Current Medical Imaging - Volume 4, Issue 2, 2008

MRI has evolved in the last 30 years as rapid, more precise, and more specific. Diffusion tensor imaging (DTI) uses the anisotropy of water motion within structures such as axons to create imaging contrast, which is then used to depict the orientation and volume of white matter fiber tracts. This technique has been propelled by the fast imaging techniques of echoplanar imaging, better gradients, and software post-processing techniques. Dr. Hubertus Axer reviews early diagnosis and also the quantification of Wallerian degeneration by DTI, which may be a crucial factor influencing the functional loss after brain damage. MRI has achieved a level of maturation that has rendered it the most accurate imaging modality for investigating abdominal diseases. Current state-of-the-art magnetic resonance cholangiopancreatography (MRCP) techniques provide high-resolution images of the native pancreatobiliary ductal system, which is a functional system. Dr. Daniel Boll describes perfusion/diffusion-weighted imaging in combination with secretininduced increase in parenchymal perfusion and also quantification of pancreatic excretory reserve. He also discusses cholecystokinin-induced stimulation of the biliary system for the quantification of gallbladder ejection fraction and also the diagnosis of biliary leakage. MRI has been shown to be superior to CT in the workup of uterine and cervical carcinomas and is a very useful problem solving tool in the characterization of gynecologic tumors. Dr. Jongchul Kim reviews MRI findings of uterine cervical carcinomas with the emphasis on the oblique axial images and also dynamic MRI for the appropriate treatment of the patients. Increasing diagnostic sensitivity and specificity proves to be the challenge for MRI. There are many ways to create contrast in MRI. The properties of the MR contrast agent will determine its binding characteristics to the molecule of interest, its tissue penetration and circulation, and potentially its cellular uptake. Exogenous contrast agents can function in many ways: altering T1 or T2 relaxation, providing a source for magnetization transfer, or offering a new nucleus to detect. The chemistry and physics of these compounds are somewhat complex because of issues like field dependence and water exchange. Dr. Yicheng Ni discusses paramagnetic metalloporphyrins, which have new applications in the evaluation of myocardial infarction, tissue viability, ablation therapy, perfusion MRI, atherosclerotic plaque, stem cell tracking and multi-organ enhancement. These porphyrin and nonporphyrin compounds are necrosis-avid contrast agents, which may be useful for target specific diagnosis and therapy of benign and malignant disorders. Altered neonatal brain white matter structure on B-mode ultrasound images has prognostic implications for certain disorders. Periventricular leukomalacia is a brain disorder of very low birth weight preterm infants. Dr. Ewout Vansteenkiste investigated a data set of 140 cases and identified the pathological group with an accuracy of 92.5%. His method can improve both the prognostic finesse and the guidance of early postnatal treatment. Optical imaging of living cells provides a powerful suite of techniques for non-invasive linking of phenotypic expression at the biochemical level to individual genotypes. Imaging of fluorescent marker distribution within living cells has enabled measurement of many active physiological processes including protein transport, membrane potential, and free ion distributions. These markers in conjuction with optical sectioning techniques such as confocal or multiphoton microscopy allow interrogation of cellular dynamics to diffraction-limited spatial resolution. Dr. Xiaoyin Xu reviews updated development of diffusion optical tomography and microendoscopy and their clinical applications............

Wallerian degeneration is the degeneration of the nerve fiber distally to damage of the axon. It classically has been described in the peripheral nervous system (PNS), but also takes place in the central nervous system (CNS). Wallerian degeneration in the CNS is much slower than in the PNS taking months to years and a secondary regeneration of the distal nerve fiber does not take place in the CNS. Thus, Wallerian degeneration may be a crucial factor influencing the functional loss after brain damage and can be detected using modern MR techniques. This article reviews the process of Wallerian degeneration in the CNS and the possibilities of imaging Wallerian degeneration in the human brain using MRI. While a decrease of signal intensities in proton-density-weighted images can be detected after 25 days and increased signal intensities in T2-weigthed images can be seen after 80 days, diffusion tensor imaging (DTI) can detect Wallerian degeneration much earlier. Several studies have demonstrated the decrease of fractional anisotropy (FA) in the corticospinal tracts after infarction of the medial cerebral artery (MCA) 2-6 month after stroke, 2-6 weeks after stroke, and 2-16 days after stroke. Moreover, a continuous decrease of FA over time in serial DTI measurements after MCA infarction could be demonstrated. Changes in FA seem to be a parameter to quantify degeneration of tracts over time in single, compact fiber bundles such as the corticospinal tract, cerebellar peduncles, spinal cord white matter, the optic radiation, and corpus callosum. Thus, the detection and quantification of Wallerian degeneration may be a tool for monitoring functional loss or functional regain after brain damage.

Non-invasive Magnetic Resonance CholangioPancreatography (MRCP) allows a direct visualization of the pancreatobiliary ductal system without administration of contrast materials or ionizing radiation by utilizing contrastrelated properties of fluids excreted by liver and pancreas into the biliary and pancreatic ducts. MRCP enables visualization of the ductal system at a resting state, thereby more accurately displaying the caliber of the native ducts. Consequently, non-invasive MRCP has essentially replaced invasive procedures such as percutaneous transhepatic cholangiography and sole diagnostic endoscopic retrograde cholangiopancreatography. Current state-of-the-art MRCP techniques provide high-resolution visualization of the native pancreatobiliary ductal system, however, many early symptoms of pancreatobiliary diseases are being compensated for and masked by pancreatobiliary parenchymal and ductal structures, emphasizing that the pancreatobiliary tree is part of a functional system. To enhance sensitivity in pancreatic imaging, pathophysiologic characteristics can be further exploited: The introduction of perfusion/diffusion-weighted imaging in combination with secretin-induced increase in parenchymal perfusion and ductal secretion will simplify the detection of generalized perfusional defects in the pancreatic parenchyma, will allow a quantification of the remaining pancreatic excretory reserve, and will help to confirm a suspected loss of ductal distensibility. Cholecystokinin-induced stimulation of functional characteristics of the biliary system in combination with MRCP imaging will allow the quantification of gallbladder ejection fraction and biliary diameter, and will lead to an increase of diagnostic sensitivity to detect microlithiasis as well as biliary leakages without application of ionizing radiation. Contrast-enhanced imaging of the biliary tree is usually part of a comprehensive evaluation of the liver utilizing hepatocyte- targeted contrast agents which undergo an active transport into the intracellular space of the hepatocytes, where they are further metabolized and eliminated through the biliary system and subsequently allow parenchymal as well as biliary assessment employing T1 - weighted pulse sequences.

Uterine cervical carcinoma is a common gynecologic malignancy, and one of the important causes of woman mortality. MR imaging, nowadays, is widely used as the single most effective modality for the cervical carcinoma. T2- weighted fast spin-echo MR images may demonstrate the high-signal-intensity cervical carcinoma. Oblique axial images perpendicular to the cervical canal may be more accurate in the evaluation of parametrial invasion and stromal involvement of the cancer. Dynamic MR imaging may improve tumor detection and depiction of the depth of stromal and parametrial invasion of the tumor. Lymph node greater than 1 cm in short-axis diameter, especially when it is necrotic, may be a malignant metastatic node rather than hyperplastic one. MR imaging with its superior soft-tissue resolution is the best helpful and useful modality in the staging of cervical carcinoma to differentiate early cervical carcinoma (stages IA and IB, or IIA) that can be treated by simple hysterectomy or radical hysterectomy with pelvic lymph node dissection from advanced tumors (stages IIB, III or IVA) that require pelvic irradiation with chemotherapy or more advanced tumors (stage IVB) that require chemotherapy with or without pelvic irradiation. Familiarity with the spectrum of MR imaging findings of the uterine cervical carcinoma will allow the clinicians to consider appropriate treatment of the patients.

Following the footprint of porphyrin-mediated photodynamic therapy (PDT), paramagnetic metalloporphyrins were originated as tumor seeking contrast agents (CAs) for magnetic resonance imaging (MRI). However, serial research has disproved their tumor selectivity, identified nonviable tissues as their real targets, and eventually elicited new applications in myocardial infarction delineation, tissue viability evaluation, ablation therapy assessment, as well as first pass or dynamic perfusion MRI, multi-organ contrast enhancement (CE), atherosclerotic plaque imaging and stem cell labeling or tracking. Furthermore, nonporphyrin analogues have been developed to reduce porphyrin related toxicities. These porphyrin and nonporphyrin compounds have been termed as necrosis-avid contrast agents (NACAs) to denote their major discovered affinity. The present review aims to document the evolving research in this particular field, to discuss possible mechanisms, to promote further preclinical and clinical development of this unique and promising class of MRI CAs, and to implicate a novel stroma targeting strategy for diagnosis and treatment of malignant and benign disorders.

Altered white brain matter structure in neonatal Ultrasound (US) images has prognostic implications for certain disorders. Commonly, physicians classify pathological white brain matter on a discrete categorical scale based on relevant qualitative characteristics. For certain pathologies, where subtle changes in structure have to be detected, this classification is too stringent. This is the case when characterizing affected white matter in the gliotic variant of Periventricular Leukomalacia (PVL), a brain disorder of very low birth weight preterm infants. The main objective of this study is to investigate quantitatively how texture information extracted from white matter regions in B-mode US images can guide physicians to a more accurate detection. A data set of 140 B-mode US images (70 non-pathological and 70 pathological) was investigated. Pathology was defined either by evolution to cystic PVL or by definite abnormality on acute MRI (ground truth). First, 7 different texture feature sets were extracted: First-Order statistics, Grey Level Co-occurrence matrix features, Run Length matrix features, Sum and Difference histogram features, Statistical features, Texture Energy Measure features and Gabor Filter features. Then, 3 classifiers were compared on these feature sets: a Bayesian Maximum A Posteriori (MAP) probability, a k Nearest Neighbor (kNN), and Fisher's Linear Discriminant (FLD) classifier. Finally, a combination of the classifiers as well as texture feature combinations based on a confidence measure, were incorporated into a multi-feature, multi-classifier algorithm. Using our method, we succeeded in identifying the pathological group with an accuracy of 92.5% and sensitivity and specificity scores that exceed those of existing non-texture based methods. Consequently, this method can improve both the prognostic finesse and the guidance of early postnatal management.

Optical molecular imaging is a fast-developing field that holds great promise in cancer diagnosis. Over the past two decades, optical imaging has been actively investigated by researchers in both academic and industrial environments. Compared with other imaging modalities like MRI, CT, and ultrasound (US), optical imaging has many advantages. Technological improvements over the past few years have brought improved tissue penetration, sensitivity, and specificity of imaging cancer by optical methods. Novel devices like diffusion optical tomography (DOT) and the miniature endoscope have either been tested in clinical trials or approved by FDA for clinical use. Many of these new technologies can be integrated into or combined with MRI, CT, or US, providing a multimodality approach to further improve the accuracy of cancer diagnosis. Fast computational methods and ever-growing powerful computers make real-time optical imaging possible, bringing us closer to the possibility of on-site diagnosis. Here we review two different optical imaging methods, namely diffusion optical tomography and microendoscopy. DOT uses near-infrared light to noninvasively measure the hemoglobin concentration and resolve spectroscopic information about the tissue, whereas a tumor may manifest itself as a heterogeneous region due to the increased blood flow to the tumor. Like MRI, CT, and US, DOT sensors interrogate the tissue from the outside. Contrast agents like indocyanine green (ICG) may be used to label the suspected tumor for enhanced DOT results. The microendoscope, however, measures fluorescence signal from inside of the body, typically a cavity like the pulmonary pathway or gastrointestinal tract. The small size of the microendoscope (a few millimeters) allows it to be inserted into small pulmonary branches inaccessible to a conventional endoscope. In addition, microendoscopy offers image resolution approximating that of the microscope, making “in-situ” optical biopsy a possibility.

The main objective of this research is to introduce a newly developed device called “Adaptive Quality Control Phantom” (AQCP) designed to perform the QC tests. This paper describes complete details of the AQCP structure and its potential as a QC phantom. AQCP is the computer-controlled phantom which positions and moves a radioactive source in the Field of View (FOV) of an imaging nuclear medicine device on a definite path to produce any spatial distribution of gamma rays to simulate QC phantoms. Different tests that include systematic uniformity, collimator hole angulation and center of rotation were conducted by this device and the results, findings and differences of these tests were compared with the QC classic method tests. The performance of systematic uniformity test show a considerable reduction in the technologist dose compared to the IAEA-TECDOC-602 method. On the other hand the average error difference between two methods for evaluating uniformity parameters of uniform images is less than 0.07%. The collimator hole angulation for three collimators was measured by using a point source and computer-controlled cylindrical positioning, the results of which show that the measurement accuracy for absolute angulation errors is better than 0.047 (degrees) or ±0.024° . The Root Mean Square (RMS) of CHA for LEHR, LEHS and LEUHR collimators was measured to be 0.290° , 0.292° and 0.208° respectively. A method for center of rotation assessment is introduced by AQCP and the results of this proposed method as compared with the routine QC test and their differences are discussed in detail. Based on the discussion made in this paper regarding AQCP, the authors believe that this device is able to simulate QC phantoms.