[Basic principles and technique of diffusion-weighted imaging and diffusion tensor imaging]

BACKGROUND

Due to their thermal energy, water molecules in tissue are in continuous random motion called diffusion. Water diffusion in pathologically modified tissue (e. g. ischemia, inflammation and neoplasia) is different from normal conditions. Diffusion-weighted magnetic resonance (MR) imaging (DWI) can measure the local strength and main direction of the diffusional motion in any picture element, thus providing diagnostic tissue information exceeding the morphological depiction.

METHODS

Diffusion-weighted MR sequences are based on the echo planar imaging (EPI) technique which is very rapid but also susceptible to artefacts. Using especially strong magnetic field gradient pulses the MR signal is sensitized to microscopic motion of water molecules resulting in a unique image contrast in addition to T1 and T2. Local deviations of the diffusion strength from normal values indicate pathological processes. The DWI sequences can measure diffusion along any direction; however, in the clinical routine only directionally averaged DWI images (trace maps) are used. Diffusion tensor imaging (DTI) represents an advanced DWI method which specifically explores diffusional anisotropy in order to obtain additional information about tissue microstructure.

CONCLUSION

Diffusion-weighted MRI is an established technique for the assessment of pathological processes. Although DWI is mainly applied in stroke diagnostics, it is increasingly being used to detect and characterize various lesions in the brain as well as in the whole body. With new sequence techniques imaging artefacts can be significantly reduced. In addition, DTI allows the reconstruction and 3-dimensional visualization of tissue fibre structure. This method has proven to be clinically important primarily for the depiction of nerve tracts in the brain and spinal cord when planning surgical interventions and radiation therapy.

[Functional MRI of speech]

Speech is a multifactorial term. In relationship with our central organ, speech is a communication medium which in a complex manner includes the production and perception of verbal information. Even within this limitation, imaging exploration of the neuronal basis of verbal communication offers an extensive investigative field which can only be presented in this article as highlights. The main focus is on the technical hurdles of investigations into speech production using functional magnetic resonance imaging (fMRI) and solution options will be presented. The difficulties contained in this aspect are that spoken words can only be poorly assessed in the noise level of the measurements and that the movements of the mouth necessary for speech motoric can lead to technical imaging artefacts. Investigation techniques and designs will be discussed which allow a representation of speech-relevant brain centers despite these problems.

[Basic principles of MR spectroscopy]

MR spectroscopy (MRS) is a non-invasive method for investigating tissue metabolism. In the clinical setting proton spectroscopy based on the hydrogen nucleus ((1)H) is primarily used for determining concentrations of important brain metabolites. This paper illustrates the physicochemical principles of MRS and the requirements for high quality spectra. Moreover, different techniques for acquiring localized spectra are demonstrated.

Neurochemical pathology in hippocampus in euthymic patients with bipolar I disorder

OBJECTIVE

Subcortical regions such as hippocampus, thalamus and ventral putamen are assumed to be involved in the pathophysiology of mood regulation. Disturbed hippocampal neuronal function indicated by reduced N-acetyl-aspartate (NAA) levels in bipolar patients was shown by several studies. Results in thalamus and putamen are inconsistent.

METHOD

N-acetyl-aspartate, choline (Cho), creatine (Cr) and myo-inositol (Ins) were measured in left hippocampus, left thalamus and left putamen using proton magnetic resonance spectroscopy in 13 euthymic patients with bipolar I disorder and 13 pairwise matched healthy control subjects. Metabolic ratios NAA/Cr, NAA/Cho, Cho/Cr and Ins/Cr were calculated.

RESULTS

Patients with bipolar I disorder demonstrated significantly reduced NAA/Cr in the left hippocampus compared with healthy control subjects. No alterations were found in thalamus or putamen.

CONCLUSION

We hypothesize that this NAA/Cr reduction might reflect neuronal dysfunction in the left hippocampus in patients with bipolar disorder.

[Supratentorial tumors]

Magnetic resonance imaging is a routine diagnostic measure for a suspected intracerebral mass. Computed tomography is usually also indicated. Further diagnostic procedures as well as the interpretation of the findings vary depending on the tumor location. This contribution discusses the symptoms and diagnostics for supratentorial tumors separated in relation to their intra- or extracranial location. Supratentorial tumors include astrocytoma, differentiated by their circumscribed and diffuse growth, ganglioglioma, ependyoma, neurocytoma, primitive neuroectodermal tumors (PNET), oligodendroglioma, dysem-bryoplastic neuroepithelial tumors (DNET), meningoangiomatosis, pineal tumors, hamatoma, lymphoma, craniopharyngeoma and metastases. The supratentorial extracranial tumors include the choroid plexus, colloid cysts, meningeoma, infantile myofibromatosis and lipoma. The most common sub-forms, especially of astrocytoma, will also be presented.

[Spinal infections]

Regardless of the compartment involved, imaging of spinal infections and other spinal inflammatory diseases must be performed by magnetic resonance imaging (MRI). This allows early detection of changes in disk space or vertebral bodies. Associated paraspinal and epidural masses are depicted. However, in spinal cord inflammation MRI has a very limited specificity and cannot usually differentiate between the wide array of possible causes of intramedullary lesions. This article covers a spectrum of inflammatory lesions of the spinal cord.

Infratentorielle Tumoren

This article gives an overview concerning the typical infratentorial tumors of adults.

MR-Spektroskopie bei Hirntumoren

MRT allows the anatomical visualization of intracerebral space-occupying lesions, and when magnetic resonance spectroscopy (MRS) is used in routine clinical practice it can give more information and be helpful in the diagnosis of such lesions. In MRS with long echo times for nerve tissue there are five metabolites that are particularly significant: N-acetyl aspartate (NAA), creatine, choline, lactate, and lipids. NAA levels are lowered in the presence of intracerebral tumors. Creatine is lowered in situations of hypermetabolic metabolism and elevated in hypometabolic conditions, but remains constant in many pathologic states and can be used as a reliable reference value. With malignant tumors there are usually elevated choline concentrations, reflecting increased membrane synthesis and a higher cell turnover. The lactate level rises following a switch in metabolism from aerobic to anaerobic glycolysis, and this is frequently observed in the presence of malignant tumors. The occurrence of lipid peaks in a tumor spectrum suggests the presence of tissue necroses or metastases. There are typical constellations that are seen on MRS for individual tumors, which are discussed in detail in the present paper.

[Imaging of spinal tumors]

Spinal tumors are often categorized into extradural, intradural extramedullary, or intramedullary. Although this classification represents somewhat of an overgeneralization as a lesion may reside in two compartments, it still helps to characterize spinal tumors. In the intradural, extramedullary space, primary tumors, such as neurofibroma and meningioma, are relatively common. Secondary tumors or leptomeningeal enhancement also occur. In the intramedullary space, primary tumors are far more common than secondary tumors or metastases.

Cognitive changes after carotid artery stenting

INTRODUCTION

We aimed to test changes in cognitive performance after carotid artery stenting (CAS).

METHOD

Ten patients were neuropsychologically tested at least 24 h before and 48 h after CAS. To diminish thromboembolic events, we used a proximal protection device. The following neuropsychological tests were selected: The Mini Mental State Examination (MMSE), symbol digit test and subtests of the Consortium to Establish a Registry for Alzheimer's Disease (CERAD) battery (verbal fluency, constructional practice, word list memory and delayed recall). Affective state was determined by the Beck Depression Score (BDS).

RESULTS

No patient suffered from depression (BDS <1) or dementia (MMSE 29.9+/-1.5). Nine of the ten patients (P=0.12) showed increased speed in the Number Connection Test (NCT) (corresponding to trail making test). Most patients showed better or similar results concerning delayed recall (P=0.31). No change was observed in the symbol digit test, word list memory, verbal fluency or constructional practice. Better results concerning NCT and delayed recall after carotid stenting might be due to improved brain perfusion.

CONCLUSION

After CAS, cognitive and memory performance seem to improve. Further studies with different time intervals and more refined testing, as well as perfusion-weighted imaging, are needed.

Time course of the apparent diffusion coefficient after cerebral infarction

The purpose of this study was to evaluate quantitative apparent diffusion changes in the center of infarction by measurement of the apparent diffusion coefficient (ADC), and to investigate the influence of ischemia on the contralateral hemisphere. By diffusion echo-planar imaging (EPI) 52 patients showing cerebral infarction were studied within 5 h to >12 months after onset of clinical symptoms. Using three diffusion gradient strengths (b1=30 s/mm(2); b2=300 s/mm(2), b3=1100 s/mm(2)) ADC maps were generated. After onset of ischemia, ADC in the center of infarction was lower than in the contralateral regions of human brain. At first ADC declined for approximately 28 h to a minimum of approximately 150x10(-8) cm(2)/s. Then the ADC reincreased and reached a "pseudonormalization" after approximately 5 days. Chronic infarctions did show much higher ADC values (2000x10(-8) cm(2)/s) than unaffected areas. Neither localization nor size of infarctions showed a significant influence on this time course. In the center of infarction diffusion is isotropic. Even brain regions of the contralateral hemisphere are influenced by cerebral ischemia. In these regions ADC is higher than for physiological conditions. The ADC also declines especially for the first 2-3 days after onset of symptoms, also followed by reincrease. The ADC calculation enables determination of the onset of infarction more exactly than is possible using only diffusion-weighted imaging. Diffusion in the center of infarction is isotropic; hence, orientation of the diffusion gradients has no significant influence on sensitivity of measurements. The calculation of the ADC ratio based on data derived from the center of infarction and the contralateral hemisphere seems to be critical because the ADC in the unaffected contralateral hemisphere also changes.

[Image fusion of MRI and immunoscintigraphy with MAb-170 in ovarian tumors]

In recent years multimodality imaging achieved growing importance. It is mostly performed by means of quite expensive software and hardware solutions. In the present pilot study a simple and low-cost procedure was developed to achieve image fusion in the pelvis. The image data of immunoscintigraphy (SPECT) and MRI were transferred to a personal computer and combined by standard software for image manipulation. The results in eleven patients with space-occupying lesions in the pelvis showed that adequate anatometabolic slices could be achieved. The results show a tendency to increased specificity and precision of multimodality imaging in comparison with SPECT and MRI alone. In conclusion, the low-cost solution, as developed by us, is feasible in clinical practice. Its results are reliable in clinical decision making.

[Proton spin tomography of the orbit in post-traumatic motility disorders]

AIM

To analyse the value of MRI for the assessment of posttraumatic disturbances of eye motility.

MATERIAL AND METHODS

We analysed retrospectively the results of 38 MR examinations of the orbit in 31 patients with posttraumatic motility impairment with preserved visus. 18 patients underwent MRI preoperatively. From this group 5 patients were additionally examined postoperatively. Another 5 patients who had not been examined prior to surgery were controlled postoperatively. Hence, a total of 10 patients with persisting disturbances was examined postoperatively. 8 patients who underwent MRI were not treated by surgery because of only minor disturbances. In all patients conventional radiography of the orbit was performed prior to MRI. Additional CT imaging was carried out in 12 patients.

RESULTS

In 18 patients examined preoperatively we found displacement of orbital fat tissue, displacement and entrapment of orbital muscles, swelling of muscles and oedema in retrobulbar fat tissue. 10 patients from the postoperative group exhibited remaining prolapsed fat tissue, oedema in fat tissue and/or swelling of muscles. 8 patients had only small soft tissue changes which did not require surgery.

CONCLUSION

Since it can image soft tissue precisely, MRI provides the decisive information in the assessment of motility impairment of the eye. In case of isolated orbital fracture with motility impairment, CT is not absolutely necessary for surgical therapy.

Bildfusion von KST und Immunszintigraphie mit MAb-170 bei Ovarialtumoren

Die Vorteile multimodaler Bildgebung sind in den letzten Jahren beschrieben worden. Zumeist wird diese unter Verwendung kostspieliger Hard- und Software im Bereich des Neurocraniums angewandt. In der vorliegenden Pilotstudie wurde ein einfaches und kostengünstiges Verfahren für Bildfusion im Bereich des Beckens entwickelt. Die Bilddaten von Immunszintigraphie (SPECT) und Magnetresonanztomographie (KST) wurden auf einen Personalcomputer transferiert und mit einem konventionellen Bildbearbeitungsprogramm fusioniert. Die Ergebnisse bei insgesamt elf Patientinnen zeigen, daß hier eine adäquate Bildfusion zu erreichen ist. Die Resultate weisen in Richtung einer höheren Spezifität und Genauigkeit der Methode im Vergleich zu SPECT und KST alleine. Zusammenfassend bestätigt diese Studie, daß alternativ zu den kostspieligen Problemlösungen der Gerätehersteller preisgünstige Alternativen auf der allgemein verfügbaren PC-Plattform möglich sind. Trotz Einschränkungen liefern sie klinisch wertvolle Zusatzinformationen.