger than the signal from the cognitive area. Though during the activation process increase in blood volume is fast, it is not as accurate as spatial resolution blood flow (Pollock, et al. 2009). Distinguishing between the signal caused by dilation of nearby veins and blood signal volume at neural activity site is extremely complex. As a result, the relative cerebral blood FLOW in fMRI serves to accomplish a better spatial resolution. Therefore, fMRI has an advantage over conventional BOLD imaging, which is that in BOLD image spatial localization of the signal function arises from venous supply while in fMRI, it originates from CAPILLARY BED. As a result, fMRI precisely characterize the function area being assessed more than BOLD imaging. The major disadvantage is that the ASL temporal resolution is lower that that of BOLD imaging. In addition, it is greatly demanding technically; therefore, its use within fMRI researches is not advisable. 7- Another role of PWI is to assess brain parenchyma, a process that is achieved by interpretation of CBV changes as neural loss and synapse, which is the cognitive dysfunction theory. This will lead to lessening of rCBF and rCBV in various cortical areas for Alzheimer and other neural diseases. References Anton, H., 2000. Elementary Linear Algebra. 8th ed. New York: John Wiley. Barbier, E., Lamalle, L. & Decorps, M., 2001. Methodology of brain perfusion imaging. 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