4 While there is a high degree of correlation between measurements of ventricular end-systolic and end-diastolic volumes measured by gated SPECT and cardiac MR imaging, it appears that cardiac MR is superior in providing more accurate chamber volumes [36, 37]. Positron emission tomography (PET) is used to assess myocardial viability by assessing its metabolism using FDG or myocardial perfusion using rubidium (Rb) 82. PET imaging has several advantages over SPECT, including better spatial resolution, higher counting eﬃciencies, and excellent attenuation correction. Computerized tomography (CT) techniques such as multi-slice CT and ultrafast electron beam tomography (EBT) are capable of providing images of the heart with high temporal and spatial resolutions. Multislice CT obtains images of the heart in many diﬀerent imaging planes and is capable of providing information on cardiac volumes and dimensions. Fast imaging combined with electrocardio- graphic triggering reduces motion artifacts. EBT imaging occurs in milliseconds, which resolves the issue of cardiac motion during imaging. It has been shown to have compa- rable diagnostic accuracy in diﬀerentiating between ischemic and nonischemic cardiomyopathy compared to radionuclide stress testing in patients with HF [38]. However, EBT requires imaging using contrast for delineation of diﬀerent chambers for determination of dimensions. This technique is also associated with high radiation exposure. As a result of great advancements in the technology of magnetic resonance imaging, it can now be used for multiple purposes in evaluation of patients with cardiac diseases. Cardiac MR (CMR) is useful in assessing cardiac, great vessels and coronary anatomy and ﬂow, ventricular function, myocardial viability, and perfusion. CMR provides the ability of imaging the heart in any desired plane in an unrestricted view, a clear advantage over echocardiography. Other advantages of CMR over echocardiography include the ease of studying the right ventricle due to its inherent three- dimensional nature and superior border detection between ventricular blood pool and the myocardium [34, 39–41]. Cardiac MR has very high accuracy and reproducibility for determination of ventricular volumes, stroke volume and ejection fraction [42, 43]. Using diﬀerent MR sequences or techniques, it is possible to detect ﬁbrosis, scarring, and inﬂammation of the myocardium. These advantages have resulted in making CMR the reference standard for ventricu- lar volumetric assessment. In fact, in many institutions where this imaging modality is available, cardiac MR is considered the alternative diagnostic test to echocardiography in cases with poor ultrasound images or when myocarditis or inﬁltrative disease of the heart is suspected [44]. 4.2. Imaging of the Kidney. Computerized tomography (CT) is the preferred imaging technique for the workup of kidney stones, renal masses, and renal arteries. Modern multide- tector CT scanners are capable of acquiring thin slices of large areas of the body in one breath hold. Advancements in postprocessing techniques have also improved the diagnostic accuracy of CT scanning. Three-dimensional reconstruction of images is commonly used without compromising on the International Journal of Nephrology quality of the images. In fact, 3D reconstruction of CT angiograms has been rapidly omitting the need for catheter angiography [45, 46]. CT urography has almost completely replaced intra- venous urograms in diagnosis of small stones and neoplasms of the kidney, ureters, and the urinary bladder. Filling defects within the collecting system might suggest a blood clot, stone, or a neoplasm. Unenhanced helical CT scan is the gold standard for diagnosing suspected renal colic secondary to kidney stones. Stones that are radiolucent on plain ﬁlms are readily detectable by CT. In addition, CT scan provides valuable information on other signs of urinary tract obstruction such as hydronephrosis or hydroureter and streaking around these structures. Contrast-enhanced CT is used to evaluate the arterial and venous supply of the kidney, renal parenchyma, and collecting system. Evaluation of the renal parenchyma occurs in the 3 phases, cortical phase, nephrographic, and excretory phase, after injection of the contrast. The detection of cortical or medullary lesions is best achieved during the nephrographic phase [47, 48]. CT imaging can also be used as a functional imaging modality for applications such as estimation of the GFR. Of course the risk of contrast-induced nephropathy is one of the main drawbacks of contrast- enhanced CT imaging, especially in individuals with acute or chronic kidney disease. The preferred agent for radionuclide imaging of the kidneys is 99mTc. It provides good image quality with low radiation and has a short half-life. 99mTc can be bound to diethylenetriaminepentaacetic acid (DTPA) for meas- urement of GFR or mercaptoacetyltriglycine (MAG3) to measure renal blood ﬂow. This imaging modality can be used in the diﬀerential diagnosis of AKI. Kidney uptake of 99mTc- MAG3 in the ﬁrst 1-2 minutes after injection is reduced in cases with acute tubular necrosis, while it would be normal in individuals with prerenal AKI. Renal uptake of the tracer during the late phase (20 minutes after injection) is expected to be high in prerenal AKI as well as ATN but low in postrenal AKI [49]. The main utility of MR imaging in the study of the genitourinary tract is in staging of malignancies. Magnetic resonance angiography may be used to study renal arteries, but its use is limited because of the risk of nephrogenic systemic ﬁbrosis in individuals with reduced renal function [50]. This risk is especially high (up to 20%) in individuals with AKI [51]. Sonographic imaging of kidneys with color Doppler and B mode has limited ability to evaluate the intrarenal arteries and arterioles. Imaging using ultrasound contrast agent can potentially overcome this limitation. The contrast agents used in renal US are gas-ﬁlled microbubbles that act similar to the red blood cells and remain in the intravascular space. In that regard, they are completely diﬀerent from contrast-agents used in CT or MRI [52]. Contrast-enhanced echocardiography has been used in the clinical setting. A few US contrast agents are approved for the study of myocardial blood ﬂow in humans. More recently, contrast enhanced ultrasonography (CEU) has been used in the study of changes in renal blood ﬂow in response to physiologic and