Dynamic studies of gadolinium uptake in brain tumors using inversion-recovery echo-planar imaging

Echo-planar imaging has been used to observe the dynamics of Gd-DTPA uptake in brain tumors. It has been possible to examine both vascular uptake and diffusion across the blood-brain barrier in a single experiment, by using the IR-MBEST echo-planar sequence which combines a high temporal resolution (approximately 3 s) with strong T1 weighting. To model the uptake it is necessary to know the arterial concentration of Gd-DTPA; in this study the signal in the sagittal sinus was measured to avoid the need to take repeated blood samples. The time constant for transfer across the blood-brain barrier was measured to be between 20 and 1050 s for different tumors. The results of the modeling correlated with the results of other assessments of tumor vascularity.

Accurate measurement of T1 in vivo in less than 3 seconds using echo-planar imaging

This paper describes a rapid and accurate method for measuring T1 in vivo using an echo-planar imaging version of the Look-Locher sequence. T1 values from 76 to 1330 ms have been measured in 3 s with a mean accuracy of 4.6%.

An assessment of the intrauterine sound intensity level during obstetric echo-planar magnetic resonance imaging

In order to assess the sound level experienced by the fetal ear during obstetric magnetic resonance imaging, a fluid filled stomach was used as an experimental model of the gravid uterus. A better than 30 dB attenuation in intensity was recorded across the frequency band of interest for all patient orientations. This was enough to reduce acoustic sound pressure from a level close to the instantaneous damage threshold (120 dB), to an acceptable level (< 90 dB). Direct mechanical coupling through the patient table was also shown to increase uterine sound pressure levels by as much as 10 dB. Much higher peak pressures could be obtained by tapping of abdomen with the fingers.

Fetal brain activity demonstrated by functional magnetic resonance imaging

Functional magnetic resonance imaging was used to study fetal brain activity. This activity was in response to an auditory stimulus.

Use of echo planar imaging to demonstrate the effect of posture on the intragastric distribution and emptying of an oil/water meal

Intragastric distribution of solids and liquids is far from uniform but until recently technical limitations have prevented detailed study. Echo planar imaging (EPI), which can provide high-quality images of intragastric contents, has been used in this study to assess the intragastric distribution of oil and water and gastric emptying in subjects lying on either their left or right side. Eight healthy volunteers underwent four gastric emptying studies after consuming either an aqueous meal (400 mL beef consomme soup + 100 mL water) or a fat/aqueous meal (400 mL soup + 100 mL olive oil) lying on either their left or right side. Using a water suppressed imaging mode the lipid phase was clearly seen layering above the aqueous phase with little evidence of mixing. Gastric emptying of the aqueous meal (A) was not significantly different when lying on the left compared to the right side. However, gastric emptying of the aqueous phase of the fat/aqueous meal (B) was considerably slower when lying on the left compared to the right side, gastric volumes actually increasing over the 90 min of the study P < or = 0.05. When lying on the right side the aqueous phase of meal B emptied significantly slower than meal A. Oil was clearly observed to layer above the water and fill the duodenal cap when subjects lay on their left and the fundus when they lay on their right side. EPI clearly demonstrates the intragastric layering of oil which causes posture to strongly influence gastric emptying. EPI is a tool with great potential to describe the intragastric events following ingestion of complex multiphase meals.

Cognitive effects of head-movements in stray fields generated by a 7 Tesla whole-body MRI magnet

The study investigates the impact of exposure to the stray magnetic field of a whole-body 7 T MRI scanner on neurobehavioral performance and cognition. Twenty seven volunteers completed four sessions, which exposed them to approximately 1600 mT (twice), 800 mT and negligible static field exposure. The order of exposure was assigned at random and was masked by placing volunteers in a tent to hide their position relative to the magnet bore. Volunteers completed a test battery assessing auditory working memory, eye-hand co-ordination, and visual perception. During three sessions the volunteers were instructed to complete a series of standardized head movements to generate additional time-varying fields ( approximately 300 and approximately 150 mT.s(-1) r.m.s.). In one session, volunteers were instructed to keep their heads as stable as possible. Performance on a visual tracking task was negatively influenced (P<.01) by 1.3% per 100 mT exposure. Furthermore, there was a trend for performance on two cognitive-motor tests to be decreased (P<.10). No effects were observed on working memory. Taken together with results of earlier studies, these results suggest that there are effects on visual perception and hand-eye co-ordination, but these are weak and variable between studies. The magnitude of these effects may depend on the magnitude of time-varying fields and not so much on the static field. While this study did not include exposure above 1.6 T, it suggests that use of strong magnetic fields is not a significant confounder in fMRI studies of cognitive function. Future work should further assess whether ultra-high field may impair performance of employees working in the vicinity of these magnets.

Fat delays emptying but increases forward and backward antral flow as assessed by flow-sensitive magnetic resonance imaging

Flow has been assessed in the gastric antrum using a velocity-sensitive version of the high-speed magnetic resonance imaging technique, echo planar imaging (EPI). Eight healthy volunteers attended fasted on three separate days and consumed 800 mL of either a 5% glucose (0.2 kcal mL-1), 10% glucose (0.4 kcal mL-1) or an isotonic mixed nutrient meal, Fresubin (1 kcal mL-1, 27.2 g fat). Gastric volumes were obtained at 10-min intervals for 1 h. Flow measurements were performed on a single slice through the antropyloric region 5 and 35 min after meal ingestion. Gastric volumes at 45 min were inversely proportional to the calorie density of the meal with (mean +/- SEM) 89 +/- 10%* of the Fresubin, 64 +/- 5%* of the 10% glucose and 41 +/- 5% of the 5% glucose remaining (*P < 0.005 vs 5% glucose). Substantial forward and backward antral flow was observed after all three meals in the initial 5-min imaging period. AT 35 min flow activity was significantly greater after both the high-calorie meals relative to the 5% meal (total number of flow events: Fresubin = 6.6 +/- 1.7,[symbol: see text] 10% glucose = 9.9 +/- 2.2, [symbol: see text] 5% glucose = 2.5 +/- 0.9,[symbol: see text] P < 0.03,[symbol: see text] P < 0.007 vs 5% glucose, n = 8). Peak forward velocities for the initial phase of emptying tended to be greater for the rapidly emptying 5% meal (5.9 +/- 0.8 cm-1) compared with the Fresubin (3.3 +/- 0.6 cm-1, P < 0.069, n = 8) and the 10% glucose (2.9 +/- 1.0 cm-1, P < 0.068, n = 8) meals. In spite of delayed gastric emptying, high-calorie meals were associated with substantial to and fro movements which may be important for meal tritruration and fat emulsification.

The investigation of placental relaxation and estimation of placental perfusion using echo-planar magnetic resonance imaging

Echo-planar imaging (EPI) is a form of magnetic resonance imaging (MRI) which acquires images in milliseconds rather than minutes as with conventional MRI. The images produced using EPI are affected by the physiological environment in which the hydrogen atoms producing the signals are found, a process referred to as relaxation. Also by producing images a matter of milliseconds apart, quantification of perfusion within the tissue being imaged is feasible. The objective of this study was to investigate T1 and T2 relaxation times along with perfusion in placentae from normal pregnancies at different gestations and also to compare these to pregnancies complicated by abnormal placental function. A cross-sectional study of normal and compromised pregnancies from 20 weeks to term and a longitudinal study of normal pregnancy were performed. Placental T1, T2 relaxation times, and perfusion were measured using echo-planar magnetic resonance imaging. Placental T1 and T2 relaxation times decreased in normal pregnancy (P<0.001). Relaxation times in pregnancies associated with placental pathology appeared to be reduced for that gestation although the numbers were too small to allow any statistical validation. No differences in placental perfusion with gestation or between normal and compromised pregnancy were demonstrated using this technique. This is the first demonstration of placental magnetic resonance relaxation and perfusion measurements in normal pregnancy using echo-planar magnetic resonance imaging. In the future it may be possible to identify compromised pregnancies by differences in placental T1 and T2 relaxation times, using this novel non-invasive technique.

Single nucleotide polymorphism in the neuroplastin locus associates with cortical thickness and intellectual ability in adolescents

Despite the recognition that cortical thickness is heritable and correlates with intellectual ability in children and adolescents, the genes contributing to individual differences in these traits remain unknown. We conducted a large-scale association study in 1583 adolescents to identify genes affecting cortical thickness. Single-nucleotide polymorphisms (SNPs; n=54,837) within genes whose expression changed between stages of growth and differentiation of a human neural stem cell line were selected for association analyses with average cortical thickness. We identified a variant, rs7171755, associating with thinner cortex in the left hemisphere (P=1.12 × 10(-)(7)), particularly in the frontal and temporal lobes. Localized effects of this SNP on cortical thickness differently affected verbal and nonverbal intellectual abilities. The rs7171755 polymorphism acted in cis to affect expression in the human brain of the synaptic cell adhesion glycoprotein-encoding gene NPTN. We also found that cortical thickness and NPTN expression were on average higher in the right hemisphere, suggesting that asymmetric NPTN expression may render the left hemisphere more sensitive to the effects of NPTN mutations, accounting for the lateralized effect of rs7171755 found in our study. Altogether, our findings support a potential role for regional synaptic dysfunctions in forms of intellectual deficits.

Genomic architecture of human neuroanatomical diversity

Human brain anatomy is strikingly diverse and highly inheritable: genetic factors may explain up to 80% of its variability. Prior studies have tried to detect genetic variants with a large effect on neuroanatomical diversity, but those currently identified account for <5% of the variance. Here, based on our analyses of neuroimaging and whole-genome genotyping data from 1765 subjects, we show that up to 54% of this heritability is captured by large numbers of single-nucleotide polymorphisms of small-effect spread throughout the genome, especially within genes and close regulatory regions. The genetic bases of neuroanatomical diversity appear to be relatively independent of those of body size (height), but shared with those of verbal intelligence scores. The study of this genomic architecture should help us better understand brain evolution and disease.

Demonstration of differences in colonic volumes, transit, chyme consistency, and response to psyllium between healthy and constipated subjects using magnetic resonance imaging

BACKGROUND

In functional gastrointestinal disorders a lack of objective biomarkers limits evaluation of underlying mechanisms. We aimed to demonstrate the utility of magnetic resonance imaging for this task using psyllium, an effective constipation treatment, in patients and controls.

METHODS

Two crossover studies: (i) adults without constipation (controls, n = 9) took three treatments in randomized order for 6 days - maltodextrin (placebo), psyllium 3.5 g t.d.s and 7 g t.d.s., (ii) adults with chronic constipation (patients, n = 20) took placebo and psyllium 7 g t.d.s. for 6 days. MRI was performed fasting and postprandially on day 6. Measurements included small bowel and ascending colon water content, colonic volume, transit time, and MR relaxometry (T1, T2) to assess colonic chyme. Stool water percentage was measured.

RESULTS

7 g psyllium t.d.s. increased fasting colonic volumes in controls from median 372 mL (IQR 284-601) to 578 mL (IQR 510-882), and in patients from median 831 mL (IQR 745-934) to 1104 mL (847-1316), P < .05. Mean postprandial small bowel water was higher in controls and patients after 7 g psyllium t.d.s. vs placebo. Whole gut transit was slower in patients than controls (P < .05). T1 of the descending colon chyme (fasting) was lower in patients (213 ms, 176-420) than controls (440 ms, 352-884, P < .05) on placebo, but increased by 7 g psyllium t.d.s. (590 ms, 446-1338), P < .001. Descending colon T1 correlated with baseline stool water content and stool frequency on treatment.

CONCLUSIONS AND INFERENCES

MRI measurements can objectively demonstrate the mode of action of therapy targeting intestinal fluid content in constipation.

Dual registration of abdominal motion for motility assessment in free-breathing data sets acquired using dynamic MRI

At present, registration-based quantification of bowel motility from dynamic MRI is limited to breath-hold studies. Here we validate a dual-registration technique robust to respiratory motion for the assessment of small bowel and colonic motility. Small bowel datasets were acquired in breath-hold and free-breathing in 20 healthy individuals. A pre-processing step using an iterative registration of the low rank component of the data was applied to remove respiratory motion from the free breathing data. Motility was then quantified with an existing optic-flow (OF) based registration technique to form a dual-stage approach, termed Dual Registration of Abdominal Motion (DRAM). The benefit of respiratory motion correction was assessed by (1) assessing the fidelity of automatically propagated segmental regions of interest (ROIs) in the small bowel and colon and (2) comparing parametric motility maps to a breath-hold ground truth. DRAM demonstrated an improved ability to propagate ROIs through free-breathing small bowel and colonic motility data, with median error decreased by 90% and 55%, respectively. Comparison between global parametric maps showed high concordance between breath-hold data and free-breathing DRAM. Quantification of segmental and global motility in dynamic MR data is more accurate and robust to respiration when using the DRAM approach.

Validation of antroduodenal motility measurements made by echo-planar magnetic resonance imaging

Echo planar imaging, a development of magnetic resonance imaging, can produce snapshot images of the stomach and antroduodenal segment in as little as 64 msec and can be more useful than conventional techniques when assessing motility. The aim of this study was to compare antroduodenal motility measured by simultaneous perfused tube manometry and echo planar imaging. Ten volunteers were studied following the ingestion of 500 mL water or 500 mL porridge. Antroduodenal images, with acquisition times of 130 msec, were taken at 3-sec intervals, synchronized with motility traces and presented as a split-screen video. This allowed direct visual comparison of gastric wall movement and motility to be made. Contractions were confined to either the stomach or the duodenum or propagated across the antroduodenal segment. Over 4550 images were available for analysis. A larger number of propagated contractions were recorded with echo planar imaging in both water (P = 0.03) and food (P = 0.02) groups, whereas manometry detected a greater number of isolated duodenal pressure waves (P = 0.005). The contraction rate for water and food studies was similar, but direct visualization indicated that the manometric technique under-detected propagated events. The ability of echo planar imaging to record antroduodenal contractile activity provides a new insight into the role of occlusive and nonocclusive contractions during gastric emptying.

Levels of HIV-infected peripheral blood cells remain stable throughout the natural history of HIV-1 infection. Swiss HIV Cohort Study

OBJECTIVE

To clarify the relationship between the number of provirus-bearing peripheral blood mononuclear cells (PBMC) and HIV-1 disease progression during the natural history of infection.

DESIGN

Twenty-four HIV-1-infected subjects with known seroconversion dates and long-term follow-up were retrospectively identified using the Swiss HIV Cohort Database. PBMC specimens from this cohort were retrieved from storage for analysis.

METHODS

Infected PBMC equivalents were determined by HIV-1 DNA quantitative competitive (QC)-PCR. The results were analysed with respect to HIV-1 disease stage and compared with a mathematical model of long-term HIV-1 disease progression.

RESULTS

PBMC HIV-1 DNA did not correlate with major indices of disease progression, including time following primary infection, time before reaching a CD4 cell count less than 200 x 10(6)/l, and time before death. The number of PBMC harbouring HIV-1 provirus was relatively constant throughout the clinical stages of HIV-1 infection, consistent with simulated data from a mathematical model of long-term HIV-1 infection. We also showed that a biased interpretation of the QC-PCR data may arise when the values are expressed as HIV-1 DNA copies per PBMC or per CD4 cell.

CONCLUSIONS

This analysis suggests that levels of provirus-bearing PBMC remain constant during the natural course of HIV-1 infection, whereas plasma virus load typically increases logarithmically during the same period. The hypothesis that plasma virus levels are directly related to the number of infected cells may deserve reconsideration.

Dynamic imaging of contrast enhancement in brain tumors

Gadolinium-DTPA has been shown to be a good probe for demonstrating defects in the blood-brain barrier, but it has a rapid rate of elimination so that peak circulating levels are short-lived. In this study ultrafast echo-planar imaging has been used in combination with a bolus injection of gadolinium-DTPA to evaluate perfusion within brain tumors and to assess the degree of disruption of the blood-brain barrier. The temporal profile of enhancement may allow discrimination between different tumor types.

Adaptation of Escherichia coli growth rates to the presence of pBR322

Changes in the growth rate of Escherichia coli K12 J62-1 in response to the presence of plasmid pBR322 have been investigated. Plasmid-free and plasmid-containing strains were grown in batch culture and their maximum specific growth rate (mu max) determined. The acquisition of pBR322 by the host resulted in a decreased mu max. Following repeated subculturing of the plasmid-containing strain on selective medium, restoration in mu max was observed. The copy number and structure of the plasmid were not significantly altered during the experiment. Growth rate measurements for a series of strains constructed using a combination of host cells and plasmids with and without culture histories, indicated that the site of the adaptive mutation was located on the host chromosome rather than on the plasmid.

DRD2/ANKK1 polymorphism modulates the effect of ventral striatal activation on working memory performance

Motivation is important for learning and cognition. Although dopaminergic (D2) transmission in the ventral striatum (VS) is associated with motivation, learning, and cognition are more strongly associated with function of the dorsal striatum, including activation in the caudate nucleus. A recent study found an interaction between intrinsic motivation and the DRD2/ANKK1 polymorphism (rs1800497), suggesting that A-carriers of rs1800497 are significantly more sensitive to motivation in order to improve during working memory (WM) training. Using data from the two large-scale imaging genetic data sets, IMAGEN (n=1080, age 13-15 years) and BrainChild (n∼300, age 6-27), we investigated whether rs1800497 is associated with WM. In the IMAGEN data set, we tested whether VS/caudate activation during reward anticipation was associated with WM performance and whether rs1800497 and VS/caudate activation interact to affect WM performance. We found that rs1800497 was associated with WM performance in IMAGEN and BrainChild. Higher VS and caudate activation during reward processing were significantly associated with higher WM performance (p<0.0001). An interaction was found between the DRD2/ANKK1 polymorphism rs1800497 and VS activation during reward anticipation on WM (p<0.01), such that carriers of the minor allele (A) showed a significant correlation between VS activation and WM, whereas the GG-homozygotes did not, suggesting that the effect of VS BOLD on WM is modified by inter-individual genetic differences related to D2 dopaminergic transmission.

Investigating the effect of blood susceptibility on phase contrast in the human brain

Recent work has shown a dramatic contrast between GM and WM in gradient echo phase images at high field (7 T). Although this contrast is key to the exploitation of phase in imaging normal and pathological tissue, its origin remains contentious. Several sources for this contrast have been considered including iron content, myelin, deoxy-hemoglobin, or water-macromolecule interactions. Here we quantify the contribution of intravascular dHb to the GM/WM contrast in the human brain at 7 T by modulating the susceptibility of the blood using a paramagnetic contrast agent. By carrying out high resolution, dynamic, gradient echo imaging before, during and after the injection of the contrast agent, we were able to follow the change in GM/WM phase contrast and to monitor simultaneously the susceptibility of the blood. Using these data in conjunction with the known susceptibility of venous blood we estimate the upper bound for the relative contribution of dHb in the vasculature to the measured GM/WM phase contrast to be 0.48 Hz for GM close to the pial surface, and 0.27 Hz for deeper GM. These values are up to 20% of the GM/WM phase difference observed in the human brain at 7 T. Furthermore, we found that the fractional blood volume differences required to account for the observed GM/WM phase contrast are 1.3% and 0.7% for GM close to the pial surface and for deeper GM, respectively.

A phase I/IIA clinical study with a chimeric mouse-human monoclonal antibody to the V3 loop of human immunodeficiency virus type 1 gp120

A phase I/IIA clinical trial with the chimeric mouse-human monoclonal antibody CGP 47,439 to the principal neutralization determinant in the V3 region of human immunodeficiency virus type 1 (HIV-1) strain IIIB envelope protein gp120 is reported. The trial was an uncontrolled single-center, open-label, multidose tolerability, immunogenicity, and pharmacokinetic study in homosexual men with advanced HIV disease. Patient groups were formed on the basis of the reactivity of the antibody with the gp120 of their HIV-1 isolates. Intravenous infusions of 1, 10, and 25 mg of antibody were followed by seven escalated doses of 50, 100, and 200 mg, every 3 weeks. The antibody was well tolerated; no toxicity was observed. Some patients showed a transient but insignificant antibody response to the antibody with no apparent adverse reactions or accelerated elimination of it. Substantial serum levels of the antibody were maintained with a mean t1/2 beta of 8-16 days. A virus burden reduction was observed in some patients.

Development and validation of a large, modular test meal with liquid and solid components for assessment of gastric motor and sensory function by non-invasive imaging

BACKGROUND

Current investigations of stomach function are based on small test meals that do not reliably induce symptoms and analysis techniques that rarely detect clinically relevant dysfunction. This study introduces the large 'Nottingham Test Meal' (NTM) for assessment of gastric motor and sensory function by non-invasive imaging.

METHODS

NTM comprises 400 mL liquid nutrient (0.75 kcal/mL) and 12 solid agar-beads (0 kcal) with known breaking strength. Gastric fullness and dyspeptic sensations were documented by 100 mm visual analogue scale (VAS). Gastric emptying (GE) were measured in 24 healthy volunteers (HVs) by gastric scintigraphy (GS) and magnetic resonance imaging (MRI). The contribution of secretion to gastric volume was assessed. Parameters that describe GE were calculated from validated models. Inter-observer agreement and reproducibility were assessed.

KEY RESULTS

NTM produced moderate fullness (VAS ≥30) but no more than mild dyspeptic symptoms (VAS <30) in 24 HVs. Stable binding of meal components to labels in gastric conditions was confirmed. Distinct early and late-phase GE were detected by both modalities. Liquid GE half-time was median 49 (95% CI: 36-62) min and 68 (57-71) min for GS and MRI, respectively. Differences between GS and MRI measurements were explained by the contribution of gastric secretion. Breaking strength for agar-beads was 0.8 N/m(2) such that median 25 (8-50) % intact agar-beads and 65 (47-74) % solid material remained at 120 min on MRI and GS, respectively. Good reproducibility for liquid GE parameters was present and GE was not altered by agar-beads.

CONCLUSIONS & INFERENCES

The NTM provided an objective assessment of gastric motor and sensory function. The results were reproducible and liquid emptying was not affected by non-nutrient agar-beads. The method is potentially suitable for clinical practice.

Elevated serum macrophage inhibitory factor-related protein (MRP) 8/14 levels in advanced HIV infection and during disease exacerbation

To assess the value of MRP 8, MRP 14, and MRP 8/14 serum concentrations as markers of disease progression in HIV infection and as markers of intercurrent infections.

DESIGN

We measured MRP 8, MRP 14, and MRP 8/14 serum concentrations in 184 HIV-infected patients in various stages of disease with or without disease exacerbation and in 50 healthy control subjects. In clinically stable HIV-infection correlations of MRP levels with stage of HIV disease, CD4 counts, p24 antigen, and beta-2 microglobulin levels were studied. In patients with intercurrent illnesses, correlations of MRP levels with type of disease exacerbation and with CRP were calculated and compared with those found in stable HIV infection.

RESULTS

MRP 8/14 levels were significantly elevated and MRP 8 levels slightly decreased in stable HIV infection compared with HIV-negative controls. The CD4 cell count and MRP 8/14 levels correlated significantly in patients with AIDS. Despite higher values of MRP 8/14 during advanced disease, these were not significant predictors of progression to death. In patients with acute infections, MRP 8/14 levels were significantly elevated, compared with patients with illnesses of noninfectious origin. Levels of MRP 8/14 associated with acute infections were significantly higher in patients with AIDS than in patients during earlier stages of HIV infection.

CONCLUSIONS

Both stable HIV infection and advanced immunedeficiency are associated with an elevation of the MRP 8/14 complex and probably with a decline of MRP 8 serum levels. MRP 8/14 is preserved as a marker of acute infection in immunecompromised patients.

Fatal outcome of a hepatitis B virus transfusion-transmitted infection

BACKGROUND AND OBJECTIVES

In 2008, hepatitis B virus (HBV) DNA testing was not yet mandatory for the screening of blood donations in Switzerland. At that time, HBsAg was the only specific mandatory marker for HBV. The importance of high sensitivity for HBV NAT screening is shown.

MATERIALS AND METHODS

Donor and recipient of a transfusion-transmitted HBV infection were followed up. Multiple samples were tested for HBV serological and molecular markers.

RESULTS

At donation, the donor appeared healthy, HBsAg was negative and had a normal ALAT level. Ten weeks later, clinical symptoms suggested acute HBV infection as was confirmed with positive HBsAg, HBeAg, anti-HBc IgG, anti-HBc IgM and anti-HBe. The archived sample from the original donation was negative for anti-HBc, but positive for HBV DNA (17 IU/ml). A recipient transfused with the red cell concentrate was HBV DNA positive (3100 IU/ml) 3 months post-transfusion. After five months, HBsAg, HBeAg, anti-HBc and HBV DNA (1.1 x 10(11) IU/ml) were positive. Two weeks later, the patient died from complications associated with HBV infection and his underlying bone marrow disease.

CONCLUSIONS

The present case illustrates the importance of introducing highly sensitive HBV NAT screening strategy to prevent possible HBV transfusion-transmitted infections from donors with low viral load.