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1.
Int J Biomed Imaging ; 2006: 35290, 2006.
Article in English | MEDLINE | ID: mdl-23165025

ABSTRACT

We systematically evaluated a variety of MR spiral imaging acquisition and reconstruction schemes using a computational perceptual difference model (PDM) that models the ability of humans to perceive a visual difference between a degraded "fast" MRI image with subsampling of k-space and a "gold standard" image mimicking full acquisition. Human subject experiments performed using a modified double-stimulus continuous-quality scale (DSCQS) correlated well with PDM, over a variety of images. In a smaller set of conditions, PDM scores agreed very well with human detectability measurements of image quality. Having validated the technique, PDM was used to systematically evaluate 2016 spiral image conditions (six interleave patterns, seven sampling densities, three density compensation schemes, four reconstruction methods, and four noise levels). Voronoi (VOR) with conventional regridding gave the best reconstructions. At a fixed sampling density, more interleaves gave better results. With noise present more interleaves and samples were desirable. With PDM, conditions were determined where equivalent image quality was obtained with 50% sampling in noise-free conditions. We conclude that PDM scoring provides an objective, useful tool for the assessment of fast MR image quality that can greatly aid the design of MR acquisition and signal processing strategies.

2.
J Magn Reson Imaging ; 18(1): 103-12, 2003 Jul.
Article in English | MEDLINE | ID: mdl-12815645

ABSTRACT

PURPOSE: To reduce the acquisition time associated with the two-point Dixon fat suppression technique by combining a keyhole in-phase (Water + Fat) k-space data set with a full out-of-phase (Water - Fat) k-space data set and optimizing the keyhole size with a perceptual difference model. MATERIALS AND METHODS: A set of keyhole Dixon images was created by varying the number of lines in the keyhole data set. Off-resonance correction was incorporated into the image reconstruction process to improve the homogeneity of the fat suppression. A perceptual difference model (PDM) was validated with human observer experiments and used to compare the keyhole images to images from a full two-point Dixon acquisition. The PDM was used to determine the smallest keyhole width required to obtain perceptual equivalence to images obtained from the full two-point Dixon method. RESULTS: In experimental phantom studies, the keyhole Dixon image reconstructed from 96 of 192 Water + Fat k-space lines and 192 Water - Fat k-space lines was perceptually equivalent to the full (192 + 192) two-point Dixon images, resulting in a 25% reduction in scan time. Clinical images of a volunteer's knee, orbits, and abdomen created from the smallest, perceptually equivalent keyhole width resulted in a 27%-38% reduction in total scan time. CONCLUSION: This method improves the temporal efficiency of the conventional two-point Dixon technique and may prove especially useful for high-field systems where specific absorption rate (SAR) limits will constrain radiofrequency (RF)-based fat suppression techniques.


Subject(s)
Image Processing, Computer-Assisted/methods , Magnetic Resonance Imaging/methods , Adipose Tissue , Humans , Phantoms, Imaging
3.
J Pharm Sci ; 92(2): 289-96, 2003 Feb.
Article in English | MEDLINE | ID: mdl-12532379

ABSTRACT

In vivo release profiles of drug-loaded biodegradable implants were noninvasively monitored and characterized using X-ray computed tomography (CT). The imaging method was adapted and optimized to quantitatively examine the release of an active agent from a model cylindrical PLGA device (the millirod) into rabbit livers over 48 h. Iohexol, a CT contrast agent, served as a model drug; optimization of CT acquisition parameters yielded a sensitivity of 0.21 mg/mL (or 95 microg iodine/mL) for this agent. In vitro validation of the method was carried out by tracking release of iohexol in gelatin gel phantoms. In vivo release in rabbit livers was characterized through quantitative analysis of CT images and compared with UV-Vis analysis of the explanted devices at three implantation times. After correction for respiratory motion, CT analysis correlates well with the extracted iohexol data at all time points. The percent error between the actual and experimental image data was below 10%. This study demonstrates the potential of using computed tomography to noninvasively quantify the rate of agent release from controlled delivery devices in vivo.


Subject(s)
Drug Implants , Pharmaceutical Preparations/metabolism , Tomography, X-Ray Computed , Animals , Biocompatible Materials , Contrast Media/administration & dosage , Contrast Media/pharmacokinetics , Gels , Image Processing, Computer-Assisted , In Vitro Techniques , Iohexol/administration & dosage , Iohexol/pharmacokinetics , Lactic Acid , Liver/metabolism , Models, Anatomic , Pharmaceutical Preparations/administration & dosage , Polyglycolic Acid , Polylactic Acid-Polyglycolic Acid Copolymer , Polymers , Rabbits , Reproducibility of Results , Respiratory Mechanics/physiology
4.
J Control Release ; 83(3): 415-25, 2002 Oct 30.
Article in English | MEDLINE | ID: mdl-12387949

ABSTRACT

In this study, X-ray computed tomography (CT) was utilized as a noninvasive method to directly examine local drug release kinetics in livers before and following radiofrequency thermal ablation. Iohexol, a CT contrast agent, was used as a drug-mimicking molecule. Release of iohexol in healthy and ablated rabbit livers over 48 h was quantified and correlated with the release profiles from phosphate-buffered saline (PBS) in vitro. The results show that iohexol release in ablated livers is significantly slower than both release in normal livers and in vitro. The time at which 50% of the drug was released (t(1/2)) into ablated liver (20.6+/-5.9 h) was 1.7 times longer than in normal liver (12.1+/-5.4 h) and approximately two times longer than that in PBS (10.1+/-1.2 h). The slower release in ablated livers is a result of severe tissue damage inflicted by thermal ablation, as supported by histological examination. This data suggests that a noninvasive imaging method provides a superior measurement over in vitro release studies in accurately quantifying the local release kinetics of an agent in an altered physiological system in vivo. Because the development of a successful local drug therapy is dependent on the understanding of the agent release kinetics at the implantation site, the noninvasive data may be indispensable in effectively predicting the implant behavior in a physiological system.


Subject(s)
Catheter Ablation/methods , Drug Delivery Systems/methods , Iohexol/pharmacokinetics , Tomography, X-Ray Computed/methods , Animals , Catheter Ablation/instrumentation , Liver/metabolism , Liver/pathology , Male , Rabbits
5.
IEEE Trans Med Imaging ; 21(10): 1310-6, 2002 Oct.
Article in English | MEDLINE | ID: mdl-12585713

ABSTRACT

Recent advances in drug delivery techniques have necessitated the development of tools for in vivo monitoring of drug distributions. Gamma emission imaging and magnetic resonance imaging suffer from problems of resolution and sensitivity, respectively. We propose that the combination of X-ray CT imaging and image analysis techniques provides an excellent method for the evaluation of the transport of platinum-containing drugs from a localized, controlled release source. We correlated local carboplatin concentration with CT intensity, producing a linear relationship with a sensitivity of 62.6 microg/mL per Hounsfield unit. As an example application, we evaluated the differences in drug transport properties between normal and ablated rabbit liver from implanted polymer millirods. The use of three-dimensional visualization provided a method of evaluating the placement of the drug delivery device in relation to the surrounding anatomy, and registration and reformatting allowed the accurate comparison of the sequence of temporal CT volumes acquired over a period of 24 h. Taking averages over radial lines extending away from the center of the implanted millirods and integrating over clinically appropriate regions, yielded information about drug release from the millirod and transport in biological tissues. Comparing implants in normal and ablated tissues, we found that ablation prior to millirod implantation greatly decreased the loss of drug from the immediate area, resulting in a higher average dose to the surrounding tissue. This work shows that X-ray CT imaging is a useful technique for the in vivo evaluation of the pharmacokinetics of platinated agents.


Subject(s)
Carboplatin/pharmacokinetics , Drug Delivery Systems/methods , Liver/diagnostic imaging , Liver/metabolism , Polymers/pharmacokinetics , Subtraction Technique , Animals , Carboplatin/administration & dosage , Carboplatin/analysis , Catheter Ablation , Drug Delivery Systems/instrumentation , Equipment Failure Analysis/methods , Imaging, Three-Dimensional/methods , Liver/chemistry , Liver/surgery , Male , Polymers/administration & dosage , Rabbits , Radiographic Image Enhancement/methods , Reference Values , Reproducibility of Results , Sensitivity and Specificity , Tissue Distribution , Tomography, X-Ray Computed/methods
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