Tissue magnetic susceptibility mapping as a marker of tau pathology in Alzheimer's disease

Alzheimer's disease is connected to a number of other neurodegenerative conditions, known collectively as 'tauopathies', by the presence of aggregated tau protein in the brain. Neuroinflammation and oxidative stress in AD are associated with tau pathology and both the breakdown of axonal sheaths in white matter tracts and excess iron accumulation grey matter brain regions. Despite the identification of myelin and iron concentration as major sources of contrast in quantitative susceptibility maps of the brain, the sensitivity of this technique to tau pathology has yet to be explored. In this study, we perform Quantitative Susceptibility Mapping (QSM) and T2* mapping in the rTg4510, a mouse model of tauopathy, both in vivo and ex vivo. Significant correlations were observed between histological measures of myelin content and both mean regional magnetic susceptibility and T2* values. These results suggest that magnetic susceptibility is sensitive to tissue myelin concentrations across different regions of the brain. Differences in magnetic susceptibility were detected in the corpus callosum, striatum, hippocampus and thalamus of the rTg4510 mice relative to wild type controls. The concentration of neurofibrillary tangles was found to be low to intermediate in these brain regions indicating that QSM may be a useful biomarker for early stage detection of tau pathology in neurodegenerative diseases.

Application of neurite orientation dispersion and density imaging (NODDI) to a tau pathology model of Alzheimer's disease

Increased hyperphosphorylated tau and the formation of intracellular neurofibrillary tangles are associated with the loss of neurons and cognitive decline in Alzheimer's disease, and related neurodegenerative conditions. We applied two diffusion models, diffusion tensor imaging (DTI) and neurite orientation dispersion and density imaging (NODDI), to in vivo diffusion magnetic resonance images (dMRI) of a mouse model of human tauopathy (rTg4510) at 8.5 months of age. In grey matter regions with the highest degree of tau burden, microstructural indices provided by both NODDI and DTI discriminated the rTg4510 (TG) animals from wild type (WT) controls; however only the neurite density index (NDI) (the volume fraction that comprises axons or dendrites) from the NODDI model correlated with the histological measurements of the levels of hyperphosphorylated tau protein. Reductions in diffusion directionality were observed when implementing both models in the white matter region of the corpus callosum, with lower fractional anisotropy (DTI) and higher orientation dispersion (NODDI) observed in the TG animals. In comparison to DTI, histological measures of tau pathology were more closely correlated with NODDI parameters in this region. This in vivo dMRI study demonstrates that NODDI identifies potential tissue sources contributing to DTI indices and NODDI may provide greater specificity to pathology in Alzheimer's disease.

•

We analyzed the microstructural changes in rTg4510 and wild type mice at 8.5 months.

•

We correlated microstructural findings with histological measures of tau burden

•

We compare two diffusion MR models: DTI and NODDI.

•

Both models revealed changes in tissue microstructure due to tau pathology.

•

The NODDI metrics demonstrated a good correlation with histological measures of tau burden.

Bimodal Imaging of Inflammation with SPECT/CT and MRI Using Iodine-125 Labeled VCAM-1 Targeting Microparticle Conjugates

Upregulation of cell adhesion molecules on endothelial cells is a hallmark of inflammation and an early feature of several neurological conditions. Here, we describe bimodal in vivo imaging of this inflammatory event in the brain using functionalized micron-sized particles of iron oxide. The particles were conjugated to anti-VCAM-1 antibodies and subsequently labeled with iodine-125. Radiolabeling of the antibody-coated particles was straightforward and proceeded in high radiochemical yields using commercially available iodination tubes. The corresponding contrast agent was evaluated in a rat model of cerebral inflammation based on intracerebral injection of tumor necrosis factor alpha and a rat model of status epilepticus. Biodistribution studies and phosphorimaging of cryosections were used to verify in vivo imaging data obtained with single photon emission computed tomography (SPECT) and magnetic resonance imaging (MRI). The contrast agent showed rapid and highly localized binding to the vasculature of inflamed brain tissue, and was effectively cleared from the blood pool within 2 min postinjection. Overall, the pattern of hypointensities observed with MRI was in good agreement with the distribution of the contrast agent as determined with SPECT and phosphorimaging; however, conspicuous differences in the signal intensities were observed. The results demonstrate that radiolabeled micron-sized particles of iron oxide enable multimodal in vivo imaging with MRI and nuclear techniques, and highlight the value of validating different imaging methods against one another.

In vivo imaging of tau pathology using multi-parametric quantitative MRI

As the number of people diagnosed with Alzheimer's disease (AD) reaches epidemic proportions, there is an urgent need to develop effective treatment strategies to tackle the social and economic costs of this fatal condition. Dozens of candidate therapeutics are currently being tested in clinical trials, and compounds targeting the aberrant accumulation of tau proteins into neurofibrillary tangles (NFTs) are the focus of substantial current interest. Reliable, translatable biomarkers sensitive to both tau pathology and its modulation by treatment along with animal models that faithfully reflect aspects of the human disease are urgently required. Magnetic resonance imaging (MRI) is well established as a valuable tool for monitoring the structural brain changes that accompany AD progression. However the descent into dementia is not defined by macroscopic brain matter loss alone: non-invasive imaging measurements sensitive to protein accumulation, white matter integrity and cerebral haemodynamics probe distinct aspects of AD pathophysiology and may serve as superior biomarkers for assessing drug efficacy. Here we employ a multi-parametric array of five translatable MRI techniques to characterise the in vivo pathophysiological phenotype of the rTg4510 mouse model of tauopathy (structural imaging, diffusion tensor imaging (DTI), arterial spin labelling (ASL), chemical exchange saturation transfer (CEST) and glucose CEST). Tau-induced pathological changes included grey matter atrophy, increased radial diffusivity in the white matter, decreased amide proton transfer and hyperperfusion. We demonstrate that the above markers unambiguously discriminate between the transgenic group and age-matched controls and provide a comprehensive profile of the multifaceted neuropathological processes underlying the rTg4510 model. Furthermore, we show that ASL and DTI techniques offer heightened sensitivity to processes believed to precede detectable structural changes and, as such, provides a platform for the study of disease mechanisms and therapeutic intervention.

Hepatic arterial spin labelling MRI: an initial evaluation in mice

The development of strategies to combat hepatic disease and augment tissue regeneration has created a need for methods to assess regional liver function. Liver perfusion imaging has the potential to fulfil this need, across a range of hepatic diseases, alongside the assessment of therapeutic response. In this study, the feasibility of hepatic arterial spin labelling (HASL) was assessed for the first time in mice at 9.4 T, its variability and repeatability were evaluated, and it was applied to a model of colorectal liver metastasis. Data were acquired using flow-sensitive alternating inversion recovery-arterial spin labelling (FAIR-ASL) with a Look-Locker readout, and analysed using retrospective respiratory gating and a T1 -based quantification. This study shows that preclinical HASL is feasible and exhibits good repeatability and reproducibility. Mean estimated liver perfusion was 2.2 ± 0.8 mL/g/min (mean ± standard error, n = 10), which agrees well with previous measurements using invasive approaches. Estimates of the variation gave a within-session coefficient of variation (CVWS) of 7%, a between-session coefficient of variation (CVBS) of 9% and a between-animal coefficient of variation (CVA) of 15%. The within-session Bland-Altman repeatability coefficient (RCWS) was 18% and the between-session repeatability coefficient (RCBS) was 29%. Finally, the HASL method was applied to a mouse model of liver metastasis, in which significantly lower mean perfusion (1.1 ± 0.5 mL/g/min, n = 6) was measured within the tumours, as seen by fluorescence histology. These data indicate that precise and accurate liver perfusion estimates can be achieved using ASL techniques, and provide a platform for future studies investigating hepatic perfusion in mouse models of disease.

Dexamethasone exacerbates cerebral edema and brain injury following lithium-pilocarpine induced status epilepticus

Anti-inflammatory therapies are the current most plausible drug candidates for anti-epileptogenesis and neuroprotection following prolonged seizures. Given that vasogenic edema is widely considered to be detrimental for outcome following status epilepticus, the anti-inflammatory agent dexamethasone is sometimes used in clinic for alleviating cerebral edema. In this study we perform longitudinal magnetic resonance imaging in order to assess the contribution of dexamethasone on cerebral edema and subsequent neuroprotection following status epilepticus. Lithium-pilocarpine was used to induce status epilepticus in rats. Following status epilepticus, rats were either post-treated with saline or with dexamethasone sodium phosphate (10 mg/kg or 2 mg/kg). Brain edema was assessed by means of magnetic resonance imaging (T₂ relaxometry) and hippocampal volumetry was used as a marker of neuronal injury. T₂ relaxometry was performed prior to, 48 h and 96 h following status epilepticus. Volume measurements were performed between 18 and 21 days after status epilepticus. Unexpectedly, cerebral edema was worse in rats that were treated with dexamethasone compared to controls. Furthermore, dexamethasone treated rats had lower hippocampal volumes compared to controls 3 weeks after the initial insult. The T₂ measurements at 2 days and 4 days in the hippocampus correlated with hippocampal volumes at 3 weeks. Finally, the mortality rate in the first week following status epilepticus increased from 14% in untreated rats to 33% and 46% in rats treated with 2 mg/kg and 10 mg/kg dexamethasone respectively. These findings suggest that dexamethasone can exacerbate the acute cerebral edema and brain injury associated with status epilepticus.

•

Dexamethasone given following seizures caused a worsening of edema and brain injury.

•

This finding also applied to doses dexamethasone as low as 2 mg/kg.

•

We show the use of automated hippocampal volume measurement for therapy monitoring.

Rapid magnetic cell delivery for large tubular bioengineered constructs

Delivery of cells into tubular tissue constructs with large diameters poses significant spatial and temporal challenges. This study describes preliminary findings for a novel process for rapid and uniform seeding of cells onto the luminal surface of large tubular constructs. Fibroblasts, tagged with superparamagnetic iron oxide nanoparticles (SPION), were directed onto the luminal surface of tubular constructs by a magnetic field generated by a k4-type Halbach cylinder device. The spatial distribution of attached cells, as measured by the mean number of cells, was compared with a conventional, dynamic, rotational cell-delivery technique. Cell loading onto the constructs was measured by microscopy and magnetic resonance imaging. The different seeding techniques employed had a significant effect on the spatial distribution of the cells (p < 0.0001). The number of attached cells at defined positions within the same construct was significantly different for the dynamic rotation technique (p < 0.05). In contrast, no significant differences in the number of cells attached to the luminal surface were found between the defined positions on the construct loaded with the Halbach cylinder. The technique described overcomes limitations associated with existing cell-delivery techniques and is amenable to a variety of tubular organs where rapid loading and uniform distribution of cells for therapeutic applications are required.

The importance of RF bandwidth for effective tagging in pulsed arterial spin labeling MRI at 9.4T

The movement towards MRI at higher field strengths (>7T) has enhanced the appeal of arterial spin labeling (ASL) for many applications due to improved SNR of the measurements. Greater field strength also introduces increased magnetic susceptibility effects resulting in marked B(0) field inhomogeneity. Although B(0) field perturbations can be minimised by shimming over the imaging volume, marked field inhomogeneity is likely to remain within the labeling region for pulsed ASL (PASL). This study highlights a potential source of error in cerebral blood flow quantification using PASL at high field. We show that labeling efficiency in flow-sensitive alternating inversion recovery (FAIR) displayed marked sensitivity to the RF bandwidth of the inversion pulse in a rat model at 9.4T. The majority of preclinical PASL studies have not reported the bandwidth of the inversion pulse. We show that a high bandwidth pulse of > = 15 kHz was required to robustly overcome the field inhomogeneity in the labeling region at high field strength, which is significantly greater than the inversion bandwidth ~2-3 kHz used in previous studies. Unless SAR levels are at their limit, we suggest the use of a high bandwidth labeling pulse for most PASL studies.

A viable isolated tissue system: a tool for detailed MR measurements and controlled perturbation in physiologically stable tissue

In vivo magnetic resonance imaging (MRI) assessment of neuronal tissue is prone to artifacts such as movement, pulsatile flow, and tissue susceptibility. Furthermore, stable in vivo scans of over 3 h are difficult to achieve, experimental design is therefore limited. Using isolated tissue maintained in a viable physiological state can mitigate many of these in vivo issues. This work describes the fabrication and validation of an MRI compatible viable isolated tissue maintenance chamber. Parameters measured from maintained rat optic nerves did not change significantly over 10 h: (i) mean axon radius [electron microscopy--0 h: 0.75±0.46; 5 h: 0.74±0.35; 10 h: 0.76±0.35 μm (P>>0.05, t-test], (ii) action potentials [grease-gap electrophysiology--4.89±0.16 mv, (P>>0.05, Pearson test], and (iii) diffusion tensor imaging parameters [fractional anisotropy: 0.86±0.02 (P>>0.05, Pearson test), mean diffusivity: 1.48E-06±9.74E-08 cm2/s, (P>>0.05, Pearson test)]. In addition, a thorough diffusion-weighted MR protocol demonstrated the comparable stability of viable isolated and chemically fixed rat optic nerve. This MRI compatible viable isolated tissue system allows researchers to probe neuronal physiology in a controlled environment by limiting in vivo artifacts and allowing extended MRI acquisitions.

MR image-guided investigation of regional signal transducers and activators of transcription-1 activation in a rat model of focal cerebral ischemia

BACKGROUND AND PURPOSE

STAT-1 is a member of a family of proteins called signal transducers and activators of transcription (STATs), and recent studies have shown its involvement in the induction of apoptosis. There is limited information on the role of STAT-1 following stroke. In this study we use MRI measurements of cerebral perfusion and bioenergetic status to target measurements of regional STAT-1 activity.

METHODS

Rats were subjected to 60 or 90 min of middle cerebral artery occlusion with and without reperfusion. MRI maps of the apparent diffusion coefficient of water and cerebral blood flow were acquired throughout the study. After the ischemia or reperfusion period, the brain was excised and samples were analyzed by Western blots using anti-phospho-STAT1 and anti-Fas antibodies. Regions were selected for analysis according to their MRI characteristics.

RESULTS

Transcriptional factor STAT-1 was enhanced in the lesion core and, to a lesser extent, in the lesion periphery, following ischemia and reperfusion. This level of activity was greater than for ischemia alone. Western blots demonstrated STAT-1 phosphorylation on tyrosine 701 and not serine 727 after ischemia and 3 h of reperfusion. Enhanced expression of the apoptotic death receptor Fas was confirmed after ischemia followed by reperfusion.

CONCLUSIONS

This study demonstrates that focal ischemia of the rat brain can induce STAT-1 activation, particularly following a period of reperfusion. The activation occurs not only in the lesion core, but also in the lesion periphery, as identified using MRI. STAT-1 may play an important role in the induction of cell death following stroke.

Simultaneous noninvasive measurement of CBF and CBV using double-echo FAIR (DEFAIR)

A new method for measuring cerebral blood flow (CBF) and cerebral blood volume (CBV) noninvasively using MRI is presented. The approach is based on the technique of arterial spin labelling (ASL), in which CBF-based contrast is generated by controlled modulation of the longitudinal magnetization of the blood. The proposed method also uses differences in T(2) between tissue and blood to differentiate the two compartments and allow assessment of the relative size of each. Two successive EPI images are acquired following spin preparation using either a slice-selective or global inversion pulse, and the technique is therefore referred to as double-echo FAIR (DEFAIR). DEFAIR is demonstrated in the normal gerbil brain and during hypothermia, where reductions of both CBF and CBV are known to occur. It is also shown theoretically that this method can be extended to include a measurement of oxygen extraction fraction. The main drawbacks of the technique are the long acquisition time and relatively low sensitivity to hemodynamic changes compared to conventional qualitative T2(*)-weighted BOLD contrast, which may limit its applicability and practical use in monitoring functional cerebral activation. However, the technique can be used repetitively in longer-term time course studies due to its noninvasive and quantitative nature.

Cerebrovascular reactivity following focal brain ischemia in the rat: a functional magnetic resonance imaging study

An essential goal of stroke research is to identify potentially salvageable regions of brain that may respond to therapy. However, current imaging methods are inadequate for this purpose. We therefore used dynamic magnetic resonance imaging of vascular reactivity following focal occlusion in the rat to determine whether measurement of perfusion reserve would help resolve this problem. We used the increase in blood-oxygen-level-dependent (BOLD) signal that occurs in normal brain following a CO2 challenge, to map vascular reactivity over the brain at 30-min intervals for 3.5 h after complete (CO) or partial (PO) focal ischemia. We assessed the regional correspondence between reactivity changes and areas of lowered apparent diffusion coefficient (ADC) and initial perfusion deficit. The area of lowered ADC was significantly smaller in the PO group compared to the CO group despite similar areas of perfusion deficit (P < 0.05). We identified four distinct areas within hypoperfused brain: a core area with low/absent reactivity and low ADC; borderzone areas with normal reactivity and either reduced ADC (CO group) or normal ADC (PO group); and an area with normal ADC and reduced/absent reactivity. In all ischemic regions, the BOLD peak arrival time in the brain was delayed or absent. There was a negative correlation between BOLD peak latency time and ADC (r = -0.42, P < 0.001), although latency alone did not differentiate individual ischemic regions. In conclusion, combining perfusion, ADC, and vascular reactivity mapping of the ischemic brain enables improved discrimination of core and borderzone regions.

High resolution MRI reveals global changes in brains of Cln3 mutant mice

Batten disease, the juvenile-onset form of neuronal ceroid lipofuscinosis (NCL), is a progressive neurodegenerative disorder of childhood with an age of onset of 5-10 years of age. JNCL is caused by mutations in the CLN3 gene which encodes a membrane protein of unknown function. Magnetic resonance imaging of the brain of juvenile NCL patients has revealed changes in signal intensity and tissue atrophy, predominantly in the cortex and cerebellum. A mouse model for Batten disease was created by targeted disruption of the murine Cln3 gene in order to further understanding of the pathophysiology of Batten disease and to evaluate potential therapeutic approaches. Several features of the disease are displayed by Cln3 mice including accumulation of characteristic storage material in neurons. The aim of this work was to investigate neurodegeneration in the Cln3 mouse model using high resolution magnetic resonance imaging to measure signal intensity ratios in selected regions of interest. Global changes were observed in the brains of 12-month-old mutant mice that mirror those seen in juvenile NCL patients. There is a decrease in signal intensity ratio in grey matter regions including cortex, hippocampus and cerebellum, tissues where neuronal storage accumulation and cell loss have been seen in the mouse model. The alterations seen in Cln3 mutant mice support the validity of further imaging studies and suggest that this method will have application in assessment of therapeutic approaches in the study of mutant mouse models of NCL including the Cln3 mouse.

Burst excitation for quantitative diffusion imaging with multiple b-values

A quantitative imaging sequence has been developed to exploit the intrinsic sensitivity of Burst NMR data to molecular diffusion. In the scan time of a single spin echo experiment, it is possible to acquire many images of the same slice, with a different T(2) and diffusion weighting. Under favorable conditions, it is possible to obtain both the diffusion coefficient and T(2) from the same experiment; or, by correcting for T(2) relaxation using a control image, more precise diffusion coefficients may be measured. The quantitative values in rat brain are in agreement with those from conventional experiments. The major gains of this method are the potentially reduced scan time, the higher number of acquired images corresponding to different diffusion weightings, the reduced sensitivity to inter-scan motion artifact and to local variations in magnetic susceptibility, and an automatic co-registration between T(2) and diffusion images. Problems with the sequence include a lower signal-to-noise ratio than is achievable with diffusion-weighted spin-echo imaging, the limitation of measuring only in-plane components of diffusion and, at present, single-slice acquisition.

Acute changes in MRI diffusion, perfusion, T(1), and T(2) in a rat model of oligemia produced by partial occlusion of the middle cerebral artery

Oligemic regions, in which the cerebral blood flow is reduced without impaired energy metabolism, have the potential to evolve toward infarction and remain a target for therapy. The aim of this study was to investigate this oligemic region using various MRI parameters in a rat model of focal oligemia. This model has been designed specifically for remote-controlled occlusion from outside an MRI scanner. Wistar rats underwent remote partial MCAO using an undersize 0.2 mm nylon monofilament with a bullet-shaped tip. Cerebral blood flow (CBF(ASL)), using an arterial spin labeling technique, the apparent diffusion coefficient of water (ADC), and the relaxation times T(1) and T(2) were acquired using an 8.5 T vertical magnet. Following occlusion there was a decrease in CBF(ASL) to 35 +/- 5% of baseline throughout the middle cerebral artery territory. During the entire period of the study there were no observed changes in the ADC. On occlusion, T(2) rapidly decreased in both cortex and basal ganglia and then normalized to the preocclusion values. T(1) values rapidly increased (within approximately 7 min) on occlusion. In conclusion, this study demonstrates the feasibility of partially occluding the middle cerebral artery to produce a large area of oligemia within the MRI scanner. In this region of oligemic flow we detect a rapid increase in T(1) and decrease in T(2). These changes occur before the onset of vasogenic edema. We attribute the acute change in T(2) to increased amounts of deoxyhemoglobin; the mechanisms underlying the change in T(1) require further investigation.

The measurement of diffusion and perfusion in biological systems using magnetic resonance imaging

The aim of this review is to describe two recent developments in the use of magnetic resonance imaging (MRI) in the study of biological systems: diffusion and perfusion MRI. Diffusion MRI measures the molecular mobility of water in tissue, while perfusion MRI measures the rate at which blood is delivered to tissue. Therefore, both these techniques measure quantities which have direct physiological relevance. It is shown that diffusion in biological systems is a complex phenomenon, influenced directly by tissue microstructure, and that its measurement can provide a large amount of information about the organization of this structure in normal and diseased tissue. Perfusion reflects the delivery of essential nutrients to tissue, and so is directly related to its status. The concepts behind the techniques are explained, and the theoretical models that are used to convert MRI data to quantitative physical parameters are outlined. Examples of current applications of diffusion and perfusion MRI are given. In particular, the use of the techniques to study the pathophysiology of cerebral ischaemia/stroke is described. It is hoped that the biophysical insights provided by this approach will help to define the mechanisms of cell damage and allow evaluation of therapies aimed at reducing this damage.

Reperfusion in a gerbil model of forebrain ischemia using serial magnetic resonance FAIR perfusion imaging

BACKGROUND AND PURPOSE

Existing methods for the quantitative measurement of the changing cerebral blood flow (CBF) during reperfusion suffer from poor spatial or temporal resolution. The aim of this study was to implement a recently developed MRI technique for quantitative perfusion imaging in a gerbil model of reperfusion. Flow-sensitive alternating inversion recovery (FAIR) is a noninvasive procedure that uses blood water as an endogenous tracer.

METHODS

Bilateral forebrain ischemia of 4 minutes' duration was induced in gerbils (n=8). A modified version of FAIR with improved time efficiency was used to provide CBF maps with a time resolution of 2.8 minutes after recirculation had been initiated. Quantitative diffusion imaging was also performed at intervals during the reperfusion period.

RESULTS

On initiating recirculation after the transient period of ischemia, the FAIR measurements demonstrated either a symmetrical, bilateral pattern of flow impairment (n=4) or an immediate side-to-side difference that became apparent with respect to the cerebral hemispheres in the imaged slice (n=4). The flow in each hemisphere displayed a pattern of recovery close to the preocclusion level or, alternatively, returned to a lower level before displaying a delayed hypoperfusion and a subsequent slow recovery. The diffusion measurements during this latter response suggested the development of cell swelling during the reperfusion phase in the striatum.

CONCLUSIONS

The CBF during the reperfusion period was monitored with a high time resolution, noninvasive method. This study demonstrates the utility of MRI techniques in following blood flow changes and their pathophysiological consequences.

Implementation of quantitative FAIR perfusion imaging with a short repetition time in time-course studies

Flow-sensitive alternating inversion recovery (FAIR) is a pulsed arterial spin labeling magnetic resonance imaging method for perfusion quantification. In its standard implementation for quantification with full longitudinal relaxation between acquisitions, its use in time-course investigations of rapidly changing flow values is limited. The time efficiency can be improved by decreasing the repetition time but quantification becomes problematic. This situation is further complicated if a whole-body radiofrequency transmit coil is not used since fresh blood spins will flow in from outside the coil. To alleviate these problems, the use of global pre-saturation is proposed. The resulting expression for the flow signal depends on the relationship between the imaging parameters and the coil inflow time and can be significantly simplified under certain combinations of these parameters. With this implementation of FAIR, quantitative flow maps of gerbil brains were obtained with a 3 minute time resolution in a study of the effects of reperfusion. The pre-occlusion flow measurements were in good agreement with values obtained by the standard FAIR implementation and by other techniques, but the low values following occlusion were underestimated due to the increased transit times.

The relationship between magnetic resonance diffusion imaging and autoradiographic markers of cerebral blood flow and hypoxia in an animal stroke model

This study examined the relationship between magnetic resonance diffusion imaging and autoradiographic markers of cerebral blood flow (99mTc-hexamethylpropylene amine oxime) and cerebral hypoxia (125I-iodoazomycin arabinoside) in a rat model of stroke. Middle cerebral artery occlusion in the rat was performed using an intraluminal suture approach. Diffusion, hypoxia, and blood flow maps were acquired 2 hr following occlusion, and were compared with T2 images and histology at 7 hr. Two hours following middle cerebral artery occlusion the lesion distributions from the diffusion maps and hypoxic autoradiographs were similar. The blood flow threshold for increased uptake of the hypoxic marker was approximately 34 +/- 7% of the normal flow. The combination of diffusion or hypoxic images with perfusion maps allowed differentiation between four regions: 1) normal tissue; 2) a region of decreased perfusion but normal diffusion and normal uptake of hypoxic marker; 3) a region of decreased perfusion, decreased diffusion and increased uptake of hypoxic marker; 4) a region of decreased perfusion, decreased diffusion and low uptake of hypoxic marker. The areas for increased uptake of hypoxic marker and decreased diffusion are equivalent, indicating similar blood flow thresholds. Regions of oligaemic misery perfusion, ischaemic misery perfusion and lesion core may be delineated with the combination of diffusion or hypoxic images and perfusion maps.

Early changes in water diffusion, perfusion, T1, and T2 during focal cerebral ischemia in the rat studied at 8.5 T

The time evolution of water diffusion, perfusion, T1, and T2 is investigated at high magnetic field (8.5 T) following permanent middle cerebral artery occlusion in the rat. Cerebral blood flow maps were obtained using arterial spin tagging. Although the quantitative perfusion measurements in ischemic tissue still pose difficulties, the combined perfusion and diffusion data nevertheless distinguish between a "moderately affected area," with reduced perfusion but normal diffusion; and a "severely affected area," in which both perfusion and diffusion are significantly reduced. Two novel magnetic resonance imaging observations are reported, namely, a decrease in T2 and an increase in T1, both within the first few minutes of ischemia. The rapid initial decrease in T2 is believed to be associated with an increase in deoxyhemoglobin levels, while the initial increase in T1 may be related to several factors, such as flow effects, an alteration in tissue oxygenation, and changes in water environment.

Assessment of various parameters in the estimation of differential renal function using technetium-99m mercaptoacetyltriglycine

Differential renal function (DRF) is an important parameter that should be assessed from virtually every dynamic renogram. With the introduction of technetium-99m mercaptoacetyltriglycine (99mTc-MAG3), a tracer with a high renal extraction, the estimation of DRF might hopefully become accurate and reproducible both between observers in the same institution and also between institutions. The aim of this study was to assess the effect of different parameters on the estimation of DRF. To this end we investigated two groups of children: group A, comprising 35 children with a single kidney (27 of whom had poor renal function), and group B, comprising 20 children with two kidneys and normal global function who also had an associated 99mTc-dimercaptosuccinic acid scan (99mTc-DMSA). The variables assessed for their effect on the estimation of DRF were: different operators, the choice of renal regions of interest (ROIs), the applied background subtraction, and six different techniques for analysis of the renogram. The six techniques were based on: linear regression of the slopes in the Rutland-Patlak plot, matrix deconvolution, differential method, integral method, linear regression of the slope of the renograms, and the area under the curve of the renogram. The estimation of DRF was less dependent upon both observer and method in patients with two normally functioning kidneys than in patients with a single kidney. The inter-observer comparison among children in either group was not dependent on either ROI or background subtraction. However, in patients with poor renal function the method of choice for the estimation of DRF was dependent on background subtraction, though not ROI. In children with two kidneys and normal renal function, the estimation of DRF from the 24 techniques gave similar results. Methods that produced DRF values closest to expected results, from either group of children, were the Rutland-Patlak plot and matrix deconvolution methods.

A quantitative method for fast diffusion imaging using magnetization-prepared TurboFLASH

For the in vivo measurement of the apparent diffusion coefficient (ADC), it is desirable for the total imaging time to be as short as possible. One technique is based on a TurboFLASH acquisition in which the diffusion gradients are inserted into a driven equilibrium Fourier transform (DEFT) combination of hard pulses. However, this sequence has the disadvantage that eddy current-induced inhomogeneities lead to incomplete refocusing of the magnetization during the diffusion preparation and to incorrect ADC values. A modification to the sequence is suggested that eliminates this error by phase-cycling the second 90 degrees pulse of the preparation. This study also investigates the effect of a reduced delay time between acquisitions on the accuracy of the measurement. The quality of the TurboFLASH sequence is demonstrated by experimental validation on an agar phantom and in vivo on the rat brain using a high-field (8.5 T) system. Reduction of the interexperiment delay time is shown to be achievable to a certain degree without compromising the measurement accuracy.

Estimation and relevance of depth correction in paediatric renal studies

Measurement of absolute renal function by gamma camera techniques requires knowledge of kidney depth to correct for soft tissue attenuation, there is debate about the need to take depth into account when only relative renal function is estimated. The aim of this study was to derive a formula for renal depth in children and to assess the importance of depth correction when relative renal function is assessed with dimercaptosuccinic acid (DMSA) on the gamma camera. In this study, kidney depths were derived from measurements on abdominal computerised tomography (CT) images in 57 children in the supine position with two normally located kidneys. Using best-subset regression analysis, one formula for both left and right kidney depth (KD, cm) was developed based on the easily measured parameters of height (H, cm) and body weight (W, kg). The inclusion of extra variables was found to significantly improve the model compared with a model using weight alone (P<0.005). A second group of 19 children who underwent technetium-99m DMSA scans, had differential function estimated from both anterior and posterior views and the geometric mean method. The mean difference in differential renal function calculated by the geometric mean method versus the posterior image was only 1.2%. In conclusion, we present a new formula for the estimation of paediatric kidney depth for the absolute quantitation of kidney uptake. Further, for normally located kidneys it appears unnecessary to use the geometric mean method or to correct for individual renal depth when calculating differential function.

Dosimetry of pediatric radiopharmaceuticals: uniformity of effective dose and a simple aid for its estimation

Formulae were investigated for predicting the effective dose to children, per unit administered activity of various pediatric radiopharmaceuticals, based only on the weight of the patient. Their influence on the uniformity of effective dose from total administered activity was also examined.

METHODS

The formulae were obtained from calculations of effective dose per unit administered activity (mSv x MBq[-1]) for five anthropomorphic mathematical phantoms applicable for newborn, 1-yr-, 5-yr-, 10-yr- and 15-yr-old children, having body weights of 3.4, 9.8, 19, 32 and 57 kg, respectively, using published biokinetic models.

RESULTS

In general, there was good linear correlation between effective dose per unit administered activity and inverse weight but, for some radiopharmaceuticals, logarithmic regression on weight provided a better fit to the data. An administered activity schedule based on body surface area, used with these formulae, resulted in reasonable uniformity of effective dose for children of all ages, with varying degrees of uniformity for different radiopharmaceuticals (coefficient of variation (COV) up to 20%). Individual activity schedules for separate radiopharmaceuticals gave best uniformity (COV < 7%) while a single general schedule, based on the mean results of the present study, yielded acceptable uniformity (COV < or = 10%) over the pediatric range.

CONCLUSION

Effective dose per unit administered activity (mSv x MBq[-1]) of pediatric radiopharmaceuticals can be predicted from body weight alone by using simple formulae, and appropriately choosing the administered activity schedule leads to similar values of effective dose for children of all ages from a given radiopharmaceutical procedure.

Development of a radiopharmaceutical activity schedule for technetium-99m dimercaptosuccinic acid in children based on image quality criteria

The aim of this study was to determine an activity schedule (amount of administered activity in relation to body weight) for technetium-99m dimercaptosuccinic acid examinations in children, from information present in renal scintigraphic images. Scans from 48 children (5 weeks to 14.8 years old) were graded for image quality according to the clarity of both kidney outline and internal structure. Numerical image data (kidney and background counts, signal-to-noise ratio) were associated with these subjective gradings to formulate three criteria, specifying the required values of the above-measured parameters to yield optimum grades of image quality. When applied to derived functions, a kidney uptake of 20% was required to satisfy the criterion based on the signal-to-noise ratio. Using this value with the other two criteria predicts the form of the weight-dependent activity schedule as a function of imaging time. Examples of schedules for imaging times of 300 and 600 s are compared with a schedule based on surface area.

Effects of diffusion anisotropy on lesion delineation in a rat model of cerebral ischemia

The effects of white and gray matter diffusion anisotropy on ischemic lesion delineation have been studied in the rat model of middle cerebral artery occlusion. Apparent diffusion coefficient (ADC) maps obtained by conventional pulsed gradient spin echo diffusion-weighted imaging (PGSE-DWI) were compared with maps of the trace of the diffusion tensor in both normal and occluded animals. Diffusion tensor trace maps were derived from the average of the ADC maps from three separate experiments with diffusion weighting along three orthogonal axes, and also from a single-scan method. A marked degree of diffusion anisotropy was observed in both cortical gray matter and white matter from ADC maps of the control animals. In the occluded animals, the systematic effects of anisotropy on ADC and lesion area influenced the delineation of the ischemic territory in the PGSE-DWI ADC maps. However, the two trace methods eliminated these effects and gave consistent ischemic lesion depiction, despite the use of differing diffusion times in the two measurements.

Autoradiographic imaging of cerebral ischaemia using a combination of blood flow and hypoxic markers in an animal model

Current routine clinical techniques, including angiography and perfusional single-photon emission tomography, can be used to indicate problems in cerebral vascular supply and areas of cerebral hypoperfusion following a stroke, but cannot distinguish between ischaemic core and penumbra. In order to image specifically the penumbra, a method or indicator should be able to define areas with reduced blood flow, and a degree of metabolic compromise. In this context, the tissue could be regarded as hypoxic rather than ischaemic, and we have therefore chosen to investigate the potential of radio-labelled hypoxic markers in the study of ischaemia. In order to combine a hypoxic marker with a blood flow marker we used technetium-99m hexamethylpropylene amine oxime (99mTc-HMPAO) and iodine-125 iodoazomycin arabinoside (125I-IAZA), during cerebral ischaemia in the rat middle cerebral artery occlusion model. 99mTc-HMPAO and 125I-IAZA were injected simultaneously 2 h following occlusion of the middle cerebral artery, and 5 h before decapitation. Paired autoradiograms were produced and compared. Three distinct patterns emerged from the autoradiograms: slightly decreased perfusion with no uptake of the hypoxic marker indicating an area of misery perfusion; moderately decreased perfusion with concomitant uptake of iodoazomycin arabinoside, a region of hypoxia; and severely decreased perfusion with no retention of the hypoxic tracer. In conclusion, we present a new use for an imaging agent in the investigation of cerebral hypoxia. This agent, IAZA together with HMPAO, provides a means of separating the penumbra into regions of misery perfusion and hypoxia. The potential impact of this may be important in the clinical investigation of stroke.

Retrospective study of plasma exchange in patients with idiopathic rapidly progressive glomerulonephritis and vasculitis

A retrospective study of 48 patients was conducted to evaluate the efficacy of plasma exchange in children with idiopathic rapidly progressive glomerulonephritis (IRPGN), and renal or non-renal vasculitis. All patients were followed up at a single centre over a 15 year period. Treatment consisted of corticosteroids and/or cytotoxic agents. Plasma exchange was used in all patients because of severe renal involvement and/or clinical deterioration. One hundred per cent of patients with renal vasculitis who started plasma exchange within one month of disease onset and 58% of cases with IRPGN had significant improvement in renal function. No relapses of vasculitis were observed after treatment with plasma exchange in patients with renal and non-renal vasculitis. The results suggest that plasma exchange associated with immunosuppressive treatment could be of benefit in cases of IRPGN or vasculitis in terms of both renal and extrarenal recovery.

Biokinetic behavior of technetium-99m-DMSA in children

After intravenous administration of 99mTc-DMSA, biokinetic data were collected from studies on 24 children aged from 5 wk to 14.8 yr (15 normal and 9 with renal pathology).

METHODS

Patients were imaged with a gamma camera up to 30 hr postinjection and the absolute activities in the kidneys, liver, spleen, bladder, knees and whole body were estimated using an attenuation-corrected conjugate counting technique. Renal uptake and elimination rates and urinary excretion of radioactivity were also measured.

RESULTS

In children with normal renal function, maximal kidney uptake was 42.4% +/- 5.4% and was taken up with a half-time of 1.0 +/- 0.2 hr. Renal excretion amounted to 18.0% +/- 4.4% at 24 hr and was lowest in children aged less than 1 yr. In children with abnormal renal function, apart from the expected reduction in renal uptake there was evidence of wider variations in uptake rate and increased urinary excretion. Mean uptakes in liver and spleen were approximately 5% and 2%, respectively, in all patients and uptake in knees, assumed to reside in the metaphyseal growth complexes, was 1.4%.

CONCLUSION

In children with normal renal function, there was little evidence of age-dependent biokinetic factors other than reduced urinary excretion and lower uptake in knees in children aged less than 1 yr. The results therefore suggest that a single biokinetic model may suffice for radiation dosimetry purposes in normal children irrespective of age.

Radiation dosimetry of technetium-99m-DMSA in children

Radiation dosimetry was performed on 24 children (aged 5 wk to 14.8 yr) who were undergoing routine diagnostic investigation of renal impairment with 99mTc-DMSA.

METHODS

Organ doses were calculated using MIRDOSE 3 with biokinetic data obtained in previously described studies, and effective doses and effective dose equivalents were estimated. Interpolation by inverse weight between pediatric anthropomorphic phantoms was compared with age-matching to discrete phantoms. Administered activities were scaled by body surface area from the adult activity of 100 MBq and the resulting radiation doses in normal children were compared with those that would have resulted from a schedule based on body weight.

RESULTS

The effective doses estimated by interpolation differed by up to 46% from those based on discrete phantoms and showed less variation. In children with normal bilateral renal function, the mean effective dose per administered activity was 0.91 +/- 0.08 mSv or 0.98 +/- 0.29 mSv by the two methods, respectively. Renal pathology reduced the effective dose, on average, by 15% of the value for normal patients.

CONCLUSION

Over the pediatric age range, the uniformity of effective dose values was improved by scaling the administered activity according to body surface area rather than to body weight.

Mercapto-acetyltriglycine (MAG 3) renography and indirect radionuclide cystography in posterior urethral valves

OBJECTIVE

To assess dynamic isotope renography with 99mTc-labelled mercapto-acetyltriglycine (MAG 3) in conjunction with indirect radionucleide cystography (IRC) in the follow-up of boys with posterior urethral valves.

PATIENTS AND METHODS

Fifty-one boys (95 renal units; mean age = 89 months, range 65-118) with previously treated posterior urethral vales underwent MAG 3 renography followed by IRC. Glomerular filtration rate (GFR) was estimated from the plasma clearance of 51Cr-ethylenediamine tetra-acetic acid following single intravenous injection. Renographic parameters noted were renal function based on the quality of renal visualization and background activity as well as renal drainage and the appearance of isotope in ureter and bladder. Following the dynamic 20 min MAG 3 renogram the child was allowed to leave the department and to return when he wished to void. IRC was then performed. Diuretics were not administered.

RESULTS

Initial drainage from 30 kidneys was normal, in 36 drainage was delayed and in 25 drainage did not occur during the renogram phase. In four kidneys function was so poor as to preclude assessment. After the IRC, drainage was noted in 17 of 25 units which had not drained during the renogram, six units did not drain and in two drainage was equivocal due to the presence of vesico-ureteric reflux. The quality of the scan was good or very good in 42 patients (mean GFR = 92 ml/min/1.73 m2 SA) and moderate or poor in nine patients (median GFR = 20 ml/min/1.73 m2 SA) (P < 0.001). A residual urine was noted following micturition in 35 boys; this was due to incomplete bladder emptying in 14 and to immediate secondary refilling from dilated upper tracts in the remainder.

CONCLUSION

IRC in conjunction with traditional dynamic renography using MAG 3 provides valuable additional information about upper tract drainage. The necessity for diuresis renography is obviated in the majority of patients. IRC also allows an excellent non-invasive, physiological assessment of both upper and lower urinary tract function which is superior to conventional single examination techniques. We recommend the use of MAG 3 with IRC in the routine follow-up of boys with posterior urethral valves.

Effect of renal maturation on the clearance of technetium-99m mercaptoacetyltriglycine

To investigate the effect of growth and maturation on the global kidney clearance of technetium-99m mercaptoacetyltriglycine (99mTc-MAG3), we investigated 509 children who attended for 99mTc-MAG3 renography in our department. In order to estimate the normal maturation of the kidney, only children who were classified as "normal" using diagnostic criteria were included in this study (n = 109). Kidney clearance was calculated using a gamma camera technique and a 20-min blood sample. There was a progressive increase in the clearance of 99mTc-MAG3 throughout childhood and into adolescence (mean clearance value below 2 month was 31.4 ml/min, mean clearance value above 12 years was 287 ml/min). When clearance was normalised to body surface area (BSA) a different pattern was seen, with a progressive increase from 3 weeks of age (mean value < 0.2 years: 208 ml/min/1.73 m2) to a plateau at the end of the first year of life (mean value > 2 years: 303 ml/min/1.73 m2). Maturation of the kidney as defined by the normalised clearance of 99mTc-MAG3 is therefore complete by the end of the 1st year of life, after which any change in non-corrected clearance values may be attributed to growth. To investigate the appropriate normalisation factor for 99mTc-MAG3 clearance in children, clearance values were compared to height, weight and BSA. The relationships were described using a linear model; the correlation coefficients demonstrated that BSA has the highest strength of relationship with 99mTc-MAG3 clearance. Body surface area may be used over 1 year of age to normalise the clearance values of 99mTc-MAG3.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertension in paediatrics: can pre- and post-captopril technetium-99m dimercaptosuccinic acid renal scans exclude renovascular disease?

In children over 1 year of age, renal disease is the commonest cause of hypertension. Arteriography is considered the reference method to establish the diagnosis of renovascular disease; however, it is an invasive technique with a high radiation burden for children. This was a retrospective study of pre- and post-capto-technetium-99m dimercaptosuccinic acid (DMSA) imaging compared with arteriography in 18 children between the ages of 3 and 17 years. Alone, the 99mTc-DMSA scan is a sensitive indicator of renal parenchymal disease, although non-specific in suggesting the pathology. The combination of pre- and post-captopril studies may increase the sensitivity and specificity in the diagnosis of renovascular disease in the presence of hypertension. This work suggests that a screening investigation with a low radiation burden can be carried out at most institutions; if the investigation is positive, there will be a high index of suspicion that renovascular disease is the cause of the hypertension.

Can technetium-99m-mercaptoacetyltriglycine replace technetium-99m-dimercaptosuccinic acid in the exclusion of a focal renal defect?

The presence of focal renal damage dictates different management of a child with urinary tract infection (UTI) compared with children who have normal kidneys. Technetium-99m-dimercaptosuccinic (DMSA) has a high sensitivity in the detection of a focal defect, and allows estimation of differential function. The introduction of 99mTc-MAG3 with high renal extraction suggests that this may be useful in children with UTI but its role remains speculative. Fifty-nine children with previous UTI underwent both 99mTc-DMSA and MAG3 within 4 wk of each other. Differential function and assessment of the images were undertaken. There is close correlation (R2 = 0.97) between the differential function. Analysis of the 99mTc-DMSA and functional MAG3 images showed that the functional image had a specificity of 88% and a sensitivity of 88% in the detection of a focal parenchymal defect. Technetium-99m-MAG3 in the clinical setting of a child with UTl allows accurate assessment of differential function and a high probability of detecting a focal renal abnormality.

Can dynamic krypton-81m imaging separate regional ventilation and volume?

This study explores the assumption that 81mKr static images represent regional ventilation. Dynamic acquisition of 81mKr ventilation images permits creation of time-activity curves and the possible separation of the confounding influences of ventilation and volume. By using a two-compartment gas mixing lung phantom, the results demonstrate that both total and tidal 81mKr are closely related to regional ventilation. In 61 children and 15 adult volunteers, there was good agreement between fractional ventilation assessed by total and tidal 81mKr. The dynamic steady-state ventilation image can be analyzed to separate tidally exchanged and resident 81mKr. This may allow regional ventilation to be distinguished from regional volume.