Current Medical Imaging - Volume 1, Issue 3, 2005

Due to insufficiencies of the currently available imaging modalities in breast cancer diagnosis the development of improved technical systems has been a main field of activities in the medical engineering community for years, including thermography, laser mammography and impedance based technologies. Up to now none of these technologies reached the break-through into routine application due to several reasons. Also electrical impedance scanning being free of side effects, non invasive, cheap, and flexible to handle, is a promising diagnostic tool especially in breast cancer detection. This review aims to describe the physical background, clinical application, already useful applications, limitations and future developments.

In vivo monitoring of apoptosis during anticancer therapy has attracted intense research interest as a potential assay to predict treatment outcome. From all apoptosis-detecting radioligands, the group of Annexin V-based tracers is currently the most widely used for real time imaging of apoptosis in patients with various malignant and non-malignant diseases. In this article, we discuss the current status of in vivo imaging of apoptosis by means of 99mTc -Annexin V scintigraphy: the biodistribution of the tracers will be illustrated and the methodological aspects of gamma camera imaging procedure will be outlined. In addition, recently developed phosphatidylserine-targeting tracers for PET imaging of programmed cell death will be highlighted. We will specify the opportunities to improve visual assessment of the scintigraphic images. A four-step grading scale and SPECT/SPECT, SPECT/CT and SPECT/MRI fusion models will be discussed in order to overcome technical limitations caused by low tumour-to-background ratio. Furthermore, possible clinical applications of this technique in oncology will be illustrated in detail, with a major accent on the group of patients with malignant lymphoma, lung cancer and head and neck squamous cell cancer.

It has been known for some time now that hypoxia is an important physiological parameter in tumor growth and response to therapy. The development and application of non invasive methods to determine the extent of tumor hypoxia as well as its modulation will improve cancer treatment strategies. Magnetic Resonance sequences that are based on the BOLD effect use the endogenous contrast agent deoxyhemoglobin as a source of contrast. This technique can be used to monitor the evolution of tumor oxygenation since there is a good correlation between the evolution of the partial pressure of oxygen (pO2) and the NMR parameters measured. The information provided by BOLD NMR is essentially qualitative in nature due to the complexity of the relationship between the pO2 and the NMR parameters measured. The factors at the origin of this complex relationship are discussed in this review. The advantages of the BOLD technique are non invasiveness and high spatial and temporal resolution. The method has been successfully applied in experimental and human tumors to monitor changes after respiratory challenges and pharmacological treatments. Additionally, the method has been used to provide maps of mature and functional tumor vessels and maps of spontaneous fluctuations of oxygenation and blood perfusion related to tumor acute hypoxia.

Positron emission tomography (PET) allows imaging of molecular and physiological events in living subjects. Recent technical advances have made it possible to use PET for small animals. A major advantage of small animal PET studies compared to necropsy studies is that the former allows longitudinal within-subject design. Fully quantitative PET studies require information on the time course of tracer delivery to the tissue. In practical terms, one disadvantage of small animal PET imaging is the need to control the animal's motion during the study in order to obtain good quantitative invivo dynamic imaging results. Thus, animals must be either physically restrained or placed under anesthesia. Anesthesia is known to have significant effects on the central nervous, cardiovascular and respiratory systems. Restraining devices lead to extremely high stress levels, which can create conditions that poorly reflect normal physiology of an animal. In this paper, we assess feasibility and limitations of quantitative metabolic brain imaging with small animal PET in mice based on our recent published papers using [18F]FDG and small animal PET. Future perspectives in quantitative metabolic brain imaging with PET in small animals are also discussed.

Most functional magnetic resonance imaging (fMRI) studies are based on the detection of small timedependent signal variation induced by changes in blood oxygenation during brain activation. However, since this signal change is very small (typically 1-5%; even under optimal conditions and high magnetic field strengths), head motion as small as 1 mm translation or 1° rotation, can also produce spurious activation, if correlated with the stimulus paradigm. Several methods have been proposed to retrospectively (during image post-processing) or prospectively (in real time) correct for motion, however these techniques generally correct only for geometric rigid-body effects. Head motion can also change the magnetic field homogeneity, altering the effective relaxation rates of tissues (R2*), and producing timedependent geometrical distortions in fMRI studies. Unfortunately these non-linear motion-related artifacts become worse at high field strengths and cannot be corrected by standard image realignment methods; therefore, the range of motion is restricted for in vivo fMRI studies, especially at high field strengths. Real-time motion monitoring during fMRI can provide highly accurate information on whether subject motion during an fMRI scan was acceptable or excessive, and whether repeat scans with excessive motion are necessary.

Purpose: To compare the image quality on low-dose urinary tract multidetector CT examinations with 80 kVp and 120 kVp. Materials and Methods: Seventy-six patients with urinary tract symptoms were evaluated for urinary tract calculi by unenhanced multidetector CT. The examinations were performed with a four-row detector CT scanner. In all patients, two low-dose CT examinations with different kVp and mAs values (120 kVp, 30 mAs and 80 kVp, 95 mAs) were performed. The other scanning parameters were kept constant. Weighted-CT dose index was equal in both examinations. Both CT examinations were quantitatively compared with calculating the HU values of kidneys, subcutaneous fat, calculus, and phleboliths and the HU ratio of urinary calculi to the kidney. Both scans were also qualitatively compared for image quality of upper and lower abdomen. Results: One hundred and two calculi were detected in 50 of 76 patients. One or more phleboliths were present in 20 patients. HU values of the phleboliths and urinary calculi were higher on images obtained with 80 kVp than those obtained with 120 kVp. HU ratio of the calculi to the kidney was higher on scans with 80 kVp. Adipose tissue was less noisy and had lower HU value on 80 kVp images. Image quality of the upper abdomen and major pelvis were better on 80 kVp images, and the majority of these patients were normal (n=28) and overweight (n=20). Minor pelvis mostly (60%) revealed poorer image quality on 80 kVp images. However, the majority were overweight (n=22) or obese (n=6). In patients with osteoporosis, image quality was relatively better when compared with similar weighed patients without osteoporosis on 80 kVp images. Small renal, proximal and middle ureteral calculi were better demarcated on 80 kVp images. Conclusion: Low-dose urinary tract multidetector CT examination with 80 kVp, compared to 120 kVp provides better image quality in the upper abdomen and major pelvis. In overweight and obese patients, minor pelvis may not be evaluated thoroughly on 80 kVp images. Detection of small renal, proximal and middle ureteral calculi could be improved by using 80 kVp setting.

Magnetic resonance imaging (MRI) infused Radiology with rejuvenating vigor in the 1980s, owing credit to a couple of magnetic resonance spectroscopy (MRS) experiments performed in 1973. MRI has since been embraced by the radiology and medical communities. If the goal of MRS is to measure many chemicals in a homogeneous magnetic field, then the function of MRI is, in general, to measure one chemical - water - in an artificially created inhomogeneous field. Combining spectroscopy principles with technologies developed over the past two decades for MRI presented the philosophical appeal of non-invasively measuring metabolic molecules in living tissue, and led to the explosive developments in the last decade of in vivo MRS, and more recently MRSI, in the settings of diagnostic radiology. This review is intended to discuss the basic technologies of the current trends in the field of in vivo MRS and MRSI, especially the inherent predilections of individual techniques to the study of certain disease states. Following a historical introduction, individual techniques and their clinical applications, found in publications between January 2000 and October 2004, are reviewed in connection with related ex vivo results, after which the practical aspects of in vivo MRS and MRSI in clinical settings are discussed.

Virtual colonoscopy (VC), also known as CT colonography, is a noninvasive imaging method for the examination of the colon. The technique is easy, less labour-intensive than barium enema and conventional colonoscopy and is inherently safer since no adverse event or procedure-related complications have ever been reported. The success rate of VC is approximately 100%, if bowel preparation and distension are optimal and no sedation is required. From the patient's perspective, the major advantages of VC include the very brief time required to perform the examination, the absence of contrast enemas and the potential for same-day colonoscopy when polyps are detected. To date several studies have reported sensitivity and specificity comparable with conventional colonoscopy in the detection of clinically significant polyps. However, the smaller the polyp size, the lower the sensitivity. The currently accepted clinical uses include the evaluation of patients who have undergone unsuccessful or incomplete conventional colonoscopy, patients with obstructing colorectal cancer, and those whose medical problems make them unsuitable for conventional colonoscopy. Current uses generally do not include the screening of asymptomatic persons, although recently published work has demonstrated no statistically significant differences between VC and conventional colonoscopy regardless of polyp size. A practical approach is to consider virtual colonoscopy 'as a currently credible alternative screening method and as a reasonable alternative to the other colorectal cancer screening tests when a patient is unable or unwilling to undergo conventional colonoscopy'.

Small transient lesions in the central part of the splenium of the corpus callosum (SCC) have been reported in MR examinations, particularly in patients with encephalitis or encephalopathy, and in patients with or without epilepsy who are receiving antiepileptic drugs. The MR findings are characteristic: an ovoid T2-high signal lesion in the SCC, no enhancement after contrast agent, restricted diffusion shown by diffusion MR imaging, and complete disappearance on follow-up MR examinations. Most noteworthy is that these lesions have restricted diffusion but are reversible, which is distinct from infarction lesions seen in energy failure. The exact mechanism of these transient lesions remains uncertain and enigmatic. We review previously reported patients to show these unique MR findings and describe possible pathogenesis of particular SCC lesions.

Retinotopic mapping is a key property of organization of occipital cortex, predominantly on the medial surface but increasingly being identified in lateral and ventral regions. The retinotopic organization of early visual areas V1-3 is well established, although anatomical landmarks can help to resolve ambiguities in poorly-defined functional maps. New morphing techniques are now available to define the metric mappings quantitatively within each retinotopic area. In the dorsal occipital regions, there is fair agreement that area V3A should be split into separate V3A and V3B maps, and that beyond them lies a further area, V7. We specify the eccentricity mapping of both V3B and V7 for the first time, showing how the latter is roughly parallel to the meridional mapping and offering formal accounts of such paradoxical behavior. In ventral occipital cortex, we support the analysis of Zeki and Bartels [1] and Wade et al. [2] that V4 maps the full hemifield, and show the existence of two more areas, a ventromedial map of the lower quadrant, emphasizing the upper vertical meridian, and an adjacent area with a dominant foveal representation. In lateral cortex, the motion area defined by a motion localizer shows pronounced retinotopy, particularly in the eccentricity parameter. A dorsolateral map between the motion area and V3B, which represents the lower quadrant with an emphasis the foveal part of the lower vertical meridian, may be a counterpart to the ventromedial map.

Neurobiological models of obsessive-compulsive disorder (OCD) have emphasized the mediation of clinical symptoms by aberrant frontal-striatal circuits. A recent neurobiological model for normal human emotion perception has proposed distinct ventral and dorsal processing systems corresponding with differential levels of emotion perception. This construct appears also relevant for elucidating the pathophysiology of OCD, being an anxiety disorder. In the present paper, we present a comprehensive review of the neuroimaging literature in OCD, with the aim of critically appraising the current status of imaging-based knowledge on neurophysiological factors in the pathogenesis of OCD, along with the methodological possibilities and limitations of each imaging paradigm for OCD research. Moreover, we intend to relate these experimental findings to the frontal-striatal model and to the model of normal emotion perception. Results of imaging studies point to the need for adjustments in current models, with respect to the involvement of frontalstriatal circuits in OCD, as a ventral-dorsal dissociation may be distinguished. Moreover, data from pediatric OCD patient samples have commenced to shed light on developmental characteristics of OCD. Furthermore, longitudinal designs, in combination with pre-post treatment comparisons, are of great value for assessing the state-trait duality. Finally, the use of multimodal study designs holds great promise for the near future.