Toxicology; in vivo x-ray fluorescence for the assessment of heavy metal concentrations in man

Review of Development and Recent Advances in Biomedical X-ray Fluorescence Imaging

The use of X-rays for non-invasive imaging has a long history, which has resulted in several well-established methods in preclinical as well as clinical applications, such as tomographic imaging or computed tomography. While projection radiography provides anatomical information, X-ray fluorescence analysis allows quantitative mapping of different elements in samples of interest. Typical applications so far comprise the identification and quantification of different elements and are mostly located in material sciences, archeology and environmental sciences, whereas the use of the technique in life sciences has been strongly limited by intrinsic spectral background issues arising in larger objects, so far. This background arises from multiple Compton-scattering events in the objects of interest and strongly limits the achievable minimum detectable marker concentrations. Here, we review the history and report on the recent promising developments of X-ray fluorescence imaging (XFI) in preclinical applications, and provide an outlook on the clinical translation of the technique, which can be realized by reducing the above-mentioned intrinsic background with dedicated algorithms and by novel X-ray sources.

X-ray Fluorescence Uptake Measurement of Functionalized Gold Nanoparticles in Tumor Cell Microsamples

Quantitative cellular in vitro nanoparticle uptake measurements are possible with a large number of different techniques, however, all have their respective restrictions. Here, we demonstrate the application of synchrotron-based X-ray fluorescence imaging (XFI) on prostate tumor cells, which have internalized differently functionalized gold nanoparticles. Total nanoparticle uptake on the order of a few hundred picograms could be conveniently observed with microsamples consisting of only a few hundreds of cells. A comparison with mass spectroscopy quantification is provided, experimental results are both supported and sensitivity limits of this XFI approach extrapolated by Monte-Carlo simulations, yielding a minimum detectable nanoparticle mass of just 5 pg. This study demonstrates the high sensitivity level of XFI, allowing non-destructive uptake measurements with very small microsamples within just seconds of irradiation time.

Review of in vivo optical molecular imaging and sensing from x-ray excitation

SIGNIFICANCE

Deep-tissue penetration by x-rays to induce optical responses of specific molecular reporters is a new way to sense and image features of tissue function in vivo. Advances in this field are emerging, as biocompatible probes are invented along with innovations in how to optimally utilize x-ray sources.

AIM

A comprehensive review is provided of the many tools and techniques developed for x-ray-induced optical molecular sensing, covering topics ranging from foundations of x-ray fluorescence imaging and x-ray tomography to the adaptation of these methods for sensing and imaging in vivo.

APPROACH

The ways in which x-rays can interact with molecules and lead to their optical luminescence are reviewed, including temporal methods based on gated acquisition and multipoint scanning for improved lateral or axial resolution.

RESULTS

While some known probes can generate light upon x-ray scintillation, there has been an emergent recognition that excitation of molecular probes by x-ray-induced Cherenkov light is also possible. Emission of Cherenkov radiation requires a threshold energy of x-rays in the high kV or MV range, but has the advantage of being able to excite a broad range of optical molecular probes. In comparison, most scintillating agents are more readily activated by lower keV x-ray energies but are composed of crystalline inorganic constituents, although some organic biocompatible agents have been designed as well. Methods to create high-resolution structured x-ray-optical images are now available, based upon unique scanning approaches and/or a priori knowledge of the scanned x-ray beam geometry. Further improvements in spatial resolution can be achieved by careful system design and algorithm optimization. Current applications of these hybrid x-ray-optical approaches include imaging of tissue oxygenation and pH as well as of certain fluorescent proteins.

CONCLUSIONS

Discovery of x-ray-excited reporters combined with optimized x-ray scan sequences can improve imaging resolution and sensitivity.

Bacterial resistance to lead: Chemical basis and environmental relevance

Natural bacterial isolates from heavily contaminated sites may evolve diverse tolerance strategies, including biosorption, efflux mechanism, and intracellular precipitation under the continually increased stress of toxic lead (Pb) from anthropogenic activities. These strategies utilize a large variety of functional groups in biological macromolecules (e.g., exopolysaccharides (EPSs) and metalloproteins) and inorganic ligands, including carboxyl, phosphate and amide groups, for capturing Pb. The amount and type of binding sites carried by biologically originated materials essentially determines their performance and potential for Pb removal and remediation. Many factors, e.g., metal ion radius, electronegativity, the shape of the cell surface sheath, temperature and pH, are thought to exert significant influences on the abovementioned interactions with Pb. Conclusively, understanding the chemical basis of Pb-binding in these bacteria can allow for the development of effective microbial Pb remediation technologies and further elucidation of Pb cycling in the environment.

The relationship between cadmium in kidney and cadmium in urine and blood in an environmentally exposed population

INTRODUCTION

Cadmium (Cd) is toxic to the kidney and a major part of the body burden occurs here. Cd in urine (U-Cd) and blood (B-Cd) are widely-used biomarkers for assessing Cd exposure or body burden. However, empirical general population data on the relationship between Cd in kidney (K-Cd), urine, and blood are scarce. Our objectives were to determine the relationship between cadmium in kidney, urine, and blood, and calculate the elimination half-time of Cd from the kidney.

METHODS

Kidney cortex biopsies, urine, and blood samples were collected from 109 living kidney donors. Cd concentrations were determined and the relationships between K-Cd, U-Cd, and B-Cd were investigated in regression models. The half-time of K-Cd was estimated from the elimination constant.

RESULTS

There was a strong association between K-Cd and U-Cd adjusted for creatinine (rp=0.70, p<0.001), while the association with B-Cd was weaker (rp=0.44, p<0.001). The relationship between K-Cd and U-Cd was nonlinear, with slower elimination of Cd at high K-Cd. Estimates of the K-Cd half-time varied between 18 and 44years. A K-Cd of 25μg/g corresponds to U-Cd of 0.42μg/g creatinine in overnight urine (U-Cd/K-Cd ratio: about 1:60). Multivariate models showed Cd in blood and urinary albumin as determinants for U-Cd excretion.

DISCUSSION

In healthy individuals with low-level Cd exposure, there was a strong correlation between Cd in kidney and urine, especially after adjustment for creatinine. Urinary Cd was also affected by Cd in blood and urinary albumin. Previous estimates of the U-Cd/K-Cd ratio may underestimate K-Cd at low U-Cd.

Quantification of bone strontium levels in humans by in vivo x-ray fluorescence

The need for in vivo bone strontium assessment arises because strontium may exert a number of effects on bone, which may be either beneficial or toxic. Measurements discussed here are noninvasive, no sample is taken, nor is there discomfort to patients. The developed source excited x-ray fluorescence system employs a 109Cd source to excite the strontium K x rays, with the source and detector in approximately 90 degree geometry relative to the sample position. The factors affecting the accuracy and minimal detectable limit for bone strontium in vivo measurements are discussed. A system calibration revealed a minimum detectible limit of approximately 0.25 mg Sr/g Ca, which is sufficient for the monitoring of strontium levels in healthy subjects and patients with elevated bone strontium concentrations. Preliminary in vivo measurements in ten healthy subjects at two bone sites (phalanx and tibia) indicated that this system can be applied for cumulative bone strontium estimation while delivering a low effective dose of 80 nSv during the measurement time. Future work will involve attempts to enhance system precision with alternative fluorescing sources and further optimization of the detection system.

In vivo determination of prostatic zinc: phantom feasibility study

This paper describes a phantom-based feasibility study for a potential in vivo determination of zinc in prostate, which could bring about improved diagnosis of prostate cancer. An x-ray fluorescence topographic technique was developed, which will permit determination of the Zn content in the prostate through the rectum, namely behind a 2-3 mm thick layer of the rectal wall. The topographic approach, together with a reconstruction method developed here, minimizes the interference of Zn from non-prostatic tissue. The phantom studies show that it will be possible to determine Zn in a prostatic compartment behind a few mm thick layer of tissue using a specially designed transrectal probe. Such a probe is currently under development in our laboratories.

Ultrasound measurements of overlying soft tissue thickness at four skeletal sites suitable for in vivo x-ray fluorescence

Due to signal attenuation in overlying soft tissue, development of x-ray fluorescence systems to measure low atomic number elements, such as strontium, in human bone required a search for a skeletal site with thin overlying tissue. This paper reports ultrasound measurements of overlying tissue on 10 subjects, at four anatomical sites. The average tissue thickness at the finger was (2.9+/-0.7) mm. The average tissue thicknesses were (3.6+/-0.7) mm, (4.8+/-2.0) mm, and (8.4+/-1.7) mm at forehead, tibia and heel, respectively. Additionally, both parametric and nonparametric approaches to the relationship between body mass index (BMI) and tissue thickness suggest that there is a significant linear correlation between the subject's BMI and overlying tissue at the finger and heel bone. These correlations might be used as a criterion to perform XRF measurements, however a larger data set is required to address these correlations more clearly.

Cadmium content in human kidney and hair in the Gdańsk region

Concentrations of cadmium were determined in the renal cortex and hair of 67 persons who died between 1996 and 1997 in the Gdańsk region of Poland. The mean concentrations in the renal cortex and the hair were: 39.8 +/- 21.45 micrograms/g and 0.35 +/- 0.33 microgram/g, respectively. The mean age of the population studied was 47.6 +/- 15.8 years. The concentration of cadmium in the renal cortex was age-dependent. In the age groups: 18-30, 31-40, 41-50, 51-60 and 61-90 it amounted to: 19.1 +/- 11.0 micrograms/g, 43.3 +/- 21.6 micrograms/g, 47.9 +/- 20.8 micrograms/g, 41.5 +/- 20.4 micrograms/g and 33.6 +/- 18.0 micrograms/g, respectively. No correlation between the cadmium contents in the renal cortex and hair has been established. Hair is not a good indicator of exposure to cadmium.

In vivo analysis of cadmium in battery workers versus measurements of blood, urine, and workplace air

OBJECTIVES

To measure in vivo the cadmium concentrations in kidney cortex (kidney-Cd) and in superficial liver tissue (liver-Cd) of nickel cadmium battery workers, and to compare the results with other commonly used estimates of cadmium exposure (current concentrations of cadmium in blood (B-Cd) and urine (U-Cd)) or repeated measurements of cadmium in workplace air (CumAir-Cd).

METHODS

The study comprised 30 workers with a range of duration of exposure of 11-51 years. 13 subjects were currently employed, whereas the other 17 had a median period without occupational exposure of eight years before the measurements. The in vivo measurements were made with an x ray fluorescence technique permitting average detection limits of 30 and 3 micrograms cadmium per g tissue in kidney and liver, respectively.

RESULTS

19 of 30 (63%) people had kidney-Cd and 13 of 27 (48%) had liver-Cd above the detection limits. Kidney-Cd ranged from non-detectable to 350 micrograms/g and liver-Cd from non-detectable to 80 micrograms/g. The median kidney-Cd and liver-Cd were 55 micrograms/g and 3 micrograms/g, respectively. Kidney-Cd correlated significantly with B-Cd (r, 0.49) and U-Cd (r, 0.70), whereas liver-Cd correlated significantly with U-Cd (r, 0.58). Neither kidney-Cd nor liver-Cd correlated with the CumAir-Cd. The prevalence of beta 2-microglobulinurea increased with increased liver-Cd.

CONCLUSIONS

Current U-Cd can be used to predict the kidney-Cd and liver-Cd measured in vivo. In vivo measurements of kidney-Cd and liver-Cd were not shown to correlate with the individual cadmium exposure estimates, obtained by integration of the cadmium concentration in workplace air. There may be several reasons for this, including uncertainties in the estimate of the individual cumulative exposures as well as in the in vivo measurements. There was a suggestion of a relation between liver-Cd and tubular proteinuria.

Biochemical tests in diseases of the intestinal tract: their contributions to diagnosis, management, and understanding the pathophysiology of specific disease states

Biochemical testing plays a major role in the complete evaluation of patients with suspected or established intestinal disease. We have classified these tests according to the medium in which they are performed: breath tests, including isotopic and nonisotopic tests, fecal tests, urine tests, serum tests, tissue tests, and other tests. The principles of various tests are outlined, and the role of each test in the evaluation of particular gastrointestinal disorders is discussed.