Current Medical Imaging - Volume 2, Issue 3, 2006

In magnetic resonance imaging (MRI), contrast between different tissues depends on a number of parameters including transversal and longitudinal relaxation times (T1, T2), susceptibility, proton density and flow. Despite this multiparameter dependence, the usefulness of contrast agents has been recognized since MRI was first introduced. Contrast agents in MRI are chemicals which by virtue of their unpaired electrons induce relaxation in the water molecules in their vicinity. Depending on their chemical structure, different contrast agents may alter the signal in specific regions of the image or in specific pathologies. Established and widely used contrast agents are small Gd chelates which, after injection into the bloodstream, rapidly extravasate into the extravascular space. More recently, a different class of contrast agents has been developed with the property of remaining in the vascular space (blood pool contrast agents). Blood pool contrast agents have been used to investigate vasculature in tumors (angiogenesis) and in the brain (cerebral blood volume and functional MRI). In the last few years, great attention has been devoted to the use of contrast agents for cellular and molecular imaging. It has been realized that cells, opportunely marked with contrast agents, can be visualized in vivo with MRI. The technique has been applied successfully to in vivo detection of stem cells after transplantation. Moreover, contrast agents have been applied to imaging phenomena at the molecular level in vivo, for instance to investigate gene expression in tissues. In this article we describe some innovative applications of contrast agents for MRI.

Assessment of ischemic heart disease is an important application of magnetic resonance imaging (MRI). This noninvasive diagnostic technique has the potential to evaluate different aspects of ischemic heart disease such as myocardial function, perfusion and the extent of acute and chronic infarctions. MRI is becoming an alternative technique for assessment of myocardial viability by predicting systolic recovery of stunned and hibernating myocardium after revascularization. MR contrast agents provide additional information on residual myocardial viability, myocardial perfusion, coronary angiography and microvascular integrity. Recent technologic improvements in scanners, sequences and post-processing of images showed the potentiality of MR-guided procedures. This review begins with a brief overview of magnetic resonance properties and MR contrast agents and proceeds to their applications in cardiac imaging and endovascular intervention. The next generation of MR contrast agents will likely focus on increase detection sensitivity so that minor tissue changes can be detected, prolonging intravascular retention (blood pool contrast agents), improving tissue targeting (tissue and molecule specific contrast agents and probes) and molecular imaging. MRI will play viable role in cardiovascular imaging and intervention in the near future and it may have the potential to complement other commonly used diagnostic modalities.

Adenosine myocardial perfusion imaging is a commonly used modality for the evaluation of patients with known or suspected coronary artery disease (CAD). In this review we discuss the pharmacology of adenosine and its effects on myocardial blood flow and the generation of perfusion defects during myocardial perfusion imaging (MPI), the indications and contraindications of MPI, stress protocols, side effects, hemodynamic and electrocardiographic changes, comparison to other stress MPI modalities, the role of adenosine MPI in special patient populations and the recent advances in the field with the emergence of the new selective adenosine agonists.

Breast cancer is a major health problem in women, worldwide. A combination of physical examination, mammography, ultrasound and fine needle aspiration cytology (FNAC) or core biopsy is currently the mainstay for preoperative diagnosis of breast lesions. Magnetic resonance imaging (MRI) has been proposed as a useful complimentary modality to provide additional information. It has shown potential in the detection, diagnosis and management of breast cancer. It can be used to improve preoperative staging, follow response to therapy, and to detect local recurrences. Contrast enhanced MRI (CEMRI) has evolved as an important tool in the evaluation of breast abnormalities with high sensitivity (94-100%), but with poor specificity. Recently, diffusion and perfusion MRI techniques have been applied to breast lesion characterization and show promise but warrant further investigations. Characterization of breast lesions may also be assisted by using information of the cellular chemistry provided by in vivo proton MR spectroscopy (MRS). Recent reports have shown that neoplastic breast tissue contains elevated levels of choline containing compounds, which can be used as a biochemical marker for differentiating benign versus malignant tumor and for monitoring treatment response. Presently, the various MR techniques show promise primarily as adjunct to the existing standard detection techniques, and its acceptability as a primary screening method will increase only if specificity can be increased. This review presents the current status of MRI and MRS in breast imaging.

Breast cancer and melanoma metastasize predominantly via the lymphatic route. It has long been known that invasion into one or a few nodes draining the primary tumour, the sentinel nodes (SN), is the most important, early sign of dissemination. If no malignant cells are detected in the SN, dissemination is unlikely to be expected. For the last 10 years SN biopsy has become an important tool in staging cancers. Two kinds of tracers are used for SN detection: The blue dye, injected during operation, and radioactively labelled colloid, injected before operation. The lymphatic drainage can then be mapped by following the blue dye by visual inspection during the operation, and with gamma camera imaging before and probe detection during the operation. The variations in the tracers used, and the injection and imaging techniques are discussed. The pathologic examination has also undergone a rapid evolution with more detailed analysis including immunohistochemistry. The use of the SN technique has quickly spread worldwide for melanoma and breast cancer but is also being tested in several other cancers. Reports on the influence on morbidity and mortality reduction are becoming increasingly convincing. The near future of SN examination is finally briefly outlined.

For some planar scintigraphic explorations, like thyroid or hip, the pinhole collimator is usually preferred to the usual parallel hole collimators because it allows a superior resolution. Progress in 3D tomography reconstruction methods allows single photon emission tomography (SPET) to be today performed using the pinhole collimator with again the advantage of a higher resolution. Pinhole collimator imposes a short distance (a few centimetres) to the target area and a restricted field of view. Over the last ten years, the potential of pinhole SPET has been essentially investigated for the ankle and the hind foot, the detection of thyroid nodules or abnormal parathyroid glands, the exploration of axillary lymph nodes. The technique was usually compared to planar scintigraphy performed with parallel hole or pinhole collimator. Sometimes a comparison to standard (with parallel hole collimator) SPET or to a morphologic imaging technique was also available. All studies highlight the improvement in resolution afforded by the use of the pinhole collimator. In bone scintigraphy, the delineation of the structures was largely superior allowing the visualisation of significant details usually never seen on scintigraphic images. In thyroid scintigraphy, the better resolution allowed to detect more nodules (essentially infra-centimetre nodules), and to reduce the number of equivocal cases. For abnormal parathyroid gland detection, the number of false negative cases decreased, with a concomitant increase of true positive cases; the glands were more sharply delineated on the images. Pinhole SPET with Tc-99m-tetrofosmin was the only scintigraphic method able to successfully reveal the number of involved lymph nodes in patients with breast cancer. In conclusion pinhole single photon appears as a promising new scintigraphic method allowing to explore small areas with a high resolution.

Polycystic ovary syndrome (PCOS) is one of the most controversial endocrine disease, for its clinical and biochemical heterogeneity. Many authors focused on the possible role of genetic, environmental and hormonal factors in the development of the syndrome, but the etiopathogenesis still remains unclear. The diagnostic criteria for the definition of PCOS are as heterogeneous as the disease itself and underwent more than one revision throughout the last years. Ultrasonography has been widely accepted as an important criterion for the detection of polycystic ovaries. Furthermore, Doppler flow analysis of both intraovarian and uterine arteries seems to provide an insight to the pathological state and the degree of progression of the disease. In this review, we provide the state of the art for a correct diagnosis of PCOS, pointing out the capacity of ultrasonography and color Doppler imaging in the early detection of the syndrome and the possibility to obtain additional information about its pathophysiology. Since PCOS patients have higher cardiovascular risk the precocity and the precision of the diagnosis is extremely important to improve the long-term prognosis.

Fractional moving blood volume (FMBV) estimated using power Doppler ultrasound (PDU) is a sensitive technique to detect slow blood movement. However, several factors as; depth, attenuation and blood characteristics can affect the PDU signals. The aim of this study was to standardized and validate the FMBV measurement compensating for the factors that can affect the final results, in order to accurate estimate blood movement in a specific region of interest (ROI) of fetal organs as an estimation of blood perfusion. FMBV measurement for the fetal lung blood perfusion was standardized in 29 healthy fetuses at 35-37 weeks of gestation. Clear definition of the ROI in both fetal lungs was proposed. FMBV values were in the range of 27-42% (mean 36.2%) and showed a coefficient of variation of 0.14. No significant differences between the FMBV values obtained from the left and the right lungs, or between cardiac systole and diastole were observed. A phantom experimental study showed good sensitivity for slow blood movement. FMBV was then compared with blood perfusion measurements evaluated with radioactive labeled microspheres (RMS) in the adrenal gland of 5 fetal lambs exposed to severe asphyxia. There was a high correlation between FMBV and RMS (r=0.90, range 0.43-0.99). The reproducibility and agreement analyses showed an intraclass correlation coefficient of 0.92 (95% confidence intervals [CI] 0.78-0.96), and a mean difference between observers of 0.06 (SD, 3.7). Conclusion. The estimation of the fractional moving blood volume measurement using power Doppler ultrasound can be reliable correlated with true fetal organ blood perfusion. FMBV is a reproducible method that can offer valuable information about adaptive fetal blood flow changes in the presence of pregnancy associated complications.

Gray-scale and color Doppler ultrasonography have limited accuracy in characterization of focal liver lesions because a significant overlapping exists between the echo pattern and vascularity of benign and malignant lesions. Several clinical studies have demonstrated that characterization of focal liver lesions that are indeterminate at conventional ultrasonography could be improved using microbubble contrast agents with contrast specific modes. In particular, latest generation ultrasound contrast agents allow evaluation of lesion vascularity in real time using low acoustic power contrast specific modes with the resolution afforded with gray-scale imaging. In this review article the enhancement pattern of different types of benign and malignant focal liver lesions using low acoustic power contrast specific modes is described, and the current role of contrast enhanced ultrasonography in liver lesion characterization is discussed.