Relevance of Combining MRI and PET Modalities - Essay Example

?Relevance of Combining MRI and PET Modalities Introduction: The research paper at hand focuses on two common modalities used in clinical practice, namely, Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET). The main objective of the paper is to bring to the fore the advantages as well as disadvantages of combining the two modalities in a single machine. The paper first gives the definition of the two clinical tools and then moves on to give a brief description about their individual uses. Later on the paper will discuss about the scenario when two modalities will be fused into a single machine, providing in detail the benefits as well as problems faced when doing the same. However on analysis of the various advantages as well as disadvantages, it is seen that the former overweighs the latter, thus it becomes clear that combining the two modalities into a single system is highly relevant in the modern clinical sphere. Definitions: Magnetic Resonance Imaging is among the most popularly deployed clinical diagnostic tools, which works by making use of a “strong magnetic field” to produce “high- resolution anatomical information with excellent soft-tissue contrast” (Metcalfe et al 2013: 2). Positron Emission Tomography is the method that determines “biochemical and physiological processes in vivo” in a quantitative way with the help of “radiopharmaceuticals labeled with positron emitting radionuclides as 11C, 13N, 15O and 18F and by measuring the annihilation radiation using a coincidence technique” (Paans n.d. 1). Whereas MRI provides imaging of the structure of tissues, PET provides insight into the functioning of these tissues in terms of biochemical and physiological processes. Both the approaches are non-invasive and do not involve the injection of any foreign substance into the human body. MRI furthermore has the advantage that it does not expose the patients to any ionizing radiation when the testing takes place. However, under both, there are a number of approaches that can be used for testing different parts of the body. In MRI, the different approaches include Dynamic Nuclear Polarization MRI, Diffusion Weighted MRI, Dynamic Contrast Enhanced MRI, Magnetic Resonance Spectroscopic Imaging etc whereas the different types of PET scans are classified into “static scan, dynamic scan” and “whole body scan” (Paans n.d. 18). These highly advanced clinical tools help in the assessment of the structure as well as functioning of soft tissues, thus differentiating which is healthy and which is not, thereby helping to determine the pathology of the same. Uses of Magnetic Resonance Imaging: MRI primarily helps in detecting abnormal and malicious cells that are a symptom or a result of some disease. Through this imaging process, clinicians are able to diagnose what disease it is that the patient is suffering from, and furthermore, they are able to suggest precise treatment for the same. MRI plays a major role in cancer management, intracranial radiotherapy, tumor delineation etc. Furthermore, it helps in developing accurate treatment plans for patients suffering from diseases ranging from those affecting the brain, head, neck, lungs etc. In terms of the brain, it provides much better visualization of any tumor that is present and also helps in treating arteriovenous malformations through magnetic resonance angiography. On the other hand, with relation to head and neck, MRI helps in determining “lymphogenic metastesis in head and neck cancer patients” (Metcalfe et al 2013: 7). Moreover, MRI is proven to be more useful than mammography for breast cancer patients, as it provides a much more sensitive imaging than the latter. Another aspect of MRI, functional MRI, helps to identify the functional organs at risk (fOAR). Uses of Positron Emission Tomography: PET is basically a nuclear imaging method, which helps in viewing the functional processes of the body through the radio-activity taking place in it. The first and foremost as well as the most common use of PET scan is to determine the possibility of cancer metastasis. Therefore, it becomes evident that it is highly used in clinical oncology both in terms of providing images of tumors in the body as well as for reaching diagnosis regarding cancer. Furthermore, it helps to detect certain brain diseases the patient is suffering from, such as those which eventually lead to dementias. PET plays an important role in differentiating Alzheimer’s disease from other types of dementias. Thus, it becomes clear that PET plays an important role in neuroimaging and in addition to this, PET is also significant with respect to cardiology as well as study of other vascular diseases. Furthermore, PET is proven to be a better method in studying skeletal muscles than electromyography, as it helps in exploring muscles that lie a little deep under the surface of the skin. The various uses of MRI and PET have been discussed, it seems logical to integrate both imaging modalities in a single machine so as to provide these benefits simultaneously, which would further reduce the cost and time of performing the modalities separately. However, there are certain disadvantages that come with this and thus, clinicians have to make a choice in the practicality of the same. Combined MRI and PET: With the advent of technology new methods developed over the “last two decades” have made it possible to integrate different modalities into the same machine, and this has far reaching consequences that in a way prove quite useful in the clinical field (Yankeelov et al 2012: 1343). In the present world, the main “cross-sectional medical imaging technologies” used in “clinical oncology” include integrated systems, which involve MRI with PET, CT with PET, Single Photon Emission Computed Tomography (SPECT) Ultrasound (US) etc (Yankeelov et al 2012: 1342). When considering the scenario of fusing MRI and PET into the same machine, it is clear that the new system would prove beneficial in many areas, but at the same time, there will arise new problems, which were not present prior to this combination. The following section explores in detail the advantages and disadvantages of integrating the two aforementioned modalities in a single machine. Advantages of Integrating MRI and PET: Ever since the success of hybrid SPECT-CT as well as PET-CT scanners, extensive research has been undertaken and a lot of effort has been concentrated to “develop hybrid PET-MRI devices” (Yankeelov et al 2012: 1343). The integration of MRI and PET into a single machine has made possible an advanced “metabolic–anatomic imaging technique” (Buchbender et al 2012: 1). In fact it has been evidenced that “anatomical information” obtained through MRI “is superior” than that obtained with the help of CT, because the proton density as well as “magnetic relaxation properties of tissue” (MRI sensitive) provide greater contrast when compared to electron density (CT sensitive) (Yankeelov et al 2012: 1343). MRI is known to provide quantitative information on tumor biology with relation to “blood flow, vascular and tissue spaces, pH and hypoxia, cellularity and metabolite concentrations” and this can be combined with PET to provide information on “glucose metabolism, cell proliferation, hypoxia, cell receptor expression” etc (Yankeelov et al 2012: 1343). Thus, it becomes clear that integrated MRI and PET system provide “complementary information” and give the “opportunity to provide a more complete picture of a patient's cancer” than either method separately (Yankeelov et al 2012: 1343). On the other hand, there is a way to obtain a joint imaging from both methods after carrying them out separately and that is by using retrospective imaging registration, however, this is operative intensive and proves to be very complicated. Therefore, “simultaneous acquisition” obtained from PET-MRI provides deep insight into “temporal correlation of changes in cell density and cell proliferation, the distribution of a radiolabeled therapeutic” in terms of underlying “tumor blood flow, microvascular permeability and proliferation” (Yankeelov et al 2012: 1343). Furthermore, the time lapse between the two measurements may be enough for the tissues to undergo significant changes, thus restricting the potential of study. The most important advantage of this integrated approach is that it is “highly accurate in T-staging of tumor entities” and it also increases the diagnostic accuracy for the “differentiation of scar tissue from recurrence of tumors such as rectal cancer” (Buchbender et al 2012: 1). Therefore, it is evidenced that PET-MRI provides “spatial and temporal co-registration of two modalities offering a wealth of complementary anatomical, physiological and molecular information” (Yankeelov et al 2012: 1343). Another advantage characterized by the fusion of the two modalities is that for the study of small animals “simultaneous scanning reduces time under anesthesia and enables scanning under identical physiological conditions” (Zaidi and Mawlawi 2007: 1526). Moreover, it helps to “measure spatially matched regional biochemical content” as well as to study “metabolic status or the presence of neoplasia and other diseases in specific tissue regions” (Zaidi and Mawlawi 2007: 1526). A major advantage that can be seen here is that since this method does not use ionizing radiation, it is best suited for pediatric oncology as well. PET-MRI system further helps in the field of oncology through “disease staging and clinical situations” by characterizing a particular region or lesion (Yankeelov et al 2012: 1343). The fact that this method can provide a whole body assessment in addition to the information about the exact disease site is what makes the integration of both modalities into one machine even more relevant. This multimodal system “allows the simultaneous monitoring of cancer localization and provides detailed information of nanoparticle distribution in vivo,” thus making it advantageous over single imaging modality (Hwang, Youn and Lee 2010: 186). Additionally, it has been evidenced that this integrated approach is “the most accurate method for evaluation of osseous and bone marrow processes” (Lentschig and Franzius 2012: 17). Disadvantages of Integrating MRI and PET: The primary disadvantage of combining the two modalities into one machine involves is that the “traditional photomultiplier tubes”, which are used in PET scanners are “highly sensitive to magnetic fields” and not exactly “functional in an MRI environment” (Mundy 2012: 3). Thus, this led to development of machines that would counter this problem, and despite the development, the problem has not been eradicated completely. Furthermore, this hybrid machine requires a “larger physical environment” than that of machines that provide single modalities (Mundy 2012: 3). Moreover, this advanced system calls for added facilities such as a “segregated injection room, uptake room and an associated hot lab” (Mundy 2012: 3). The estimated time for the completion of this testing is one hour, which is more than the time taken up by a PET-CT scan, thus, there is a disadvantage of increased time. Furthermore, in addition to providing extra facilities for this multimodality testing, there are other cost factors involved. For the operation of the machines, “both nuclear medicine technologists and radiographers” are essential in order to read as well as interpret the images, thus, this results in additional costs (Mundy 2012: 11). Although an integrated system featuring both MRI and PET modalities is very useful for attenuation correction in terms of the brain, this is not the case in terms of other tissues of the body since they are not as homogenous as the tissues of the brain. Thus, this proves to be a challenge, as automated segmentation becomes quite difficult in such a scenario. Therefore, the disadvantages of combining the two modalities can be summed up by concluding that it involves a time-consuming examination process, is highly complex and is not cost effective as well. However, the most basic challenge faced here is that PET-MRI has not exactly proven to outperform other combined imaging modality. There is not enough research material to support the practicality of the same, and this area still remains to be explored fully. Thus, it can be said that PET-MRI will be used as an adjunct to other modalities, but will not replace them. Despite this, truth remains that MRI being a non-invasive superior imaging modality, when combined with PET, will obviously provide benefits that outweigh the disadvantages of this multi-modality imaging system. Conclusion: The paper sheds light on the scenario of integrating MRI and PET into the same system and it explains in detail the advantages as well as disadvantages that arise as a result of the same. Although this multimodality imaging method has significant disadvantages, they can be overcome by future research as well as novel introduction of various elements that will help reduce the cost incurred, as well as the overall time required for the examination. Therefore, since the advantages of this method outweigh its disadvantages, it becomes clear that this option is viable, as MRI will provide superior imaging of soft tissues, whereas PET will provide the functioning of these tissues. However, in order for it to be incorporated into daily clinical practice, this integrated system needs more development. References Buchbender, C., Heusner, T. A., Lauenstein, T. C., Bockisch, A., Antoch, G. 2012. Oncologic PET/MRI, Part 1: Tumors of the Brain, Head and Neck, Chest, Abdomen, and Pelvis. The Journal of Nuclear Medicine, Vol.53 (6): pp. 1-11. Hwang, D. W., Youn, H., Lee, D. S. 2010. Molecular Imaging Using PET/MRI Particle. The Open Nuclear Medicine Journal, Vol.2: pp. 186-191. Lentschig, M. and Franzius, C. 2012. Case Report: Combining [18F] PET with MR for Detection of Bone Metastases in One Simultaneous Examination. Clinical Body Imaging: pp. 13-18. Metcalfe, P., Liney, G. P., Holloway, L., Walker, A., Barton, M., Delaney, G. P., Vinod, S., Tome, W. 2013. The Potential for an Enhanced Role For MRI in Radiation-Therapy Treatment Planning. Technology in Cancer Research and Treatment. Mundy, L. 2012. PET-MRI Integrated Hybrid Scanners. Health Policy Advisory Committee on Technology, State of Queensland (Queensland Health): pp. 1-14. Paans, A. n.d. Positron Emission Tomography. Netherlands: Groningen University Hospital. pp. 1-45.  Yankeelov, T. E., Peterson, T. E., Abramson, R. G., Garcia-Izquierdo, D., Arlinghaus, L. R., Li, X., Atuegwu, N. C., Catana, C., Manning, H. C., Fayad, Z. A., Gore, J. C. 2012. Simultaneous PET–MRI in Oncology: A Solution Looking for a Problem? Magnetic Resonance Imaging: pp. 1342–1356. Zaidi, H., and Mawlawi, O. 2007. Simultaneous PET/MR will Replace PET/CT as the Molecular Multimodality Imaging Platform of Choice. Medical physics, Vol.34 (5): pp. 1525-1528. Read More