Current Technologies and Future Directions

Werner Jung; Vlada Zvereva; Bajram Hajredini; Sebastian Jäckle

Magnetic resonance imaging (MRI) is the imaging modality of choice in many clinical situations, and its use is likely to grow due to expanding indications and an ageing population. Many patients with implantable devices are denied MRI except in cases of urgent need, and when scans must be performed they are complicated by the need for burdensome and costly personnel and monitoring requirements that have the net effect of restricting access to scans. Several small studies, enrolling a total of 344 patients, suggest that some patients with conventional systems may undergo MR examinations without clinically overt adverse events. However, a number of potential interactions exist between implantable cardiac devices and the static and gradient magnetic fields and modulated radio frequency (RF) fields generated during MR scans; nearly all studies have reported pacing capture threshold changes, troponin elevations, ectopy, unpredictable reed switch behaviour, and other ‘subclinical’ issues with pacemakers and implantable cardioverter-defibrillators (ICDs) in patients who have undergone MRI. Attention has turned to devices that are specifically designed to be safe in the MRI environment. A clinical study of one such device documented its ability to be exposed to MRI in a 1.5 T scanner without adverse impact on patient outcomes or pacemaker system function. Such new technologies may enable scanning of pacemaker and ICD patients with reduced concerns regarding the short- and long-term effects of MRI. As importantly, these devices may increase the number of centres that are able to safely perform MRI and, thus, expand access to scans for patients with these devices.

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Many studies have shown that cerebral atrophy is significantly greater in patients with Alzheimer disease (Alzheimer’s disease) than in persons without it. However, the variability of atrophy in the normal aging process makes it difficult to use MRI as a definitive diagnostic technique. (See the images below.)

Coronal, T1-weighted magnetic resonance imaging (MRI) scan in a patient with moderate Alzheimer disease. Brain image reveals hippocampal atrophy, especially on the right side. Axial, T2-weighted magnetic resonance imaging (MRI) scan of the brain reveals atrophic changes in the temporal lobes. Axial, T2-weighted magnetic resonance imaging (MRI) scan shows dilated sylvian fissure resulting from adjacent cortical atrophy, especially on the right side. Axial, T1-weighted magnetic resonance imaging (MRI) scan shows a dilated sylvian fissure caused by adjacent cortical atrophy. Axial, T1-weighted magnetic resonance imaging (MRI) scan shows bilateral cortical atrophy with accentuated cortical sulci; there is decreased involvement in the posterior aspect. Axial, T1-weighted magnetic resonance imaging (MRI) scan shows bilateral cortical atrophy with accentuated cortical sulci; there is decreased involvement in the posterior aspect.

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Current diagnosis of Alzheimer disease (Alzheimer’s disease) is made by clinical, neuropsychological, and neuroimaging assessments. Routine structural neuroimaging evaluation is based on nonspecific features such as atrophy, which is a late feature in the progression of the disease. Therefore, developing new approaches for early and specific recognition of Alzheimer disease at the prodromal stages is of crucial importance.

Alzheimer disease was first described in 1907 by Alois Alzheimer. From its original status as a rare disease, Alzheimer disease has become one of the most common diseases in the aging population, ranking as the fourth most common cause of death. Alzheimer disease is a progressive neurodegenerative disorder characterized by the gradual onset of dementia. The pathologic hallmarks of the disease are beta-amyloid (Aß) plaques, neurofibrillary tangles (NFTs), and reactive gliosis. (See the images and slideshows below.)

Coronal, T1-weighted magnetic resonance imaging (MRI) scan in a patient with moderate Alzheimer disease. Brain image reveals hippocampal atrophy, especially on the right side. Axial, T2-weighted magnetic resonance imaging (MRI) scan of the brain reveals atrophic changes in the temporal lobes.

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The problem of implanted metals causing tissue damage by movement in patients exposed to MRI fields has produced a confusing welter of erroneous, pseudoscientific publications about magnetics, metals, medical equipment, and tissue compatibility. Quite simply, among the devices made for implantation, only those fabricated of stainless steel have the ferromagnetic properties capable of causing such accidents. The author, who introduced the basic design of the modern aneurysm clip in the late 1960s and then a cobalt nickel alloy as an improvement over steel, while chairing the neurosurgical committee assigned to the task of establishing neurosurgical standards at American Society for Testing and Materials, exposes this flawed information and offers clear guidelines for avoiding trouble.

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Focusing on the clinical applications of fMRI, this chapter will present methods to identify characteristic anatomical landmarks, and describe the course and shape of some gyri
and sulci and how they can be recognized on MR imaging.

Some redundancy is desired in order to course over cortical landmarks. If fMRI is not performed during clinical routine imaging, usually a 3D data set is acquired to overlay the results. Nowadays, fMRI is performed using echo planar imaging (EPI) with anisotropic distortion, whereas 3D T1-weighted data sets, such as MPRage (magnetization prepared rapid acquisition gradient echo) or SPGR (spoiled gradient recalled acquisition in steady state) sequences, are usually isotropic.

Normalization of the fMRI data may reduce this systemic error to some extend that is more pronounced at the very frontal aspect of the frontal lobe and the very posterior aspect of the occipital lobe. However, for individual data, normalization and overlaying fMRI results on anatomy remains crucial. No two brains, not even the two hemispheres within one subject, are identical at a macroscopic level, and anatomical templates represent only a compromise (Devlin and Poldrack 2007).

Usage of templates like the Talairach space (based on the anatomy of one brain) or the MNI template (based on 305 brains) can cause registration error as well as additional variation, and reduce accuracy; indeed, it does not warrant the shammed anatomical precision in the individual case.

Radicular pain often is the result of nerve root inflammation +/- mechanical irritation. Clinical practice and research demonstrate that mechanical compression alone to the nerves causes only motor deficits and altered sensation but does not necessarily cause pain. Inflammation within the epidural space and nerve roots, as can be provoked by a herniated disk, is a significant factor in causing radicular pain.

Amy Argus, MD; Mary C. Mahoney, MD

Mammography is the primary modality used for imaging the breast. However, it has known limitations. Dynamic contrast-enhanced breast magnetic resonance imaging (MRI) provides superior sensitivity to detect breast cancer and, when used in the appropriate clinical setting, it has become a useful adjunct to mammography.

Mammography is the primary modality used for imaging the breast. However, it has known limitations. Dynamic contrast-enhanced breast magnetic resonance imaging (MRI) provides superior sensitivity to detect breast cancer and, when used in the appropriate clinical setting, it has become a useful adjunct to mammography. Overlap in the MRI appearance of some benign and malignant diseases limits the specificity of breast MRI. The false-positive findings which result then prompt additional imaging and/or biopsies for benign disease. This, combined with the higher cost and limited availability of breast MRI vs. conventional imaging, requires appropriate use of the imaging modality. The American Cancer Society has outlined recommendations for the use of breast MRI for breast cancer screening and the American College of Radiology practice guidelines include 12 indications for the performance of breast MRI.^[1,2]

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George D. Lundberg, MD

Cervical spine injury can have very serious consequences and can be difficult to diagnose. Five investigators in Wisconsin with a meta-analysis selected 5 prospective or retrospective published diagnostic protocol results that included magnetic resonance imaging. They studied reports of 464 victims of blunt trauma with clinically suspicious or unevaluatable cervical spines. Clinical follow-up was the gold standard. Sensitivity of MRI for cervical spine abnormalities was 97%, with a negative predictive value of 100% and zero false negatives; specificity was 99% with a positive predictive value of 94%; 97 patients, that is, 21%, had abnormalities by MRI that were not found by radiographs or CT. This study, reported in 2008 in The Journal of Trauma,^[1] establishes MRI as the new gold standard for conclusively excluding cervical spine injury in a patient with blunt trauma.