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Biomedical subjects

Jens H Jensen

Publications and source records attributed to Jens H Jensen.

10 recordsLinked to original sources
PubMed2006

Magnetic field correlation imaging.

A magnetic resonance imaging (MRI) method is presented for estimating the magnetic field correlation (MFC) associated with magnetic field inhomogeneities (MFIs) within biological tissues. The method utilizes asymmetric spin echoes and is based on a detailed theory for the effect of MFIs on nuclear magnetic resonance (NMR) signal decay. The validity of the method is supported with results from phantom experiments at 1.5 and 3 T, and human brain images obtained at 3 T are shown to demonstrate the method's feasibility. The preliminary results suggest that MFC imaging may be useful for the quantitative assessment of iron within the brain.

Algorithms↗
PubMed2006

Diffusional kurtosis imaging in the lung using hyperpolarized 3He.

Diseases of the small airspaces represent an increasingly important health problem. Asthma is primarily a disease of airway dysfunction, while chronic obstructive pulmonary disease (COPD) is associated with abnormalities in both the small airways and the alveoli. Conventional diffusion magnetic resonance imaging (MRI) of hyperpolarized noble gases, because of the short T(2)* of the gas, is only capable of monitoring diffusion over short times and hence only short distances. Diffusion imaging is therefore only sensitive to changes in small structures of the lung (primarily the alveoli), and will not adequately interrogate diffusion along the longitudinal axes of bronchi and bronchioles. In this communication we present a new method, termed diffusional kurtosis imaging (DKI), that is particularly sensitive to diffusion over longer distances. DKI may therefore be more sensitive to abnormalities in the bronchioles and bronchi than conventional diffusion imaging. Preliminary DKI measurements on healthy human subjects and one patient with symptoms suggestive of small airway disease are presented. Although the apparent diffusion coefficient (ADC) in the patient was similar to that in the normal controls, diffusional kurtosis was markedly reduced. This suggests that DKI measurements may be useful for assessing diseases of the small airways.

Artifacts↗
PubMed2006

Microvessel density estimation in the human brain by means of dynamic contrast-enhanced echo-planar imaging.

Animal studies have shown that in vivo estimates of microvessel density in the brain may be obtained from an MRI-measurable index (Q) provided that a sufficiently high dose of an intravascular paramagnetic contrast agent is employed. Q is determined from the shifts in the transverse relaxation rates induced by the contrast agent, and a high dose is required for the validity of analytic expressions relating Q to the microvessel density. However, the steady-state imaging techniques used in these prior investigations are not appropriate for humans, as the required contrast agent dose is too large. Here results of a pilot study with three subjects are reported. The results suggest that reliable Q measurements can be performed in the human brain at 1.5 T by using an interleaved spin-echo (SE)/gradient-echo (GE) echo-planar imaging (EPI) sequence and a bolus injection of a triple dose of Gd-DTPA. Lower- and upper-bound estimates for the microvessel density were derived from the Q-values, and were found to be in reasonable accord with previously cited values determined by histology.

Adult↗
PubMed2006

Three-dimensional characterization of non-gaussian water diffusion in humans using diffusion kurtosis imaging.

Conventional diffusion tensor imaging (DTI) measures water diffusion parameters based on the assumption that the spin displacement distribution is a Gaussian function. However, water movement in biological tissue is often non-Gaussian and this non-Gaussian behavior may contain useful information related to tissue structure and pathophysiology. Here we propose an approach to directly measure the non-Gaussian property of water diffusion, characterized by a four-dimensional matrix referred to as the diffusion kurtosis tensor. This approach does not require the complete measurement of the displacement distribution function and, therefore, is more time efficient compared with the q-space imaging technique. A theoretical framework of the DK calculation is established, and experimental results are presented for humans obtained within a clinically feasible time of about 10 min. The resulting kurtosis maps are shown to be robust and reproducible. Directionally-averaged apparent kurtosis coefficients (AKC, a unitless parameter) are 0.74 +/- 0.03, 1.09 +/- 0.01 and 0.84 +/- 0.02 for gray matter, white matter and thalamus, respectively. The three-dimensional kurtosis angular plots show tissue-specific geometry for different brain regions and demonstrate the potential of identifying multiple fiber structures in a single voxel. Diffusion kurtosis imaging is a useful method to study non-Gaussian diffusion behavior and can provide complementary information to that of DTI.

Adult↗
PubMed2006

Quantitative MR imaging in Alzheimer disease.

Alzheimer disease (AD) is the most common type of dementia. It currently affects approximately 4 million people in the United States. AD is a progressive neurodegenerative disorder characterized by the gradual deposition of neuritic plaques and neurofibrillary tangles in the brain, which is thought to occur decades before the onset of clinical symptoms. Identification of people at risk before the clinical appearance of dementia has become a priority due to the potential benefits of therapeutic intervention. Although atrophy of medial temporal lobe structures has been shown to correlate with progression of AD, a growing number of recent reports have indicated that such atrophy may not be specific to AD. To improve diagnostic specificity, new quantitative magnetic resonance (MR) imaging methods are being developed that exploit known pathogenic mechanisms exclusive to AD. This article reviews the MR techniques that are currently available for the diagnostic assessment of AD.

Alzheimer Disease↗
PubMed2005

Diffusional kurtosis imaging: the quantification of non-gaussian water diffusion by means of magnetic resonance imaging.

A magnetic resonance imaging method is presented for quantifying the degree to which water diffusion in biologic tissues is non-Gaussian. Since tissue structure is responsible for the deviation of water diffusion from the Gaussian behavior typically observed in homogeneous solutions, this method provides a specific measure of tissue structure, such as cellular compartments and membranes. The method is an extension of conventional diffusion-weighted imaging that requires the use of somewhat higher b values and a modified image postprocessing procedure. In addition to the diffusion coefficient, the method provides an estimate for the excess kurtosis of the diffusion displacement probability distribution, which is a dimensionless metric of the departure from a Gaussian form. From the study of six healthy adult subjects, the excess diffusional kurtosis is found to be significantly higher in white matter than in gray matter, reflecting the structural differences between these two types of cerebral tissues. Diffusional kurtosis imaging is related to q-space imaging methods, but is less demanding in terms of imaging time, hardware requirements, and postprocessing effort. It may be useful for assessing tissue structure abnormalities associated with a variety of neuropathologies.

Anisotropy↗
PubMed2005

Methods for noninvasive measurement of tissue iron in Cooley's anemia.

To examine the relationship between myocardial storage iron and body iron burden, as assessed by hepatic storage iron measurements, we studied 22 patients with transfusion-dependent thalassemia syndromes, all being treated with subcutaneous deferoxamine, and 6 healthy subjects. Study participants were examined with a Philips 1.5-T Intera scanner using three multiecho spin echo sequences with electrocardiographic triggering and respiratory navigator gating. Myocardial and hepatic storage iron concentrations were determined using a new magnetic resonance method that estimates total tissue iron stores by separately measuring the two principal forms of storage iron, ferritin and hemosiderin. In a subset of 10 patients with beta-thalassemia major, the hepatic storage iron concentration had been monitored repeatedly for 12-14 years by chemical analysis of tissue obtained by liver biopsy and by magnetic susceptometry. In this subset, we examine the relationship between hepatic iron concentration over time and our current magnetic resonance estimates of myocardial iron stores. No significant relationship was found between simultaneous estimates of myocardial and hepatic storage iron concentrations. By contrast, in the subset of 10 patients with beta-thalassemia major, the correlation between the 5-year average of hepatic iron concentration and the current myocardial storage iron was significant (R = .67, P = .03). In these patients, myocardial storage iron concentrations seem to reflect the control of body iron over a period of years. Magnetic resonance methods promise to provide more effective monitoring of iron deposition in vulnerable tissues, including the liver, heart, and endocrine organs, and could contribute to the development of iron-chelating regimens that more effectively prevent iron toxicity.

Adult↗
PubMed2004

High-resolution MR imaging of mouse brain microvasculature using the relaxation rate shift index Q.

Magnetic resonance imaging (MRI) is a powerful method for in vivo quantification of tissue properties. It has been previously proposed that the index Q identical with Delta R2/(Delta R2*)2/3, where Delta R2 and Delta R2* are the spin echo and gradient echo relaxation rate shifts caused by the injection of an intravascular contrast agent, may be useful for characterizing microvasculature. In particular, Q is expected to correlate well with the density of microvessels. This study presents high-resolution in vivo Q-maps of normal mouse brain obtained with a superparamagnetic iron oxide contrast agent at a field of level of 9.4 T. Normative Q values are derived for several regions of interest and significant interregional variations are observed. Microvessel densities estimated from the Q-maps are found to be in reasonable accord with histologically determined values. A possible application of Q-maps is the assessment of angiogenic activity in tumors.

Animals↗
PubMed2004

Applications of ultrasmall superparamagnetic iron oxide contrast agents in the MR study of animal models.

Ultrasmall superparamagnetic iron oxide nanoparticles have been widely used during the past decade as MR intravascular contrast agents in the study of animal models. Such agents enhance both T1 and T2/T2* relaxation, although for animal studies it is the later type of enhancement that is most commonly exploited. Their strong microscopic intravascular susceptibility effect enables the local blood volume distribution to be mapped in various organs. High spatial resolution and sensitivity can be achieved, because the long half-life of these agents in blood, combined with anesthetization, permits steady-state measurements over extended periods. This capability has been utilized to study the cerebrovascular blood volume distributions and their changes in normal, activated, pathologic and pharmacologically or genetically modified states, particularly in rodent animal models. It has also been applied to study blood volume changes in other tissues, such as the myocardium. The relaxation rate shifts Delta R2 and Delta R2* induced by iron oxide agents may differ depending on certain morphological characteristics of the microvascular network, and sensitive Delta R2 and Delta R2* mapping can potentially provide, in addition to blood volume, measurement of other important microvascular parameters such as blood vessel density and size. This work aims to review the applications of ultrasmall superparamagnetic iron oxide contrast agents in MR animal studies, with an emphasis on the investigation of microvascular parameters.

Animals↗