Perfusion-Weighted Imaging in MRI - Assignment Example

? Perfusion-weighted imaging in MRI Key Words: Endogenous Tracers, Exogenous Tracers, Gadolinium (Gd-DTPA), Perfusion, Cerebral Blood Volume, Protein, Endogenous Hypoxia Makers, Protons, Diffusion, Perfusion Weighted Imaging – PWI, 1. Discuss the use of exogenous and endogenous tracers in perfusion-weighted imaging. Perfusion imaging mainly entails overseeing flow of blood throughout venous system. Mainly, this assumes varied distinct categories of tracers essential in conducting perfusion weighted imaging (Module 4, 2013). The tracers mainly include endogenous and exogenous tracers (Module 4, 2013). Exogenous traces DSC imaging process Perfusion-weighted Imaging mainly entails magnetic receptiveness integrated with inflow effects to obtain hemodynamic data. In this process, an exogenous tracer like gadolinium in form of (Gd-DTPA) compound when introduced into the venous system prompts occurrence of transient signal. This occurs especially when Gadolinium passes through a tissue and there is existence of susceptibility difference between capillaries and immediate tissues in the human body (MRES7007, module 4). Afterwards, the process results to the forming of a strong field gradient around blood walls, which yield to express signal appearing in gradient echo results. In addition, the latter results in diffused dephasing in spin echo images. Exogenous tracers are vital in determining hemodynamic quantities in human body. For instance, blood flow, blood volume, and mean time whereby a given tracer molecule ought to pass through the tissue commonly this process referred also as mean transit time (Faroh, Mohammed & Law, 2011, p. 48). In addition, relative cerebral blood volume is extremely essential and most popular DSC perfusion metric in brain imaging (Faroh, Mohammed & Law, 2011, p. 55). The signal loss that occurs as a result of passage of gadolinium takes place in the below equation: S = S0e -TEDR2 Where TE = Echo Time controlling T2, T2 = weighting in the images, R2 = Relaxation rates difference and S0 = Original signal without contrast agent. DSC process also serves the purpose of measuring passage of non-diffusible contrast bolus when passing through human brain. This process mostly occurs due to signal’s decrease as bolus makes its way through the imaging slice. Signal time curve obtained from the passage of contrast bolus through the intended tissue of interest its conversion is via statistical analysis mainly utilizing change in the concentration of contrast agent (Faroh, Mohammed & Law, 2011). Combination of the final resultant time curve usually comes up with an index that is commensurate to cerebral blood volume of a certain pixel essential in producing a respective image (Faroh, Mohammed & Law, 2011). The arterial signal obtained in this case aids in computing main transit time and regional cerebral flow. Figure 1: Images (A-C) exhibits bolus as it navigates through the vowel, hence yielding to signal decrease. Consequently, signal time curve (A) turns into a concentration- time curve (B). The arterial input function(C) takes place in order to alter signal -time curve measures near a major art. Perfusion imaging requires a certain series consisting of high resolution and is applicable in both spin echo as well as gradient echo (Module 4, 2013). However, in spin echo, reduction in the large vessels appearance occurs that interferes with the sequence. Besides, the resulting to difference in echo is at heightened contrast incomparable to noise ratios (Module 4, 2013). The use of echo difference in this sequence also resulting to heightened signal change, which in turn supports working of shorter TE, small though insignificant gadolinium as well as other varied contrast agents. Measurement of blood flow via exogenous tracers is mainly through the below formula: F=V/MTT Where F= the blood flow, V= the volume of spread contrast agent in the tissue, MTT= time taken by the tracer to reach the venomous system. Measurement of CBV mainly entails the following equation: Measurement of CBF is via the below equation: Ct (t) =CBF *Ca * R (t) In this equation, R(t)= the residual function that calculates amount of tracer in the tissue ,Ca= the arterial concentration of the contrast agent, Ct= tissue concentration of the different agent Calculation of MTT: MTT=CBV/CBF In this equation, CBV and CBF must be known in order to effectively calculate the MTT Figure 2: Echo EPI images indicating change in signals as the tracer navigates via the venomous system in exogenous method or DSC process whose main purpose encompasses producing images in PWI. Tumor Staging Exogenous tracers are extremely essential due to their role, which they undertake during tumor staging (Boazon et al. 2012). This is because they give vital information concerning one’s volume of blood as well as its flow within the human body (Boazon et al. 2012). In addition, exogenous tracers display the occurrence of an interrupted blood brain barrier via contrast enhancement. Hypoxia microenvironment visualization majorly depends on detection of exogenous tracers localized in hypoxic tissues or the hypoxia-induced exogenous makers (Boazon et al. 2012). In this process, most used exogenous tracers are pimonidazole, EF5 and CCI-103F (Boazon et al. 2012). In addition, their state of being rampant and popular in the medical field is due to 2-nitroimidazole compounds, which are selective based on reduction role in hypoxic inhabited regions that comprise of tumors binding to intracellular molecules (Boazon et al. 2012). In this context, exogenous hypoxia makers have become popular in clinics as well as used in conducting experimental studies of tumor hypoxia. Hypoxia regulates usually undertakes the role of regulating numerous cellular proteins. Potential hypoxia’s endogenous markers embrace carbonic hydrase 9 (CA9) and glucose transporters 1(GLUT1) (Boazon et al. 2012). In addition, integration of endogenous protein makers and exogenous tracers is very essential in detecting alterations in tumor hypoxia. Hence, resulting to a beneficial combination of both endogenous markers and exogenous tracers, this is suitable especially when supervising effects caused by interventions responsible for agitating intra-tumor distribution of hypoxia. Endogenous Arterial Spin Tagging Some people may exhibit hypertensive reactions to exogenous tracers such as Gadolinium or even have severe nerve infections if their excretion from the body does not take place in a proper manner via the kidney (Module 4, 2013). They mainly fall under two categories, which are pulsed or continuous (Faroh, Mohammed & Law, 2011, p 72). Continuous arterial spin mainly entails inversion of the inflowing blood mainly supported by RF and steady difference evident in the event of protons’ flowing period. In pulsed arterial spin inflowing labeled or ASL protons usually accesses the designated region within the specified inversion period, which leads to signal lapse (Module 4, 2013). In this scenario, experts use another approach to perfusion weighted imaging which entails water as a diffusible tracer (Module 4, 2013). In this method, preparation of water protons contained in the blood takes place by inversion in large vessels. After a short period, protons enter capillaries, which is the targeted region of interest. However, these protons in their nature bear varying magnetism compared to other images obtained normally (Module 4, 2013). The prepared and normal image sets aside signals of incoming arterial blood to the targeted organ. Therefore, in this case divergence in terms of signal already detected varies directly with the quantity of blood that has flowed in the targeted region. Figure 3: Image illustrating signals inflowing through Middle cerebral Artery in human brain to produce images in Endogenous method or Arterial spin Tagging process Endogenous tracers do not entail involvement of an externally added product but solemnly rely on the ability to bring out contrast from specific excitation or diffusion mechanisms. For instance, labeling of inflowing blood protons found in the same blood as contrast agents. Additionally, tagged protons not found in the imaging volume perfuse into the tissues, resulting in a drop in the signal intensity (Module 4, 2013). Monitoring of these events is through quantitative analysis. In this process, arterial transit times may have a fundamental impact on the measured signal. However, this effect depends on the method applied and varies anatomically. Hence, explained using this relationship ? M=2Mob * F * T * e-t/T1 This is the equation for Pulsed Arterial Spin Labeling. ?M=2Mob * F * T1 This is the equation for Continuous Arterial Spin Labeling. Important assumptions in ASL: Alteration of labeled blood and the tissue spins is completely however, it does not occur in high flow cases (Module 4, 2013). The measurement of ASL depends majorly on transit times (Module 4, 2013). Relaxation period in this case remains unchanged, which is not real leading to erratic measurements (Module 4, 2013). Perfusion Weighted Imaging - PWI clinical applications PWI is essential in differentiating between some cysts that resemble tumors in appearance. Conversely, cysts do not contain any blood compared to tumors (Module 6, 2013). PWI exhibits a heightened sequence in its quest of ascertaining degree of tumor inflammation by use of angiogenesis examination. It is also vital in showing ischemic regions in the heart via use of exogenous tracers to conduct cardiac perfusion imaging (Module 6, 2013). 2. Compare and contrast the two methods. A significant similarity that characterizes these two methods is the use of venous system during perfusion process. However, each method ought to undergo varied modifying procedures based on the tracers each one utilizes to ensure attainment of the required data is effective besides having heightened accuracy. Venous system ensures during perfusion process selected tracers are capable of reaching their respective organs of interest, for instance, brain. In addition, the two methods despite being renowned in obtaining the required data with heightened efficiency, they are also prone to errors mostly caused by signal loss (Module 4, 2013). In exogenous tracers, this is evident especially when gadolinium permeates through tissues to initiate MRI signals as shown in Fig 1 (Module 4, 2013). Figure 4: Brain's EPI images indicating signal alterations as tracer permeates through. This is because when the bolus of tracer is on its way towards the tissue, it yields to a delay that in turn ends up causing signal loss (Module 4, 2013). Mainly, this is especially when tracer passes through the tissue but reverts to its normal state after it passes out. Signal loss in this case varies directly with gadolinium concentration found in the capillaries and can be described using the equation, S = S0e -TE ?R2 Whereby TE = Echo Time controlling, T2 = weighting in the images, S0 = Original signal without contrast agent, ?R2 = Relaxation rates difference, which varies directly with contrast agent concentration, hence represented by the equation, ?R2 =K*CT (2) Similar phenomenon is also evident when ascertaining brain images using ASL, which represents endogenous tracers. In this case, signal loss depends on the amount of T2 mostly influenced by the duration of the delay (Module 4, 2013). The cause of this delay results from prolonged transit between feeding artery and the targeted tissue especially among patients experiencing cerebrovascular diseases or those who have advanced in age. Exogenous tracers usually utilize magnetic susceptibility and inflow effects in their quest to attain haemodynamic data. This is by introducing gadolinium (Gd-DTPA) into the venous system, which in turn induces paramagnetic susceptibility (HongBin, Kai, Yan, Zhu & Fu, 2012). This phenomenon takes place between capillaries containing Gd-DTPA and the neighboring or immediate tissues (Module 4, 2013). Hence, leading to extremely high gradient fields, which produces direct signal dephasing in gradient echo pictures as well as diffusion-induced dephasing in echo images as, indicated in Fig 2. Figure 5: Perfusion-weighted MRI technique essential when utilizing intravascular tracers. Conversely, ASL, which is a type of endogenous technique, utilizes water as tracer in the blood to obtain the required data. This is because water is freely diffusible tracer compared to numerous solvents though it ought to undergo inversion process during its preparation in order to produce protons (Module 4, 2013). After production, protons spin within a given tissue section based on carotid arteries’ level before proceeding into the human brain. However, these protons ought to undergo some delay where they eventually end up in capillaries comprising the destined region where they usually permeate into the liquid space (Module 4, 2013). Hence, end up producing the required images due to their respective varying magnetization images though the amount relies on the amount of blood introduced during the delayed period (Module 4, 2013). ASL indication is evident in Fig shown below, Figure 6: Arterial Spin Labeling Other differences characterizing these two techniques encompass their respective assumptions. Endogenous diffusible tracer (water) assumptions include, ASL measurements rely on transit times Relaxation times for tissues usually remain constant throughout, whereby it is prone to errors. This is because T1 arterial blood does neither rely on vessels’ volume nor the amount of oxygenation (Module 4, 2013) Complete exchange of both blood and tissue spins though in heightened flow rates this premise does not hold or untrue. (Module 4, 2013) In addition, Gillard, Waldman and Barker (2009) in their study cite ASL suffers from potential artifacts emanating from overestimation of brain perfusion. These are detrimental to the effective production of required images because their effects affect magnetization process evident in the imaging plane or on the intended area of study (Gillard, Waldman & Barker, 2009, p. 711). To shun this scenario, scholars have advocated several strategies, which are to compensate or do away with MT problem in both pulsed (Gillard, Waldman & Barker, 2009). Exogenous non-diffusible tracer techniques hold onto the assumption that the given tracer does not relocate or move but remains in the intravascular section. Hence, this leads to the establishment of a unique relationship between intravascular susceptibility agent concentration and MR relaxation rate (Gillard, Waldman & Barker, 2009). According to Gupta (2013), this phenomenon prompts medical practitioners currently prefer using Dynamic Susceptibility Contrast (DSC) due to its added benefits. In addition, its unique characteristics of self-propagation is simple to operate especially in clinical setting and studies in the current world whereby everything including data keeping has embraced technology (Gupta, 2013). Exogenous tracer methods assumptions include, Perfusion in this technique always remains constant and tracer does not affect it in any way (Module 4, 2013). Input acts as the only source of blood in this case. It is also assumed when this operation is in progress, tracer mixes with blood evenly to give the right images results. After the commencement of this method’s process, there is no pooling. Therefore, output acts as the sole sinks throughout imaging procedure. Significant assumption made when utilizing endogenous tracer or ASL is a premise that holds complete swap between labeled blood and tissue (Faroh, Mohammed & Law, 2011). However, this is not applicable in all cases especially those involving animals whose their respective blood flow is extremely high. In addition, local relaxation properties are constant always, which is far from reality thus leading to serious errors. For instance, T1 arterial blood characteristics in this case ought to be independent of vessel’s size and oxygenation rate, hence implying the following T2 does not experience any alterations especially in the respective tissue of interest (Module 4, 2013). However, this is not true in reality because T2 ought to indicate effects or impacts initiated by T1 during perfusion process. The labeled water protons in ASL or endogenous method have a longer decay of signal (T1) of approximately 1 second than exogenous (Module 4, 2013). However, this is essential in allowing adequate and maximum time intended to detect tissue’s perfusion as well as microvasculature whereby its representation encompasses (1s at 1.5T) (Module 4, 2013). To increase scan time while using ASL method, it is essential to use high SNR, which will in turn increase NEX. This is because small difference evident between signal intensity and ordinary images of approximately 2%, hence causing signal deficiency (Module 4, 2013). In addition, the presence of small difference in intensity may yield to large artefacts whose solution includes using either FSE or EPI with the intention of avoiding shunning motion effects (Module 4, 2013). However, the use of EPI despite cited to be of great essence in shunning motion artefacts, sensitivity to susceptibility characterizing certain instances like hemorrhages, paranasal sinus as well as categories might yield to improper interpretations. Hence, rendering the process declining in its efficiency despite cited to be advantageous. Application of exogenous tracers despite studies and numerous experts in the medical field citing to be effective and of more value compared to other alternatives, it may be detrimental to people with a history of allergy and asthma. Hence, cause them to experience certain side effects like queasiness, headache, and lightheadedness besides tingling. Contrary to exogenous tracer, ASL has varied predicaments that reduce its efficiency while in use mostly caused by decay charge evident especially when the velocity is extremely slow or in the events of prolonged transit durations. The latter despite cited during ASL process to be essential in attaining the required imaging data, its prolonged delay results to signal loss especially when imaging a cerebrovascular disease victim, hence implying without proper modifications there is absence of the required images. Medical practitioners in most cases prefer utilizing exogenous tracers in ascertaining tumor stage compared to endogenous tracers. The reason behind this cite CBV is capable of providing adequate and exact information besides contrast agents exhibiting any slight interruption that might occur mostly caused by brain acting as a barrier (Module 4, 2013). 3. How is the final perfusion information obtained and what value does it have in a clinical setting? Part 1 In obtaining the final perfusion data, it is essential to contact analysis based on tracer kinetics that developed and started in 19Th Century. The method has found essential applications in varied and numerous disciplines like PET, nuclear medicine besides MRI in the medical field. Premise used in this analysis encompasses that of conservation of mass, whereby inflow blood ought to be equal to the outflow blood. Therefore, vascular system in this case acts as a linear system with no wastage of material inside the provided volume space. However, this is an ideal system and in order to attain almost exact results there ought to be compensation for residue blood that enters the system at t=0 with a function of R(t). Hence, the residue formula; RTotal (t) = ?F(i)R(t-i) then after integrating mathematically ends being Cr(t) = F[Cin(t’)R(t-t’)]dt’ between 0 and time t Leading to Volume being V = F x MTT In attaining the required data, fMRI entails repetitive bolus injections of contrast coupled with increment of blood flow in the neural cavity. However, in the cognitive respective regions signal despite the cited increment of blood flow continues to exhibit weak characteristic. Hence, there is need to maintain heightened activation of cerebral throughout the entire process, which in turn yield to high blood flow, volume and spatial resolution in the vessels. Consequently, this aids measurement of fMRI emanating from the floor of capillaries by sending signals to the respective interpretation machine. Part 2 MR perfusion method Clinical uses It is effective in evaluating metabolism while an individual is doing exercise or at rest and assessing the extent of ischemic disease. The technique is also effective in ascertaining the extent of malignant neoplasm by augmenting the involved tissue’s metabolism. In the event of delayed enhancement of MR perfusion imaging, it acts as an essential tool in showing ischemic region within the cardiac though solely utilizing exogenous tracers. In the medical filed, practitioners mostly prefer this technique in ascertaining tumor staging. It plays an essential role in distinguishing neurosis and tumor reappearance. This is because compared to neurosis tumor exhibits a heightened extent of perfusion. References Boazhong, 2012. A model system for validating of PET Radiopharmaceuticals: Focusing on Tumor Microenvironment. [pdf]Available at: [ Accessed 27th September 2013]. Faro, S. H., Mohamed, F. B. & Law, M. 2011. Functional Neuroradiology: Principles and Clinical Applications. New York: Springer. Gillard, J. H., Waldman, A. D., & Barker, P. B. (2009). Clinical MR neuroimaging: physiological and functional techniques. Cambridge, Cambridge University Press. Gupta, A. K. (2013). Diagnostic radiology: recent advances and applied physics in imaging. [S.l.], Jaypee Brothers Medical P. HongBin, H., Kai, L. Yan, J., Zhu, K. & Fu. 2012. An in vivo study with an MRI tracer method reveals the biophysical properties of interstitial fluids in the rat brain. China Science - Life Science, 55(9), p.782-787 DOI: 10.1007/s11427-012-4361-4 Magnetic Resonance Imaging Course ,MRES7007, module 4 and 6. Marco, E. et al. 2013. Perfusion MRI: The Five Most Frequently Asked Technical Questions. American Journal of Roentgenology, 200(1), p.24–34. DOI:10.2214/AJR.12.9543 Read More