D. Lanens

Proton relaxation enhancement by means of serum albumin and poly-l-lysine labeled with DTPA-Gd3+: Relaxivities as a function of molecular weight and conjugation efficiency

A series of poly-L-lysine chains, with molecular weight ranging from 3300 up to 102,000 Da, were labeled with DTPA-Gd3+. No significant differences in longitudinal and transversal relaxivity, could be demonstrated as a function of the chain length. The R1 and R2 relaxivities were respectively 2.5 and 5 times superior to those of plain DTPA-Gd3+ (at 2.4 T). Bovine serum albumin was also labeled in a way that a wide (DTPA-Gd3+)/BSA range (3-39) was obtained. The longitudinal relaxivity, of these paramagnetically labeled albumins, increased with increasing (DTPA-Gd3+)/BSA ratios. This effect was most pronounced at very low (DTPA-Gd3+)/BSA ratios.

IN-VITRO NMR MICRO IMAGING OF THE SPINAL-CORD OF CHRONIC RELAPSING EAE RATS

Chronic relapsing experimental allergic encephalomyelitis (Cr-EAE) was induced in Lewis rats with an emulsion of guinea pig spinal cord tissue in complete Freunds adjuvant enriched with Mycobacterium tuberculosis H37 RA. The sensitized rats developed Cr-EAE showing two to three relapses during the first 40 days. In vitro transverse T2-weighted spin echo images of the spinal cord of Cr-EAE rats, sacrificed at the clinical height (hind leg paralysis and urinary incontinence) of the third bout and their controls, were compared with the corresponding histopathology. Lesions extended over the entire spinal cord, however, the larger lesions were predominantly present in the cervical and upper thoracic regions. In the white matter only areas of demyelination and large perivascular demyelination were discernable on the MR images. Size and shape of these lesions correlated well with the morphological characteristics revealed by histopathology. Plaques in the ventrolateral funiculus were generally located peripherally, while plaques in the dorsal funiculus were mainly present in the medial part. The NMR images, however, could not distinguish between demyelination, remyelination, inflammation, and oedema. Also lesions in the gray matter could not be distinguished with MR imaging techniques. However, if lesions were localized at the interface of the gray and white matter the boundary between the gray and white matter was less well defined.

Complementary use of T2-weighted and postcontrast T1- and T2∗-weighted imaging to distinguish sites of reversible and irreversible brain damage in focal ischemic lesions in the rat brain

The evolution of a photochemically induced cortical infarct was monitored using T2-, postcontrast (GdDOTA) T1-, and postcontrast (DyDTPA-BMA) T2*-weighted NMR imaging techniques. Data acquired with these different NMR imaging types were compared, both qualitatively and quantitatively. The T2*-weighted NMR images after spordiamide injection (DyDTPA-BMA) were perfusion-weighted images that allowed the differentiation between several infarct-related areas in terms of different degrees of perfusion deficiency. No quantitative information on cerebral blood flow (CBF) was obtained. A clear distinction was made between areas with a complete lack of CBF located in the core of the lesion and temporary CBF insufficiencies in the rim surrounding this core. Concomitant observations on T2-weighted and postcontrast T1-weighted images revealed the same temporary rim characterized by an increased water content, and an intact blood-brain barrier (BBB), as well as by reduced perfusion. This rim appeared within the first hours after infarct induction, reached a maximum 24 h later, and lasted between 3-5 days, when its size gradually decreased until complete disappearance. These observations suggest the existence of an area at risk. Only on postcontrast T1-weighted images, the core of the lesion remained visible during the whole experimental period (10 days) and reflected in all likelihood the irreversibly damaged ischemic central core. The combined application of different NMR imaging techniques when studying focal cerebral infarctions in the rat brain allowed us to distinguish, in terms of NMR characteristics, zones of reversible from irreversible brain damage and to estimate the severity of the damage. This might offer an appropriate experimental setup for the screening of cerebroprotective compounds.

Complementary use of T2- and postcontrast T1-weighted NMR images for the sequential monitoring of focal ischemic lesions in the rat brain

The noninvasive nature of NMR imaging enables serial studies on a single animal. In 12 male Wistar rats, the dynamic progression of a photochemically induced (Rose Bengal) infarct was studied starting immediately after induction and up to 10 days. The results demonstrated that both T2- and postcontrast T1-weighted NMR images are required to discern the time dependent dynamics of the ischemic process. The ischemic lesion was already visible on T2-weighted images within 30 min after the induction. Twenty-four hours after the insult, both area and intensity reached maximum values. Hereafter and up to day 10, both parameters decreased. Postcontrast T1- weighted images revealed a blood-brain barrier (BBB) rupture immediately after the induction which persisted until 10 days after the insult. The application of contrast agents such as Gd-DOTA or Gd-HP-DO3A also allowed the detection of 10-day-old lesions which were not always discernable on T2-weighted images. The penetration of both contrast agents in the affected area proceeded slower the first 5 days after the insult while at day 10, maximum contrast enhancement was reached almost immediately after administration of the contrast agent. At 24 hr after the insult, the discrepancy between the lesion area as determined on T2-weighted images and on postcontrast T1-weighted images was maximal. At this stage, the lesion was characterized by central core with a leaky BBB surrounded by a reversible zone which appeared enhanced on T2-weighted images.

Noninvasive in vivo 13C-NMR spectroscopy of a 13C-labeled xenobiotic in the rat

This study demonstrates that the xenobiotic product, 1-(o-chlorophenyl)-1-(p-chlorophenyl)-2,2-dichloro-3-13C-propane can be monitored in the liver of an intact animal by in vivo 13C surface coil NMR spectroscopy after intraperitoneal administration. The carbon-13 label could be detected after a single dose of only 200 mg/kg of the product. The intrahepatic changes of the signal intensity of the labeled product were monitored as a function of time. No signals corresponding to metabolites could be detected.

High resolution NMR imaging: Gd-DTPA labeled enzyme as a probe for permeability studies in polyacrylamide gels

The penetration of horse liver alcohol dehydrogenase (HLAD) molecules into polyacrylamide gel beads, which are used to immobilize the enzyme, was studied. HLAD was labeled with gadolinium diethylene-triamine-pentaacetic acid (Gd-DTPA), using the N-hydroxy-succinimide active ester of DTPA as a chelating agent. The HLAD-(Gd-DTPA)27 has a 3.7-fold larger longitudinal (R1) and a 14-fold larger transversal relaxivity (R2) (at 2.4 T) than the plain Gd-DTPA. A series of dry polyacrylamide gel beads, with total monomer concentration ranging from 5% to 30% were synthesized and swollen in a buffered solution of HLAD-(Gd-DTPA)27. The gel beads were examined with high resolution NMR imaging. The T1- and T2-weighted images revealed that the permeability for the labeled HLAD decreased with increasing total monomer concentration of the gel beads. These imaging results correlate fairly well with the enzymatic reactivities measured for the same range of gel beads but swollen in a solution of non labeled HLAD and NAD+ (nicotinamide adenine dinucleotide). It is concluded that Gd-labeling can be used to monitor the distribution of weakly concentrated, water soluble products in a solid matrix.

Optimalization Of NMR Micro Imaging (1.9T, 4.7T, 7.4T) Of The Central Nervous System Of An Animal Model For Multiple Sclerosis

Nuclear magnetic resonance (NMR) imaging of the central nervous system (CNS) is by far the most sensitive technique for the visualization of multiple sclerosis (MS) lesions. At this moment there is no clear correlation between NMR characteristics and neuropathological data. In this study, in vivo and in vitro high resolution NMR imaging was optimized in order to investigate the NMR characteristics of lesions in an animal model for MS. These characteristics will be correlated with the specific neuropathologic features of the lesions.

A Stereotactic Apparatus For The Optimalisation Of Brain NMR Micro Imaging In Tbe Study Of Multiple Sclerosis And Ischemia

NMR imaging was proven to be an effective method for the visualization of the central nervous system. In performing NMR micro imaging on small laboratory animals it is mandatory to achieve an excellent correlation between the NMR images and the corresponding histological sections to avoid erroneous interpretation of results. We developed therefore a stereotactical apparatus and an adapted RF coil to overcome the minute differences in the angles between the NMR images and the histologic sections. This apparatus was used in the study of two different animal models and allowed an excellent correlation between the NMR micro images and the histological sections.

NMR micro imaging of lesions of the central nervous system in animal models

An animal model of multiple sclerosis (MS) was studied to establish the neuropathological correlation of abnormal nuclear magnetic resonance (NMR) signals. Appropriate radio frequency coils are used to allow NMR imaging experiments with very high spatial resolution. Materials and methods for experiments on chronic recurrent experimental allergic encephalomyelitis (CREAE) in guinea pits are discussed, and a morphological study is described. CREAE was chosen as the animal model because of its similarities to MS. It was found that the lesions could be visualized by NMR imaging in vivo and in vitro. For the in vivo measurement there was a mini-extent required for the lesion to be detected. In vitro the sites and sizes of the areas with abnormal NMR signal correlated closely with inflammation and demyelination.<<ETX>>