Techniques for the coherent generation and detection of electromagnetic radiation in the far infrared, or terahertz, region of the electromagnetic spectrum have recently developed rapidly and may soon be applied for in vivo medical imaging. Both continuous wave and pulsed imaging systems are under development, with terahertz pulsed imaging being the more common method. Typically a pump and probe technique is used, with picosecond pulses of terahertz radiation generated from femtosecond infrared laser pulses, using an antenna or nonlinear crystal. After interaction with the subject either by transmission or reflection, coherent detection is achieved when the terahertz beam is combined with the probe laser beam. Raster scanning of the subject leads to an image data set comprising a time series representing the pulse at each pixel. A set of parametric images may be calculated, mapping the values of various parameters calculated from the shape of the pulses. A safety analysis has been performed, based on current guidelines for skin exposure to radiation of wavelengths 2.6 μm–20 mm (15 GHz–115 THz), to determine the maximum permissible exposure (MPE) for such a terahertz imaging system. The international guidelines for this range of wavelengths are drawn from two U.S. standards documents. The method for this analysis was taken from the American National Standard for the Safe Use of Lasers (ANSI Z136.1), and to ensure a conservative analysis, parameters were drawn from both this standard and from the IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields (C95.1). The calculated maximum permissible average beam power was 3 mW, indicating that typical terahertz imaging systems are safe according to the current guidelines. Further developments may however result in systems that will exceed the calculated limit. Furthermore, the published MPEs for pulsed exposures are based on measurements at shorter wavelengths and with pulses of longer duration than those used in terahertz pulsed imaging systems, so the results should be treated with caution.

MR tagging is a recent imaging development that, in cardiac applications, makes possible the tracking of points in the myocardium during the cardiac cycle. Researchers have developed semiautomated, computer-based methods for analyzing tagged images, but the images are complex and present a challenge to automated tracking systems. Simulation can provide an inexhaustible supply of images for testing and validation of tag tracking software and preview the effect of parameter changes in acquisition. SIMTAG is an interactive computer program that simulates two-dimensional tagged-MR experiments. The mathematic model used in the simulation and algorithms for simulating image noise and object deformation are described. Examples of the use of simulated images in SPAMM parameter selection, a comparison of tag contrast in signal-averaged SPAMM and CSPAMM, and simulated images as test sets for tag-tracking software are presented.

Simulation of MR images is a useful tool for offline sequence development and as an aid to understanding image formation. One particular application of simulation is MR tagging, which is used for tracking myocardial motion. Simple spatial-domain methods cannot adequately represent effects common in these images, such as motion artifact and signal wrap. An existing frequency-domain model is shown to be inappropriate for tagged images, and an extension based on the Bloch equations and Fourier shift theorem is described to correct this. Software incorporating the new model is used to generate ideal tag intensity profiles and to accurately simulate tagged images. The shifted k-space patterns associated with tagged images, and their dependence on the order of the binomial tagging sequence, are explained. An application of the Fourier shift theorem is suggested that allows more rapid simulation of static tagged images.

1289Discussion A high incidence of hypertension, ranging from 30 to 80% has been reported within two to 12 months after transplantation.2-6 We have extended these observations by showing that about half of a large series of patients were hypertensive up to six years after transplantation.Interestingly, the stability of the incidence of hypertension over the years was only apparent, as blood pressure had returned to normal by three years in a third of the patients who were hypertensive at three months, whereas over a third of the initially normotensive subjects had become hypertensive by three years. As a result, although never more than 66% of the patients had hypertension at any one time, 82% of all transplanted patients were hypertensive at one time or another during follow-up.The causes of hypertension in renal transplant recipients are multiple. Our data suggest that, contrary to previous findings,5 the dose of corticosteroids is not by itself an important factor. A significant correlation between blood pressure and steroid dosage was found during follow-up in only a quarter of the hypertensive patients. Three months after transplantation no relation was found between steroid dose and diastolic blood pressure-an observation that agrees with others.24 Finally, there was no significant difference in the steroid intake of patients whose blood pressure remained consistently raised over five years compared with values in patients with normal pressures over five years. The patients' own kidneys1 6 could not be incriminated because 80 of the 85 patients had undergone bilateral nephrectomy. Renal artery stenosis was observed in only two patients, but renal angiography was performed only in patients with severe refractory hypertension. In a prospective study,8 incidences as high as 23% were reported, though renal artery stenosis was also noted in normotensive patients, which underlines the fact that the association between renal artery stenosis and hypertension may not always be causal. At the very least, our data suggest that renal artery stenosis is not a common cause of hypertension after renal transplantation.In our series renal failure was the most significant factor associated with the development of hypertension, especially among severely hypertensive patients. Many more of the patients with severe hypertension had an abnormal creatinine concentration, and the mean concentration in this group was significantly higher than that of the normotensive patients.Our data should not obscure the fact that 20-25% of hypertensive patients had both normal renal function and a daily steroid dose of 10 mg or less 24 to 60 months after transplantation. Smellie et al9 and Malekzadeh et al'0 have also reported hypertension in transplanted patients with a normal renal function. Arteriography in some of their patients showed intrarenal arterial lesions that were tentatively attributed to subclinical rejection episodes. Similar lesions might have caused hypertension in some of our patients. They might have accounted for the progr...