The Anatomy and Normal MR Appearance of the Hippocampus - Research Paper Example

The Anatomy and Normal MR Appearance of the Hippocampus Anatomy of hippocampus Structural characteristics The hippocampus, together with amygdala, fornix, mammillary bodies and cingulate gyrus, is a part of the limbic system, which is composed of cortical structures near the corpus callosum and thalamus. Found at the medial side of the temporal lobe (Saladin, 2003), the hippocampus is a curved elevation of gray matter extending throughout the whole length of the floor of the interior horn of the lateral ventricle for both hemispheres. As the name implies, the structure resembles a seahorse, particularly in coronal sections of the brain. This vital structure for memory is closely associated with the fornix and mammillary bodies. First, the postero-ventral hippocampal fimbriae, named because of its brush-like appearance on coronal section, are composed of fibers from the white matter. The fibers then proceed anteriorly and superiorly, forming the crus of the fornix. The left and right crura join together superiorly to form the body of the fornix, located directly beneath the corpus callosum. The fibers further proceed anteriorly, dividing into two again to form the columns of the fornix. As it curved posteriorly, the columns end inferiorly as mammillary bodies of the hypothalamus (Durbridge, 2012; Saladin, 2003). Figure 1 shows a diagram of the mid-coronal cut of the brain, showing the hippocampus, in relation to other brain structures, particularly of the limbic system. It should be noted that the corpus callosum is directly above the body of the fornix, while the third ventricle is immediately beneath the body of the fornix. On the other hand, figure 2 provides an isolated view of the amygdala, hippocampus, fornix and mammillary bodies. Normal appearance in MRI In determining whether the hippocampus is normal or not, the MRI slices presenting its head, body and tail are looked at (Figure 3). A criteria for the assessment of hippocampus in MRI has been developed by Baulac et al. in 1998. Later studies then made minor modifications to it. Bernasconi et al. (2005), for example, evaluated the hippocampal head, body and tail using the following qualifications. Each criterion is listed in Table 1, and the images representing each criterion are in Figure 4. Shape and positioning of hippocampus are abnormal when at least three of the eight criteria present in the same individual. Table 1. The Criteria for MRI-imaged Hippocampus Evaluation 1) Medial positioning with respect to temporal horn, such that the hippocampus, its head and anterior part of the body, being proximal with the crus cerebri, and with the collicular plate proximal to the tail and the posterior portion of the body. 2) Round, globular shape with vertical orientation (Figures 4a, 4b, 4c, and 4f) 3) Empty choroid fissure (Figures 4c, 4d and 4e) 4) Fimbriae at the hippocampal body is misplaced on the dorsolateral edge of Ammon’s horn (Figures 4d and 4e) 5) The collateral sulcus is deep and oriented vertically (Figures 4a and 4f) 6) The collateral sulcus protrudes into the empty choroid fissure (Figure 4e) 7) Reduction of the upper horizontal portion of the parahippocampal gyrus adjacent to the hippocampal fissure. 8) Subiculum is bulging upward to look thicker than normal Aside from these characteristics, the medial positioning and the vertical orientation of the hippocampus are also looked into to determine the structure’s normalcy. The medial positioning is analyzed using medial distance, or the distance between the midline and the fimbriae (a) divided by the distance from the midline to the temporal lobe neocortex (b) passing through the temporal horn of the lateral ventricle (arrowhead). Normal medial distance is approximately 0.23-0.29. On the other hand, differences in vertical orientation are analyzed through the parahippocampal angle, or the angle between the descending and ascending portion of the parahippocampal gyrus. The normal value ranges between 100°-135° (Figure 5) (Bernasconi et al., 2005). MRI Neuroimaging of Epilepsy MRI Findings Patients with generalized tonicoclonic seizures (status epilepticus) can also present with focal T2 signal intensity increase, swelling and increased volume at the right hippocampus as early as three days after the onset of seizures. Pathologic MRI findings on the hippocampus may also persist even after a successful treatment. Structural defects detected in MRI include hippocampal sclerosis and increased T2 signal intensity. Atrophic change of the right hippocampus was also seen in a patient without prior history of epilepsy (Kim et al., 2001). Figure 6 shows an example of the images seen in epilepsy protocol MRI of a patient with epilepsy. MRI Protocol in generalized epileptic seizures Kim et al. (2001) used the following parameters in their study of epilepsy. Images taken in peri-ictal period included T1-weighted oblique coronal or axial view spin-echo imaging, with TR = 400 and TE = 9-10. Matrix is 256 x192, section thickness is 7 mm, and field of view is 16 x 16 cm. On the other hand, T2-weighted oblique coronal view used fast-spin echo imaging, with TR = 4, 000, TE = 102, and matrix is 512 x 256. Section thickness is 5 mm, field of view is 16 x 16 cm. Similarly, axial view has TR = 4000, TE = 102, matrix = 256 x 256, section thickness of 5 mm, section gap of 2 mm, and field of view of 16 x 16 cm. Diffusion-weighted oblique coronal or axial view imaging (TR = 5000, TE = 100, matrix = 128 x 128, section thickness = 5 mm, and field of view = 24 x 24 cm. Finally, fluid-attenuated inversion recovery (FLAIR) oblique coronal or axial view imaging can be obtained with TR = 11000, TE = 127-140, matrix = 256 x 192, section thickness = 5 mm, and field of view = 16 x 16 cm) were added. Importance of epilepsy protocol MRI The use of this epilepsy protocol MRI allows visualization of hippocampal sclerosis and cortical malformations that may be missed in conventional MRI alone. In fact, studies have shown that in patients who had a standard MRI, only half were found to have brain lesions. On the other hand, this value increased to as much as 90% when epilepsy protocol was used. Knowing the presence or absence of these lesions facilitates better prognostication and subsequent development of a more effective therapeutic plan for patients. Indeed, the patients diagnosed early have a higher chance of epilepsy remission than those diagnosed later in the history of the disease (Passaro, 2011). 2. Discuss the role of MRI in the assessment of Alzheimer's Disease Alzheimer’s Disease MRI Findings Alzheimer’s disease is clinically described as a progressive neurodegenerative disease manifesting as a gradual onset of dementia. Radiographically, the gyri of hippocampus (Saladin, 2003) and the cerebrum (Ramachandran, 2012) atrophy, as shown in figure 7. MRI of patients in the initial stages of Alzheimer’s disease also show large reductions in hippocampal volumes (as much as half), increase in the size of temporal horns, as well as the third and lateral ventricles. The hippocampal atrophy has a sensitivity of 77% and specificity of 80% for detecting Alzheimer’s disease. Abnormalities seen in MRI involving the hippocampus, amygdala, cingulate gyrus, head of the caudate nucleus, temporal horn, lateral ventricles, third ventricle, and basal forebrain have a 77% prediction rate of Alzheimer’s disease. Changes in blood perfusion, using dynamic susceptibility contrast (DSC), echo-planar imaging and signal targeting with attenuation radiofrequency (EPISTAR), or blood oxygenation level-dependent (BOLD) imaging can be used (Ramachandran, 2012). MRI Protocol To image the hippocampus in the diagnosis of Alzheimer’s disease, sagittal 3D T1-fast field echo sequence can be done, with TR = 18, TE = 10, 1 acquisition, average pulse sequence, matrix size = 256 x 256, FOV = 256, slice thickness = 1 x 1 x 1 mm3) (Bernasconi et al., 2005). Role of MRI in Assessment of Alzheimer’s Disease The presence of the changes characteristic of Alzheimer’s Disease in healthy elderly patients makes MRI less contributory in the diagnosis of this neurologic pathology. In fact, hippocampal atrophy can also be seen in multi-infarct dementia, fronto-temporal dementia and Parkinson disease, although not as progressed as those seen in Alzheimer’s disease. Many have thus looked into improving the functionality of MRI in clinical setting. For example, a study has found that yearly MRI scans of individuals with the disease shows a greater rate of change than that of normal subjects. Such differences between patients with Alzheimer’s disease and healthy individuals can be seen even as early as three years before the clinical signs manifest (Ramachandran, 2012). Functional MRI (fMRI) measuring cerebral perfusion is also useful in the assessment of Alzheimer’s disease. Common areas of hypoperfusion are the posterior parieto-temporo-occipital regions, as well as the hippocampal and prefrontal regions (Ramachandran, 2012). 3. Review the anatomy and MR appearance of the brachial plexus. Suggest an MRI protocol for a patient presenting with thoracic outlet syndrome Anatomy of the brachial plexus The brachial plexus forms from the ventral rami of spinal nerves C4 to T2. It passes over the first rib into the axilla. It is subdivided into 5 roots, 3 trunks, 6 divisions and 3 cords, respectively. The five roots are the ventral rami of nerves C5 to T1. These roots coalesce to form the upper, middle and lower trunks (Saladin, 2003). The trunks can be found lateral to the middle scalene muscle and superior to the subclavian artery. The demarcation of the divisions is found near the crossing of the brachial plexus with the clavicle. On the other hand, the cords are formed from division at the level when the subclavian becomes axillary blood vessels (van Es, Feldberg, Ramos, and Witkamp, 1995). Each trunk divides into an anterior and posterior division, and the resulting six divisions merge to form three large fiber bundles: the posterior, medial and lateral cords (Saladin, 2003). The brachial plexus ends with ulnar, median, musculocutaneous, radial, and axillary nerves (van Es, Feldberg, Ramos, and Witkamp, 1995). The nerves innervate the muscles of the upper limb, neck and shoulder for cutaneous sensation, muscular contraction and proprioception (Saladin, 2003). Figure 8 shows the nerves with impulses coming from the brachial plexus. Appearance of brachial plexus in MRI With coronal slices of MRI, specifically using the coronal and sagittal planes, the anatomic components of the brachial plexus, such as the roots (Figure 9), trunks (Figure 10), divisions, and cords (Figure 11 and 12), can be seen. The imaging modality has been used in symptoms such as pain, neural deficits, or muscular atrophy that can be attributed to a brachial plexus injury. Sagittal T1-weighted spin-echo images, with TR = 600-700 and TE = 20, proton-density images, with TR = 1800 and TE = 30, as well as T2-weighted spin-echo images, with TR = 1800 and TE = 90. The slice thickness is 6 mm, and a gap of 4 mm, and cover the brachial plexus from the myelum to the medial side of the humeral head. On the other hand, 13 coronal images per plane are obtained using thin T1-weighted spin-echo images, with TR = 600-700, TE = 20, slice thickness of 3 mm, gap of 0.3 mm (van Es, Feldberg, Ramos, and Witkamp, 1995). Thoracic Outlet Syndrome The thoracic outlet syndrome, usually due to trauma or excessive arm activity, is defined as upper extremity weakness and/or numbness caused by compression of the neurovascular bundle by various structures surrounding the thoracic outlet, composed of the interscalene triangle, costoclavicular space and retropectoralis minor space, through which neural, arterial and venous structures pass through, as seen in figure 13. The arterial TOS (aTOS) is compression of the subclavian artery, venous TOS (vTOS) is affecting the subclavian vein, and neurogenic TOS (nTOS) is impingement of the brachial plexus. aTOS is uniquely caused by a cervical rib or anomalous first rib. The symptoms of aTOS include digital ischemia, claudication, pallor, coldness, paresthesia, and pain in the hand, but seldom in the shoulder or neck. On the other hand, vTOS manifests as arm edema, cyanosis, pain, and paresthesia. Finally, nTOS present with pain, paresthesia, weakness of the upper extremity, neck pain, occipital headaches, coldness, and color changes (Sanders, Hammond and Rao, 2007). MRI Protocol for Thoracic Outlet Syndrome aTOS and vTOS are located between the anterior and middle scalene muscles. As such, MRI, possibly together with MR angiography, becomes a more useful diagnostic tool of TOS. In particular, studies suggest that the best way to view the thoracic outlet is through T1-weighted MRI images (Demondion et al., 2006). However, a number of factors, such as abrupt changes in the path of the vessel, turbulent flow and change of motion can result to formation of artifacts on the image (Antani, 2011). It must be further noted that MR angiography in possible nTOS is an unnecessary diagnostic procedure (Sanders, Hammond and Rao, 2007). MRI imaging is conducted in sagittal and coronal planes from medial-superior to lateral-inferior. The axial plane can also be taken to provide information about the nerve roots as they exit from foramina (van Es, Feldberg, Ramos, and Witkamp, 1995). The sagittal MRI is most useful in identifying vascular and nervous compressions, since these structures travel along antero-posterior and cranio-caudal direction (Demondium et al., 2006). In imaging the whole brachial plexus of one side in the sagittal plane, from the roots to the origin of the peripheral nerve branches in the axilla, it is important that relatively thick slices (6 mm) with large inter-slice gaps (4 mm) should be obtained (van Es, Feldberg, Ramos, and Witkamp, 1995). On the other hand, coronal images are useful in analyzing the brachial plexus. In TOS, it is important that the gadolinium-enhanced angio-MRI should be performed with the patient in different postural maneuvers, like abduction, adduction and arm elevation, since structures may obscure the area of compression. However, the size of the tunnel and the supine position of the patient limit arm movement during the imaging. It is also difficult to use the imaging in very thin individuals because of the little adipose tissue that helps distinguish and individualize the anatomic structures in question (Demondium et al., 2006). Vascular TOS may be determined by comparing the vessel’s cross-sectional area at the suspected location with the arm in anatomic position and at arm elevation. Venous thrombosis and collateral circulation are also signs of vTOS. The caliber of the vessel is also important. On the other hand, nTOS is suggested if there is disappearance of fat around the brachial plexus close to bony structures. A combination of aTOS/vTOS and nTOS is also possible, as shown in figure 14 (Demondium et al., 2006). References Antani, M. 2011. Thoracic outlet syndrome imaging. [online] Available at: < http://emedicine.medscape.com/article/418670-overview> Bernasconi, N. et al. 2005. Analysis of shape and positioning of the hippocampal formation: an MRI study in patients with partial epilepsy and healthy controls. Brain, 128, pp. 2442-2452. Demondion, X. et al. 2006. Imaging Assessment of Thoracic Outlet Syndrome. RadioGraphics, 26. pp. 1735-1750. Durbridge, G. 2012. MRES7013. Fundamental MRI of Brain and Spine. Queensland, Australia: University of Queensland.   Kim, K. et al. 2001. Transient MR Signal Changes in Patients with Generalized Tonicoclonic Seizure or Status Epilepticus: Periictal Diffusion-weighted Imaging. AJNR, 22, pp. 1149-1160. Passaro, E. A. 2011. Neuroimaging in Epilepsy. [online] Available at: Ramachandran, T. S. 2012. Alzheimer Disease Imaging. [online] Available at: Saladin, K. S. 2003. Anatomy & Physiology: The Unity of Form and Function. 3rd ed. New York: McGraw-Hill Sanders, R. J., Hammond, S. L. and Rao, N. M. 2007. Diagnosis of thoracic outlet syndrome. J Vasc Surg, 46. pp. 601-604. van Es, H. W., Feldberg, M. A. M., Ramos, L. M. P., and Witkamp, T. D. 1995. MRI of the brachial plexus. MedicaMundI, 40. pp. 84-90. Vemuri, P. and Jack Jr., C. R. 2010. Role of Structural MRI in Alzheimer’s Disease. Alzheimer’s Research and Therapy, 2, pp. 23-33. Read More