Magnetic Resonance Imaging, Effects of Gadolinium - Essay Example

?Introduction The acronym MRI stands for Magnetic Resonance Imaging is one of the best knows methods of medical imaging and tissue characterization. This theory was proposed by two eminent scholars of science, Raymond Damadian and Paul Lauterbur, who had been working independently and proposed this new concept roughly in late 1960s. This technique is basically nuclear magnetic resonance imaging, which involves computation of resonance signals obtained when an atomic nucleus is exposed to static magnetic field. MRI makes use of the natural magnetic proprieties of atoms within the body. Due to huge abundance of water in the body, hydrogen atoms are focused upon in this process. Upon applying the magnetic field, the hydrogen atoms align in the direction of the magnetic field. Next, when radio waves are added to the magnetic field, it results in the deflection of the magnetic field and resonation of the hydrogen atoms. Resonation frequency of hydrogen atoms is dependent upon the strength of the magnetic field. Upon the switching off of the radio frequency, the atoms get aligned again, generating a signal which is used to create MRI image. After giving subtle time for atoms to relax, multiple signals are produced to detect abnormalities in the tissues. (Berger, 2002) These signals are generated in the form of an image depicting significant information. (Dawson, 2008) (Blink, 2004) It has wide range of applications in evaluating blood flow in the arteries (Clinical Policy, 2001) and diagnosis of soft tissue problems such as brain tumours, multiple sclerosis, torn ligaments, shoulder injuries and earlier stages of stroke. MRI offers multiple privileges to probe deep into soft tissues which CT scans or X-ray do not provide. Secondly, they don’t use any ionising radiations and are thus claimed safe as radio waves are found all around and do not damage the tissues. (Berger, 2002) Thirdly, MRI offers diagnosis of a specific area in the patient at various planes unlike CT scan that is limited only to one plane. (WebMD, 2009) However, there are several limitations to it such as 1) high cost of an MRI scan; 2) victims of claustrophobia are hesitant to undergo the diagnosis and condition to hold breath for long durations, 90 minutes or so, is quite uncomfortable for aged patients. (Michaelmas, 2004) Nevertheless, MRI is one of the major breakthroughs in medicine, allowing the doctors and researchers to accurately diagnose disease conditions of ailing humans. Magnetic Resonance Venography (MRV) is another technique based on nuclear magnetic resonance that is used particularly for monitoring the blood flow in veins of the body. (Vega, 2008) A detailed account of the different scenarios at which MRA and MRV are being used is discussed in a report by Clinical policy Bulletins. (Clinical Policy, 2001) The only difference between MRI and MRV is that MRV uses special computer software to generate the images. This paper will discuss the various techniques used in MRV namely Phase Contrast 3D PC MRV, Time of Flight 3D TOF MRV and Contrast enhanced (3D MRV CE) and MRI scanners in general. Phase Contrast 3D PC MRV This technique makes use of velocity induced shifts to measure blood flow. The velocity of blood flow is proportional to the phase shift, thus giving true quantification of blood flow and direction of blood flow. (Lirng, 2010) (Srichai MB, 2009) Signal intensity is encoded by velocity on the MR image, where stationary protons appear in gray colour. Blood flow in one direction is brighter as compared to the flow in the opposite direction. (Srichai MB, 2009) A significant factor called VENC factor, which is targeted to visualize arteries or veins in the region of interest. The higher the VENC factor, the arteries become more clear and vivid on the MRI image; lower VENC factor values help in visualizing veins. The direction of flow and flow velocities can be calculated using the MRI images. (Medical Physicist, n.d.) Various implications of 3D-PC have been explored in the past decade. Stoquart-ElSankari et al investigated the accuracy of this technique in cerebral venous flow. In the light of the limitation reported by various researchers, this technique seems promising.(Stoquart-ElSankari, 2009) Recently, Ebbers et al discussed another innovation in phase contrast technique that allows evaluation of cardiac system including myocardial, valvular and vascular disorders. (Ebbers, 2011) P van Ooij compared the efficacy of 3D- Phase Contrast MRI with patient- specific computational fluid dynamics (CFD) in assessment of blood flow within intracranial aneurysms. The results showed that 3D PC MRI was as effective as CFD in terms of flow pattern visualization, thereby suggesting the wide use of this technique. (van Ooij P, 2013) Time of Flight 3D TOF MRV This is one of the earliest techniques relying on the blood flow (Glockner, 2010) and proton movement in the blood. The basic principle involves the measurement of the TOF effect so that a drastic contrast is created so as to distinguish between stationary and flowing blood. (Muhs, 2007) A 2D TOF involves collection of signals in a sequential manner from a series of image slices, per slice at a time whereas a 3D TOF involves the simultaneous gathering of signals from the entire area of interest. 3D images have more advantages than the 2D images in terms of greater signal to noise ratio (SNR) and greater resolution. The technique involves the exposure of the area of interest to multiple RF pulses. Transfer of magnetization is carried out in order to suppress the signal from the stationary tissue and thereby enhance the contrast of the flowing tissue. The echo time is kept low so as to neglect artifacts due to disturbed blood flow. (Kim, 2012) Figure 1: A 3D TOF image obtained from healthy volunteer. (Kim, 2012) A comprehensive array of protocols required in various clinical conditions is given Elmaoglu et al. (Elmaoglu, 2012) TOF has a few limitations such as extended imaging time ranging from minutes to hours depending upon patient cooperation and sensitivity to artifacts. (Muhs, 2007) Several innovations are being made such as MOTSA technique. Generally, 2D TOF images have greater contrast than 3D TOF image but possess lower resolution. Parker et al developed a hybrid approach that has the benefits of both 2D and 3D, thus maintaining low signal saturation and high signal to noise ratio (SNR). (Parker, 1995) 3D Contrast Enhanced MRV This is one of the most widely applied techniques that makes use of a 3D spoiled gradient echo sequence in addition to a gadolinium-based contrast. Gadolinium chelate agents have a unique property of shortening T1 relaxation of the blood as compared to background tissue thereby increasing the contrast. The technique involves the injection of bolus and MRA is performed only when the concentration of gadolinium increases in the arteries. At the start, a non contrast data set is obtained so as to take into account the background signal. The imaging of the chest and abdomen is carried out in a breath hold so as to reduce motion artefact in the image. This is followed by subtraction of the post-contrast image from the non-contrast image. Using a fluoroscopic agent, the signals are enhanced in the veins rather than the arteries. The contrast is deepened by the impact of gadolinium, and thus a better resolution image is obtained. (Glockner, 2010) (Muhs, 2007) Image optimization is to be carried out with great care and caution taking into account timing, amount and rate of the injection of contrast agent. The ultimate goal is to record the central region of k-space as this contains the major details of the image. The timing of the bolus concentration needs to coincide with central k-space data acquisition, and increases sensitivity to artifacts. (Zhang H, 2007) Thus, this is to be recorded the instant when the concentration of the contrast agent is the greatest. The amount of the contrast agent injection is kept short so as to ensure central k-space is obtained at the right time. (Muhs, 2007) Effects of Gadolinium Gadolinium as already discussed above is used as a contrast medium that makes certain tissues more vividly visible on a MRI scan. For details, check (3D Contrast Enhanced MRV). This agent is known to improve diagnostic accuracy in certain conditions such as inflammation as well as infectious diseases affecting bones, brain, spine and soft tissues. The contrast medium comprises of gadolinium ion bonded by a carrier molecule called cheating agent. (Figure 2) Physical and Chemical characteristics are illustrated in Table 1. Certain reported reactions that rarely occur are brief headache, dizziness a few minutes following the injection. However, patients with reduced kidney function or kidney failure and hepatorenal syndrome should not be administered with this agent .Some patients were also found to be suffering from nephrogenic systemic fibrosis (NSF), a disease resulting in skin thickening and tightening as well as internal organ damage, occurring in people previously suffering from kidney abnormalities(Molan, 2009). People with normal kidney function are insusceptible to this disease. (ISMRM, 2010) Pregnant ladies are advised to have detailed meetings with the doctor considering the risks. (Molan, 2009) Figure 2: Chemical structure of Gadolinium contrast medium. (Perez OD, 2002) Gadolinium containing agents are available in the following names: Omniscan (Gadodiamide), Magnevist (Gadopentetate dimeglumine) ,MultiHance (Gadobenate dimeglumine), Gadovist (Gadobutrol),Vasovist (Gadofosveset), Dotarem (Gadoteric acid) ,ProHance (Gadoteridol) and Primovist (Gadoxetic acid disodium). Out of these, five are FDA approved which comprise of (Magnevist, MultiHance, Omniscan, OptiMARK, and ProHance). (FDA, 2007) Among all these agents, Omniscan and OptiMARK4 have been found major cause of side-effects owing tot htier similar structures. Very few cases have been reported from Magnevist. (ISMRM, 2010) Table 1 - Uses of Gadolinium in various industries. (RSC, 2013) GE Neurovascular Array Coil - Excite HD 1.5T A single GE neurovascular array coil reduces the cost of coil changeovers and patient repositioning during time of scanning. (Invivo, 2013) (MRI Coils, n.d.) An array of coils allows scanning of various body parts with the same machine, using different configurations. (Medrad, n.d.) The geometry of a coil is a crucial factor for excellent results i.e it must be. large enough to hold the patient’s body and capture a good field of view. Thus, the design of the coil is taken into account for the body part to be measured. (GE Health Care, 2005) Chan et al recently patented a multiple degree of freedom adjustable array coil system which seems to confer numerous benefits. (Chan, 2004) 1.5 Tesla MRI Machine The strength of the magnetic field used in this machine is measured in units called Tesla. 1.0 Tesla equals 10,000 Gauss; this is termed as High- Field MRI. (Tesla Memorial Society, n.d.) This provides ample coil configurations thus augmenting the possibility of imaging sequences. (Medrad, n.d.) The potential of this machine was studied in prediction of pathological stage in prostate cancer in patients undergoing radical prostatectomy. The results showed that 1.5 tesla machine a.k.a standard MRI does not give much detail. However, MRI with higher filed strength along with use of dynamic contrast-enhanced MRI or spectroscopy would be a good protocol to apply in hospitals. (Johnston, 2013) However, recently 3 Tesla MRI machine has gained much popularity in terms of even better MRI image and finer detail. Radiologists at MIT could identify a developmental problem only when 3 Tesla MRI was performed. On the same footprints, University of Illinois, Chicago and University of Zurich, Switzerland have started experimentation on 3-tesla scanners. Signal changes observed are greater as compared to 1.5 Tesla machines thus giving better precision and fineness of detail. As technology is advancing, it’s likely that this will be paved by even robust technologies. (Sandrick, 2010) Wardlaw et al conducted a comprehensive review and compared the working of 1.5 Tesla machine with 3 Tesla machine. 3 Tesla MRI produced a better image termed as ‘ crisp’ but in terms of diagnosis, the two machines performed equally better. However, due to presence of artefacts and longer time to attain the image as well as cost of 3 Tesla MRI image, it still seems unclear which machine is better than the other. (Wardlaw, 2012) Read More