First-pass renal perfusion imaging using MS-325, an albumin-targeted MRI contrast agent

RATIONALE AND OBJECTIVES

MR angiography is proving to be a useful clinical study for the diagnosis of vascular disorders of renal arteries. However, its utility in terms of stenosis characterization is still limited. Renal perfusion could provide supplemental information that could allow for a comprehensive evaluation of renal artery stenosis by MR imaging.

METHODS

MS-325 is a small-molecule blood pool agent that reversibly binds with serum albumin and hence leads to higher relaxivity and longer residence times in the blood. In this study, the authors evaluated the use of MS-325 to perform first-pass perfusion imaging and contrast-enhanced MR angiography in the characterization of renal artery stenosis in an animal model.

RESULTS

Quantitative perfusion estimates were obtained in the renal cortex (258 +/- 19.8 mL/min/100 g) and are comparable to microsphere measurements (198 +/- 12.2 mL/min/100 g), given the practical constraints. Based on these measurements, perfusion showed minimal changes even when the diameter reductions reached 75%.

CONCLUSIONS

MS-325 could provide quantitative perfusion estimates that when combined with MR angiography may lead to comprehensive evaluation of renal artery stenosis.

Three-dimensional MRI of coronary arteries using an intravascular contrast agent

To assess the effectiveness of an intravascular contrast agent, MS-325, for enhancing the vascular signal in coronary MR angiograms, six minipigs were studied using a three-dimensional, gradient-echo sequence with retrospective respiratory gating. To suppress the myocardial signal, preparatory RF pulses were applied before data acquisition. With the administration of MS-325, the blood signal-to-noise ratio increased by 97-276%, depending on the region of interest in which the blood signal was measured and the precontrast imaging sequence structures. The blood/myocardium contrast-to-noise ratio also significantly increased. High-resolution images (0.58 x 0.58 x 1 mm3) obtained from postmortem pig hearts demonstrated the potential delineation of coronary arteries with MS-325. In conclusion, this study supports further evaluation of the utility of MS-325 in improving coronary MR angiography in humans.

Steady-state and dynamic MR angiography with MS-325: initial experience in humans

PURPOSE

To evaluate the imaging performance and patient tolerance of a blood-pool contrast agent (MS-325) for magnetic resonance (MR) angiography.

MATERIALS AND METHODS

Imaging of peripheral and carotid vessels was performed in seven healthy volunteers in a phase I clinical trial of the gadolinium chelate MS-325. Each volunteer received an intravenous injection of 0.05 mmol/kg MS-325 over 30 seconds. Dynamic (arterial phase) and steady-state (arterial-venous phase) three-dimensional gradient-echo MR angiograms were acquired during, immediately after, and approximately 50 minutes after injection. Images were ranked (1 [poor] to 5 [excellent]) for overall image quality, and signal-to-noise ratio (S/N) and contrast-to-noise ratio (C/N) were measured by using standard techniques.

RESULTS

All volunteers tolerated the procedure well. The MS-325-enhanced studies demonstrated intense vascular signal. Mean peripheral arterial C/N was 12.9 +/- 4.8 (standard deviation), 78.8 +/- 29.4, 46.1 +/- 10.9, and 41.9 +/- 14.1 for the two-dimensional (2D) time-of-flight (TOF) and the contrast material-enhanced dynamic, early steady-state, and late steady-state images, respectively. Image quality of steady-state postcontrast images was statistically significantly (P < .02) higher than that of 2D TOF images. Image quality of early and late postcontrast images was similar, but a small (10%) decrease in C/N was noted from early to late images.

CONCLUSION

MS-325 provides excellent vascular and selective arterial enhancement during dynamic MR angiography. The long blood residence time also allows acquisition of steady-state images of the arteries and veins with excellent spatial resolution.

MS-325: albumin-targeted contrast agent for MR angiography

PURPOSE

To evaluate the protein-binding and signal enhancement characteristics of MS-325, a gadolinium-based magnetic resonance (MR) imaging blood pool agent that binds to albumin, and compare results with those obtained with existing gadolinium- and iron oxide-based agents.

MATERIALS AND METHODS

Protein binding in human plasma was measured by means of ultrafiltration. T1 relaxation times (20 MHz) were measured in human plasma or ex vivo samples from rabbits and monkeys injected with 0.1 mmol of MS-325 per kilogram of body weight. Imaging (three-dimensional fast imaging with steady-state precession, or FISP) was performed at 1.0 T in phantoms, which contained varying concentrations of different agents, or rabbits after injection of 0.015-0.100 mmol/kg MS-325.

RESULTS

MS-325 is 80%-96% bound in human plasma and exhibits a relaxivity approximately six to 10 times that of gadolinium diethylenetriaminepentaacetic acid (DTPA). Images of phantoms containing MS-325 were significantly brighter than those containing existing gadolinium chelates or iron particles (monocrystalline iron oxide nanoparticle, or MION) at equivalent concentrations. Findings of in vivo studies indicated strong, persistent plasma T1 reduction with MS-325 for 1 hour (T1 of MS-325, 50-100 msec; T1 of Gd-DTPA, 200-400 msec) and strong vascular enhancement on MR images.

CONCLUSION

MS-325 is highly protein bound after injection and provides vascular signal enhancement superior to that provided with other agents. As the first gadolinium-based blood pool agent in human trials, MS-325 has the potential to enhance both dynamic and steady-state MR angiograms.

Intravascular contrast agent improves magnetic resonance angiography of carotid arteries in minipigs

This study was designed to optimize three-dimensional (3D) time-of-flight (TOF) magnetic resonance angiography (MRA) sequences and to determine whether contrast-enhanced MRA could improve the accuracy of lumen definition in stenosed carotid arteries of minipigs. 3D TOF MRA was acquired with use of either an intravascular (n = 13) and/or an extravascular contrast agent (n = 5) administrated at 2 to 4 weeks after balloon-induced injury to a carotid artery in 16 minipigs. Vascular contrast, defined as signal intensity differences between blood vessels and muscle normalized to the signal intensity of muscle, was compared before and after the injection of each contrast agent and between the two agents. Different vascular patencies were observed among the animals, including completely occluded vessels (n = 5), stenotic vessels (n = 3), and vessels with no visible stenosis (n = 8). Superior vascular contrast improvement was observed for small arteries and veins and for large veins with the intravascular contrast agent when compared with the extravascular contrast agent. In addition, preliminary studies in two of the animals showed a good correlation for the extent of luminal stenosis defined by digital subtraction angiography compared with MRA obtained after administration of the intravascular contrast agent (R2 = .71, with a slope of .96 +/- .04 by a linear regression analysis). We concluded that use of an intravascular contrast agent optimizes 3D TOF MRA and may improve its accuracy compared with digital subtraction angiography.

Visual aftereffects and the consequences of visual system lesions on their perception in the rhesus monkey

This study examined the consequences of visual system lesions on visual aftereffects produced by achromatic stimuli of various luminance contrasts and chromatic stimuli of various wavelength compositions. The effects of repeated exposure to such adapting stimuli were assessed using probes whose luminance contrast and wavelength composition were systematically varied using both detection and discrimination paradigms. Interocular tests revealed that both peripheral and central mechanisms contribute to the visual aftereffects produced by the adapting stimulus arrays used in this study. Contrary to the hypothesis according to which the midget system of the retina is the conveyor of visual afterimages, we found that blocking this system with lesions of parvocellular lateral geniculate nucleus, through which the midget cells make their way to the striate cortex in primates, did not eliminate the visual aftereffects. It appears therefore that the parasol system of the retina, which courses through the magnocellular layers of the lateral geniculate nucleus to cortex, can convey the necessary signals for the generation of visual aftereffects. Lesions of areas V4 and MT did not have significant effects on the visual aftereffects studied suggesting that the central factors that contribute to the visual aftereffects occur either already in area V1 or are conveyed to higher centers through regions other than areas V4 and MT.

Broad-band visual capacities are not selectively impaired in Alzheimer's disease

Histological examination of the optic nerves of Alzheimer's disease (AD) patients has revealed a selective degeneration of large axon ganglion cells. This morphological abnormality raises the possibility of a selective impairment of broad-band channel visual function. To test this hypothesis, we administered visual psychophysical tests associated with either the color-opponent or the broad-band retinocortical channel to 14 AD patients and 29 elderly control subjects (ECS). In previous studies in monkeys, these tests had been sensitive to the effects of either parvocellular or magnocellular LGN lesions. In the present study, the color-opponent channel was assessed by tests of texture and color discrimination; the broad-band channel was assessed by tests of flicker and motion detection. Logistic regression analysis indicated that all tests collectively discriminated diagnostic groups at a borderline level of significance (p = 0.09). ANOVA also indicated a trend towards overall depressed function for AD patients on some capacities tested. Analyses comparing the prevalence of deficits in the AD and ECS groups showed that a significantly greater number of AD patients than ECS had deficits on texture discrimination, blue-violet discrimination, and 4.72 degrees/s motion detection. No individual subject demonstrated a selective impairment of broad-band channel function. The visual deficits in AD did not resemble those caused by lesions of magnocellular LGN in monkeys, indicating that the visual impairment in AD is not a functional reflection of damage limited to the broad-band channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ON channel blockade with 2-amino-4-phosphonobutyrate (APB) on brightness and contrast perception in monkeys

Four experiments were performed to assess the effects of ON channel blockade with the glutamate analog 2-amino-4-phosphonobutyrate (APB) on brightness and contrast perception in monkeys. In Experiment 1, we demonstrate that stimuli brighter than background (incremental stimuli) appear less bright following ON channel blockade. This decrease in brightness is not enough to account for the previously observed threshold increase for detection of incremental stimuli following APB administration (Schiller et al., 1986; Dolan & Schiller, 1989). Experiment 2 examines the role of the ON and OFF channels in the interaction between local contrast and apparent brightness. The phenomenon of simultaneous contrast was examined under normal conditions and following APB administration. We find that even following ON channel blockade, the brightness of a stimulus is determined primarily by its contrast with its immediate background. This indicates that the lateral processes involved in simultaneous contrast can operate even when one channel has been compromised. In Experiment 3, we examined the role of the ON channel in detection of stimuli that appear by virtue of changes in background vs. foreground luminance. We find that the ON channel selectively conveys information pertaining not only to the temporal nature that defines the stimulus as incremental but also to the spatial features that define it as incremental. In Experiment 4, we test the hypothesis that incremental and decremental temporal luminance ramps are differentially processed by the ON and OFF channels to a higher degree than are step-luminance changes. We find that the detection of incremental ramps is no more affected than is the detection of incremental steps following APB administration.

Evidence for only depolarizing rod bipolar cells in the primate retina

The mammalian rod bipolar, for which only one class has been identified, has been described as being hyperpolarizing by some investigators and depolarizing by others. We now report the effects of 2-amino-4-phosphonobutyrate (APB), a potent blocker of depolarizing bipolar cells, on visual behavior in the dark-adapted monkey. While in mesopic and photopic conditions only the monkeys' ability to detect incremental stimuli is impaired, under scotopic conditions all light mediated response in the monkey is eliminated. Assuming APB is acting on rod bipolars in the same fashion as it does on cone bipolars, we conclude that the primate rod bipolars all depolarize to light and that the ON and OFF channels are formed by the amacrine cell network.

Cell generation, death, and retinal growth in the development of the hamster retinal ganglion cell layer

During the early postnatal period in the hamster, the retinal ganglion cell layer grows, establishes its central connections, and undergoes substantial cell loss. In this study, we describe the development of the retinal ganglion cell layer with particular attention to the creation of local specializations in cell density. Changes in the number and spatial distribution of cells identified by a single 3H thymidine injection were examined through the period of maximal cell loss (postnatal days 4-10) and at adulthood. The cells of the retinal ganglion cell layer are generated from embryonic day 10 to postnatal day 3. Overall, cell number in the ganglion cell layer increases by approximately 108,000 cells (223%) from postnatal day 1 to 5, because of continued migration of cells generated prenatally. Cell number decreases from postnatal day 5 to 10 (25%), coincident with the presence of degenerating cells. Cell type is correlated with day of generation: the largest cells, all having retinal ganglion cell morphology, are generated on embryonic days 10 and 11; intermediate-sized cells predominantly of ganglion cell morphology on embryonic day 12; and smaller cells of displaced amacrine or glial cell morphology thereafter. At adulthood, the hamster retina shows a streaklike elevation of cell density through central retina. However, at the time of maximal cell number (postnatal day 5), cell density is uniform across the retina. During the period of cell degeneration, cells are lost in greater relative numbers from the retinal periphery. This cell loss occurs principally from the first-generated cells (embryonic days 10 and 11), as shown by both changes in the distribution of labeled cells and by the spatial pattern of labeled degenerating cells. From postnatal day 10 to adulthood, relative cell density continues to decline in the periphery of the retina, thus suggesting that differential growth completes the production of the adult cell density distribution.