Andrew Webb

In vivo human coronary magnetic resonance angiography at 7 Tesla

Introduction. Cardiac MRI at high fields faces many challenges including the lack of commercially available body RF coils, increased sample-induced B1 inhomogeneity, increased magnetic susceptibility effects which make imaging with balanced sequences difficult, and SAR limitations. However, if these can be at least partially overcome, then the higher signal-to-noise is advantageous both for imaging and localized spectroscopy. Here we investigate the feasibility of acquiring coronary magnetic resonance angiography (CMRA) scans, a promising technique for the non-invasive visualization of the coronary anatomy (1), in volunteers at 7 tesla (T) on a time-scale acceptable for clinical studies.

G12 From premanifest to manifest Huntington's disease: a 2-year follow-up study with magnetic resonance spectroscopy at 7 Tesla

Background Previous cross-sectional magnetic resonance spectroscopy (MRS) studies in Huntingtons disease (HD) have demonstrated differences in metabolite concentrations compared to controls in several regions of interest, especially the putamen and caudate nucleus. It has been suggested that metabolite changes could be used as biomarker in future therapeutic trials. The aim of the present study was to assess metabolite changes in both premanifest and early HD over a 2 year follow-up period using MRS at 7 Tesla. Methods In 13 HD gene carriers (10 premanifest and 3 manifest HD) proton MRS was performed at baseline and after 24 months. At follow-up, four of the premanifest HD gene carriers had progressed into manifest HD, as assessed by clinical measures. 7T MR proton spectroscopy was performed in three regions of interest; the caudate nucleus, putamen and prefrontal cortex. Six metabolites were quantified for each region at each time point. Statistical analysis was performed using paired t-tests. Results In the caudate nucleus a decrease in creatine (p=0.032) and myo-inositol (p=0.006) concentrations was observed. A significant decrease in the putamen was seen in the total N-acetylaspartate (tNAA) (p=0.022) and choline concentrations (p=0.007). Premanifest HD converters showed higher rates of tNAA decrease in the putamen (p<0.003) than non-converting premanifest HD. Conclusion Over a period of 2 years we have demonstrated metabolite changes in the caudate nucleus and putamen of HD gene carriers around disease onset. This demonstrates the potential of MRS for providing a biomarker of disease progression and for evaluating future therapeutic interventions.

Introduction to Medical Imaging: X-ray planar radiography and computed tomography

Introduction X-ray planar radiography is one of the mainstays of a radiology department, providing a first ‘screening’ for both acute injuries and suspected chronic diseases. Planar radiography is widely used to assess the degree of bone fracture in an acute injury, the presence of masses in lung cancer/emphysema and other airway pathologies, the presence of kidney stones, and diseases of the gastrointestinal (GI) tract. Depending upon the results of an X-ray scan, the patient may be referred for a full three-dimensional X-ray computed tomography (CT) scan for more detailed diagnosis. The basis of both planar radiography and CT is the differential absorption of X-rays by various tissues. For example, bone and small calcifications absorb X-rays much more effectively than soft tissue. X-rays generated from a source are directed towards the patient, as shown in Figure 2.1(a). X-rays which pass through the patient are detected using a solid-state flat panel detector which is placed just below the patient. The detected X-ray energy is first converted into light, then into a voltage and finally is digitized. The digital image represents a two-dimensional projection of the tissues lying between the X-ray source and the detector. In addition to being absorbed, X-rays can also be scattered as they pass through the body, and this gives rise to a background signal which reduces the image contrast. Therefore, an ‘anti-scatter grid’, shown in Figure 2.1(b), is used to ensure that only X-rays that pass directly through the body from source-to-detector are recorded.

Introduction to Medical Imaging: Magnetic resonance imaging (MRI)

Introduction Of the four major clinical imaging modalities, magnetic resonance imaging (MRI) is the one developed most recently. The first images were acquired in 1973 by Paul Lauterbur, who shared the Nobel Prize for Medicine in 2003 with Peter Mansfield for their shared contribution to the invention and development of MRI. Over 10 million MRI scans are prescribed ever year, and there are more than 4000 scanners currently operational in 2010. MRI provides a spatial map of the hydrogen nuclei (water and lipid) in different tissues. The image intensity depends upon the number of protons in any spatial location, as well as physical properties of the tissue such as viscosity, stiffness and protein content. In comparison to other imaging modalities, the main advantages of MRI are: (i) no ionizing radiation is required, (ii) the images can be acquired in any two- or three-dimensional plane, (iii) there is excellent soft-tissue contrast, (iv) a spatial resolution of the order of 1 mm or less can be readily achieved, and (v) images are produced with negligible penetration effects. Pathologies in all parts of the body can be diagnosed, with neurological, cardiological, hepatic, nephrological and musculoskeletal applications all being widely used in the clinic. In addition to anatomical information, MR images can be made sensitive to blood flow (angiography) and blood perfusion, water diffusion, and localized functional brain activation.

Introduction to Medical Imaging: Nuclear medicine: Planar scintigraphy, SPECT and PET/CT

Introduction In nuclear medicine scans a very small amount, typically nanogrammes, of radioactive material called a radiotracer is injected intravenously into the patient. The agent then accumulates in specific organs in the body. How much, how rapidly and where this uptake occurs are factors which can determine whether tissue is healthy or diseased and the presence of, for example, tumours. There are three different modalities under the general umbrella of nuclear medicine. The most basic, planar scintigraphy, images the distribution of radioactive material in a single two- dimensional image, analogous to a planar X-ray scan. These types of scan are mostly used for whole-body screening for tumours, particularly bone and metastatic tumours. The most common radiotracers are chemical complexes of technetium ( 99m Tc), an element which emits mono-energetic γ-rays at 140 keV. Various chemical complexes of 99m Tc have been designed in order to target different organs in the body. The second type of scan, single photon emission computed tomography (SPECT), produces a series of contiguous two-dimensional images of the distribution of the radiotracer using the same agents as planar scintigraphy. There is, therefore, a direct analogy between planar X-ray/CT and planar scintigraphy/SPECT. A SPECT scan is most commonly used for myocardial perfusion, the so-called ‘nuclear cardiac stress test’. The final method is positron emission tomography (PET). This involves injection of a different type of radiotracer, one which emits positrons (positively charged electrons).

Introduction to Medical Imaging: General image characteristics, data acquisition and image reconstruction

Introduction A clinician making a diagnosis based on medical images looks for a number of different types of indication. These could be changes in shape, for example enlargement or shrinkage of a particular structure, changes in image intensity within that structure compared to normal tissue and/or the appearance of features such as lesions which are normally not seen. A full diagnosis may be based upon information from several different imaging modalities, which can be correlative or additive in terms of their information content. Every year there are significant engineering advances which lead to improvements in the instrumentation in each of the medical imaging modalities covered in this book. One must be able to assess in a quantitative manner the improvements that are made by such designs. These quantitative measures should also be directly related to the parameters which are important to a clinician for diagnosis. The three most important of these criteria are the spatial resolution, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). For example, Figure 1.1(a) shows a magnetic resonance image with two very small white-matter lesions indicated by the arrows. The spatial resolution in this image is high enough to be able to detect and resolve the two lesions. If the spatial resolution were to have been four times worse, as shown in Figure 1.1(b), then only the larger of the two lesions is now visible. If the image SNR were four times lower, illustrated in Figure 1.1(c), then only the brighter of the two lesions is, barely, visible.