| | MR imaging of the kidneys and adrenal glands
Advances in MR imaging technology, including high-performance gradients, faster pulse sequences, and phased-array coils, provide for the acquisition of near isotropic three-dimensional data sets during a single breath-hold. Volumetric rendering of these data provides surgically relevant information for minimally invasive renal-adrenal surgery and allows MR imaging to compete with multidetector CT. Further advantages of MR imaging include lack of ionizing radiation, direct multiplanar capability that enables more accurate localization of masses, and superior intrinsic soft tissue contrast augmented by the use of extracellular gadolinium chelates. Gadolinium chelates have been shown to be exceedingly safe, may be administered to patients without concern for contrast-induced nephrotoxicity, and are well tolerated in those patients with a history of iodinated contrast allergy [1], [2]. Finally, the ability of MR imaging to detect both gross and microscopic fat provides for accurate characterization of adrenal and renal masses.
Technique  With the evolution of MR imaging technology, the protocols used to evaluate the kidneys and adrenal glands have also evolved. At the authors' institution, all abdominal MR imaging examinations are performed with a torso phased-array coil. Phased-array coils increase signal to noise by a factor of two to three, which allows for the use of smaller fields of view with concomitant increased spatial resolution. Breath-hold sequences are used exclusively to minimize artifacts secondary to respiratory motion. Studies are performed during end expiration to optimize image co-registration for subtraction algorithms. For those patients in whom the sequences are longer than their breath-hold capability, the authors administer 2 L/minute of oxygen by nasal cannula. Finally, before starting the MR imaging examination, cushions are used to elevate the patient's arms anterior to the level of the kidneys and out of the imaging plane of a coronal acquisition. This minimizes wraparound artifact (in the phase-encoding direction) when performing three-dimensional coronal acquisitions and allows the use of smaller fields of view with improved resolution. Comprehensive examination of the kidneys entails evaluating the renal vasculature, parenchyma, and collecting system. Precontrast imaging includes an axial breath-hold T1-weighted gradient echo (GRE) sequence performed in and out of phase. This sequence provides an excellent anatomic overview of the abdomen and is useful to evaluate for adenopathy and characterize an incidental adrenal lesion. Used in conjunction with a frequency-selective fat-suppressed T1-weighted sequence, this also allows differentiation of fat from hemorrhage, both of which may occur in a renal or adrenal mass. A coronal breath-hold T2-weighted half-Fourier single-shot turbo spin echo sequence is performed to help characterize cystic lesions of the kidney, to assess for hydronephrosis, and to determine if an adrenal mass is present. The coronal plane is advantageous in evaluating exophytic lesions that occur at the poles of the kidneys. These may not be optimally demonstrated in the axial plane. In addition, coronal plane images are more helpful in establishing the association of the lesion to its surrounding organs. To evaluate the renal vasculature, a high-resolution breath-hold fat-suppressed three-dimensional T1-weighted spoiled GRE sequence is performed in a coronal-oblique plane before and after the intravenous administration of gadolinium. Using the proper scan delay and acquiring the low spatial frequency (high contrast) lines of k-space during peak arterial enhancement are critical to minimize venous contamination and to prevent scanning before sufficient contrast reaches the aorta. Optimization of the arterial phase may be determined by using fluoroscopic triggering [3], an automated bolus detection technique [4], a timing run, or with a “best guess” method. As gradient strength and pulse sequences improve, a time-resolved approach can be used, which obviates the need for a timing strategy [5]. By scanning with rapid temporal resolution (approximately 5 seconds) at least one phase shows optimal arterial opacification. The authors prefer a timing run using 1 mL of gadolinium followed by a 20-mL saline flush injected at 2 mL/second by a power injector [6]. The MR angiogram is then performed with 19 mL of gadolinium followed by a 20-mL saline flush. Approximately 30 seconds after the angiogram, the same sequence is repeated to obtain an MR venogram. With the near isotropic resolution of a three-dimensional sequence, it is possible to evaluate and reformat the images in innumerable planes using a workstation. It is also possible to display the data as maximum-intensity-projection (MIP) images, which have a similar appearance to conventional angiography. With the MIP algorithm, however, small vessels may not be depicted and stenoses may be overestimated. It is always necessary to review the three-dimensional source data to confirm the findings of the MIP images. Evaluation of the renal parenchyma is performed with a second breath-hold three-dimensional fat-suppressed T1-weighted spoiled GRE sequence in the axial plane [7]. This is performed before and after the administration of intravenous gadolinium. The postcontrast acquisition is obtained after the MR venogram, approximately 3 to 5 minutes after the gadolinium bolus. This is the most important sequence the authors use in characterizing a renal mass as an enhancing neoplasm or a cyst. In the authors' experience, some cystic neoplasms are so hypovascular that at least 2 minutes are needed to demonstrate enhancement. In many cases, it is possible to determine qualitative enhancement of a lesion with side-by-side comparison of the precontrast and postcontrast acquisitions. In those cases in which a lesion is hyperintense on the precontrast images, however, qualitative enhancement is difficult, if not impossible, to appreciate. In this instance, a subtraction algorithm may be applied to help assess enhancement characteristics (Fig. 1) [8]. With good image co-registration, subtracted images appear similar to fat-suppressed postcontrast images. In those cases of poor image co-registration, however, a careful evaluation of the nonsubtracted images is necessary. MR urography may be performed with T2-weighted turbo spin echo sequences using a thick slab projection technique or with multiple contiguous thin sections. Alternatively, a delayed three-dimensional T1-weighted GRE sequence after the administration of gadolinium contrast material can be performed [9]. The authors prefer the latter technique because the spatial resolution is much higher and the voxels are near isotropic, which provides for excellent image quality when viewed at any projection. Immediately after the timing run, all patients receive 10 mg of intravenous furosemide to augment diuresis. After the “parenchymal” axial three-dimensional T1-weighted GRE sequence, the “vascular” coronal-oblique three-dimensional T1-weighted sequence is repeated a final time to evaluate the collecting system and ureter (approximately 12 minutes after the initial injection of contrast). The precontrast acquisition is then subtracted from the delayed MR urographic acquisition. A MIP image is created, which has a similar appearance to a conventional urogram (Fig. 2). Renal mass characterization With the exception of angiomyolipoma, routine MR sequences have not been shown to be sensitive or specific in the characterization of renal masses. The most important aspect in analyzing a renal mass is to demonstrate the presence or absence of enhancement. An enhancing mass implies a vascular mass, consistent with a neoplasm. Once a lesion has been shown to demonstrate enhancement, it is necessary to characterize it as a surgical lesion (renal cell carcinoma, oncocytoma, or transitional cell carcinoma) or a nonsurgical lesion (metastases, lymphoma, or angiomyolipoma). Renal cell carcinoma Renal cell carcinoma is the most common renal neoplasm accounting for 80% to 85% of all malignant renal tumors and for 2% of all cancers [10]. The widespread use of cross-sectional imaging and the incidental detection of asymptomatic neoplasms have increased the incidence of renal cell carcinoma [10]. Combined with the improved characterization of small renal lesions and earlier surgical intervention, there has been a slight improvement in the 5-year survival of renal cell carcinoma [10]. Renal cell carcinomas have variable signal intensity on T1- and T2-weighted sequences. With regard to the background renal parenchyma, they are often slightly hypointense on T1-weighted images and isointense to slightly hyperintense on the T2-weighted images. Renal cell carcinomas, however, may demonstrate any signal intensity depending on their content of hemorrhagic material. The diagnosis of renal cell carcinoma rests on demonstrating enhancement within a renal mass. Unlike CT, in which Hounsfield units are standardized, MR intensity units are arbitrary and vary from sequence to sequence. It is difficult to accurately quantify enhancement. Qualitative enhancement by means of a subjective comparison of the precontrast and postcontrast acquisitions or more accurately by means of subtraction algorithms needs to be performed. This is most important for hypovascular lesions or those that are hyperintense on the precontrast T1-weighted images. The prognosis of renal cell carcinoma is related to the tumor stage. MR imaging has been shown to be accurate for the staging of renal cell carcinoma and more accurate than CT for the evaluation of tumor extension into the renal vein and inferior vena cava [11]. It is important to demonstrate the most cephalad extent of thrombus in the inferior vena cava, because the surgical approach is altered if thrombus approaches the right atrium. For this the multiplanar capability of MR imaging is ideally suited. In addition, it is often possible to differentiate enhancing tumor thrombus from bland thrombus (Fig. 3). MR imaging does not offer any advantage when compared with CT for evaluating retroperitoneal adenopathy. The signal characteristics of metastatic lymph nodes are similar to those of normal lymph nodes. As with CT, evaluation for metastatic adenopathy is size dependent, with lymph nodes greater than 1 cm considered abnormal and suspicious for metastatic disease. After surgery, gadolinium-enhanced MR imaging may be used to evaluate for early postoperative complications including hemorrhage or urinary leak in those patients who undergo partial nephrectomy (Fig. 4). In addition, MR imaging is useful in the routine postoperative surveillance for recurrent neoplasm or metachronous lesion for which these patients are at increased risk. In these patients with reduced numbers of nephrons, this can be performed safely with MR imaging without exposure to nephrotoxic contrast agents. Angiomyolipoma Angiomyolipoma (renal hamartoma), a benign tumor, is composed of varying amounts of fat, smooth muscle, and blood vessels. They are uncommon lesions with a prevalence of 0.3% to 3% and occur more commonly in women than men [12]. Angiomyolipomas occur in two different clinical scenarios. More commonly, they are sporadic (80%); however, they may be associated with tuberous sclerosis (20%) in which they tend to be multiple and bilateral [13]. Patients are usually asymptomatic and angiomyolipomas are usually incidentally discovered when the patient is imaged for another reason. When large, angiomyolipomas may exert mass effect on the adjacent organs and cause symptoms. In addition, patients with large angiomyolipomas may present with acute flank pain caused by spontaneous hemorrhage. This may be life threatening and require emergent laparotomy. Angiomyolipoma is the only renal tumor that may be characterized on the basis of its tissue composition and signal characteristics. The relative amounts of fat, smooth muscle, and vessels within the tumor establish its MR imaging appearance. The diagnosis of angiomyolipoma rests on demonstrating the presence of macroscopic fat within the lesion. When an angiomyolipoma is predominately composed of fatty tissue, it demonstrates hyperintense signal on the T1-weighted images. Other renal masses, however, including hemorrhagic cysts may also show similar signal characteristics. It is imperative to compare the T1-weighted images obtained with frequency-selective fat-suppression with those obtained without fat-suppression, to establish the presence or absence of macroscopic fat (Fig. 5). The use of frequency-selective fat-suppression is essential, because hemorrhage and other tissues with a short T1 lose signal on inversion recovery pulse sequences and may be diagnosed erroneously as containing fat. Some angiomyolipomas contain only a tiny amount of macroscopic fat and a concerted effort should be made to identify even small amounts of fat. In rare instances these lesions may not contain any fat. In such cases, the diagnosis of angiomyolipoma cannot be made and the lesion is indistinguishable from a renal cell carcinoma. Angiomyolipoma may also be diagnosed with the use of chemical-shift imaging techniques. Exploiting the precessional frequency differences of fat and water, this technique provides images when fat and water signal are in phase (additive) or out of phase (destructive). This produces the characteristic India ink artifact on the T1-weighted out-of-phase images, manifested as a low signal intensity rim at any soft tissue (water) and fat interface. Both hemorrhagic cysts and angiomyolipomas are hyperintense on T1-weighted in-phase images and may be indistinguishable from each other. They are readily differentiated, however, on the T1-weighted out-of-phase images. For angiomyolipomas, the India ink artifact appears at the interface of the tumor (fat) with the kidney (water) (Fig. 6). For hemorrhagic cysts, the India ink artifact occurs at the interface of the cyst (fluid) and the perirenal fat (fat) (Fig. 7), not at the interface of the cyst and the kidney. Caution should be used in diagnosing a renal mass as an angiomyolipoma if it loses signal on out-of-phase imaging, because clear cell carcinoma of the kidney may show identical findings [14]. Clear cell carcinoma does not contain bulk fat, however, and does not lose signal on frequency-selective fat-suppressed T1-weighted images. Lymphoma Lymphoma may involve the kidneys by hematogenous spread, in which a single mass or multiple bilateral masses are present, or by direct extension of retroperitoneal lymphoma. Generally, most patients with renal lymphoma have systemic involvement and the diagnosis should not be difficult, given the appropriate clinical history. The MR imaging appearance of lymphoma is nonspecific; however, the most common appearance is that of multiple homogeneous solid masses that may be well defined, but tend to have infiltrative margins with the kidney. When lymphoma diffusely infiltrates a kidney, the kidney enlarges, but maintains its reniform shape [15]. Metastases The most common tumor to metastasize to the kidney is carcinoma of the lung. Renal metastases tend to be multiple and bilateral, and frequently are associated with metastases to other organs. Although they have nonspecific MR imaging features, renal metastases may demonstrate infiltrative growth patterns. With the proper clinical history, the diagnosis should be obvious. In a patient with a history of malignancy (without other metastases) and a solitary renal mass, however, the renal mass is more likely to represent a renal cell carcinoma, and not a metastasis [16]. Nevertheless, it is possible that a single renal metastasis could occur, and differentiation from a renal cell carcinoma may not be obvious. In this situation, a renal biopsy is indicated to determine the exact etiology of the lesion. Simple and complex renal cysts The appearance of complex cystic renal masses is diverse and the proper management of these lesions is frequently not clear-cut. Bosniak [17], [18], [19], [20] has proposed a classification system designed to help categorize cystic lesions into surgical and nonsurgical cases. Although the classification scheme is based on CT criteria, the same approach provides a useful framework for MR imaging [21]. It should be stressed, however, that there is not always a clear correlation between the findings at MR imaging and the CT images, and further work is needed to identify these differences. For example, just as septations may be identified more readily with ultrasound than with CT, the same may be true for MR imaging. When evaluating a complex cystic renal mass on an MR examination, it is necessary to analyze the various components of the lesion. This includes the number of septae, the thickness of the wall or septae, the interface of the lesion with the kidney, the contents of the lesion, and most importantly, the presence or absence of enhancing soft tissue components. Simple cysts (category I) are common and demonstrate hypointense signal on T1-weighted images and are uniformly hyperintense on T2-weighted images. After gadolinium, they do not enhance. Category II cysts are benign and mildly complex. They may contain very thin septae that are best depicted on the T2-weighted images, where they appear as thin low signal intensity curvilinear structures against the hyperintense cystic fluid (Fig. 8). When these lesions contain hemorrhagic or proteinaceous material, they demonstrate hyperintense signal on the T1-weighted images. MR imaging is ideally suited for characterizing hemorrhagic cysts, particularly in those patients who cannot receive iodinated contrast secondary to a history of renal failure or allergy. This is especially true in patients with acquired cystic disease of dialysis or with autosomal-dominant polycystic kidney disease, in which hemorrhagic cysts are very common. Using a subtraction algorithm, it is possible to demonstrate that these lesions do not enhance, and thereby characterize them as benign hemorrhagic cysts (see Fig. 1). Category III lesions are more complex. They may demonstrate thick enhancing walls or thick enhancing septae, but do not contain nodular enhancing soft tissue components associated with the wall or septae (Fig. 9). These lesions are indeterminate and in most cases surgery is indicated. Approximately 50% of these lesions are malignant [19]. Category IV lesions are those cystic masses that are clearly malignant and demonstrate unequivocal enhancing soft tissue components. Differentiation of a more complex category II cyst from a less complicated category III cyst is where the most disagreement occurs among radiologists. This differentiation is critical, however, because category II lesions are benign and do not require treatment, whereas category III lesions, in most cases, require surgery. A subcategory, category IIF, was proposed. Category IIF cysts can be followed-up with additional examinations, and if there is any interval growth, the lesion has to be considered as category III or higher and needs surgical evaluation. Without exposure to radiation or nephrotoxic contrast material, MR imaging is ideal for following these lesions. A limitation of MR imaging in characterizing cystic renal masses is the inability to depict calcification within the wall or septum of a lesion. With CT, it is sometimes difficult to determine enhancement of a heavily calcified lesion. Theoretically, MR imaging would be helpful in characterizing these lesions because the calcification would not be depicted on the MR imaging examination, and any possible enhancement could be appreciated better [22]. Role of MR imaging in preoperative planning With the recent advances in minimally invasive surgical techniques and improved characterization of small renal masses, many patients with renal cell carcinoma are eligible for laparoscopic nephrectomy or nephron-sparing surgery. By performing a comprehensive renal MR imaging examination with three-dimensional sequences, it is not only possible to stage the tumor, but also to demonstrate accurately the vascular supply and the relationship of the tumor to the collecting system and the surrounding renal parenchyma. This helps the surgeon decide which treatment option is most appropriate and helps minimize any potential complications. With the limited field of view of the laparoscope, this information is especially valuable to the surgeon. The authors encourage their urologists to view and interact with the three-dimensional data sets at a workstation before surgery. MR imaging of the adrenal glands Similar to the increased detection of asymptomatic renal masses, the widespread use of cross-sectional imaging has also increased the detection of incidental adrenal masses. Benign and malignant lesions of the adrenal glands are common and characterization of these lesions is of great clinical importance. With MR imaging, it is possible to characterize some adrenal lesions by means of their signal characteristics on different pulse sequences or by their enhancement characteristics. These include adenoma, myelolipoma, hematomas, and cysts. In addition, the multiplanar capability of MR imaging allows improved depiction of the relationship of the adrenal gland to the kidney. It is sometimes difficult to differentiate an exophytic lesion arising from the upper pole of the kidney from an adrenal lesion, a relationship not optimally demonstrated with conventional axial images. The capability of obtaining images in innumerable planes is an advantage of MR imaging. Adrenal adenoma The incidence of adrenal adenomas in the general population is estimated at 2% to 8% [23]. The adrenal gland is also the most common site of metastases per unit weight of any organ [24]. Within the oncologic population, it is common to find an adrenal mass, and a frequent clinical problem is to determine the etiology of such a lesion. MR imaging accurately can distinguish an adenoma from a metastasis in most cases. This allows for more accurate staging of cancer patients, decreases the number of adrenal biopsies, and allows the appropriate treatment regimen to be instituted earlier. Various MR techniques have been proposed to characterize adrenal masses as adenoma or a metastasis. Early work demonstrated that calculating T2 values could characterize these lesions [25], [26]. Sufficient overlap exists, however, to render this cumbersome technique unreliable. Some authors advocate using gadolinium chelates to help characterize adenomas. Krestin et al [27] showed that adenomas tend to washout faster when compared with metastasis. Although some overlap between benign and malignant lesions occurs, it is time consuming to quantify enhancement on MR, and can be done quickly with CT. Other authors have reported that adenomas have a capillary blush seen on arterial-phase imaging, whereas metastases do not [28]. The easiest, fastest, and most reliable way to diagnosis an adrenal adenoma, however, rests on demonstrating intracellular lipid within the mass (lipid-rich adenoma) [29]. By using chemical-shift techniques (breath-hold T1-weighted GRE images in phase and out of phase) it is possible to characterize lipid-rich adenomas. These adenomas contain intracellular lipid and water protons within the same imaging voxel. On out-of-phase images, the signal from these protons cancel each other out and result in signal loss when compared with the in-phase images (Fig. 10). Frequently, signal loss on opposed-phase imaging is obvious. There are cases, however, in which the signal loss is subtle and not readily apparent. In these cases, it is necessary to compare the adrenal mass with an internal standard. In general, the liver is not a reliable standard secondary to the possibility of coexisting steatosis. The authors prefer to use spleen as an internal standard for subjective analysis of signal loss [29], [30]. It is important to remember that the echo time of the out-of-phase image should be shorter than the in-phase image to eliminate T2 decay as a confounding variable of signal loss. It is important to carefully evaluate the entire adrenal mass for signal loss on opposed-phase imaging. This is especially true for patients with a known neoplasm that has a high pretest probability to metastasize to the adrenal glands (lung cancer). A collision tumor results when a metastasis and an adenoma are contiguous [31]. In this instance, the adenomatous portion of the lesion, which contains intracellular fat, loses signal on out-of-phase images, whereas the metastatic (nonadenomatous) portion does not (Fig. 11). A much more common scenario is coexistence of lipid-rich and lipid-poor regions within the same adenoma. Under these circumstances the lesion cannot be characterized definitively as benign and may require further imaging. Adrenal adenomas that do not contain intracellular lipid (lipid-poor adenomas) do not lose signal on opposed-phased imaging. Furthermore, they cannot be characterized with CT densitometry (they measure greater than 17 HU) [32]. These lesions are indeterminate, and are especially troublesome in the oncologic patient because metastasis cannot be excluded. It has been demonstrated that these lipid-poor adenomas can be characterized by means of their washout characteristics on contrast-enhanced CT or MR imaging [33], [34], [35]. Alternatively, positron emission tomography scanning accurately can characterize lipid-poor adenomas from metastasis [36], [37]. Adrenal adenomas may be classified as hypersecreting or, more commonly, as nonhypersecreting. Hypersecreting adrenal adenomas may produce aldosterone (Conn's syndrome), cortisol (Cushing's syndrome), or androgens (hyperandrogenism). Because intracellular lipid may be present in both hypersecreting and nonhypersecreting adenomas, it is not possible to differentiate them with MR imaging alone and correlation with the appropriate laboratory values is necessary. Adrenal cortical carcinoma Adrenal cortical carcinoma is a rare neoplasm that most commonly occurs in the fourth to fifth decades of life with equal prevalence in men and women. They are typically large (>5 cm) at presentation, may contain varying degrees of hemorrhage and necrosis, and often contain calcium [23], [38]. Some adrenal cortical carcinomas are hypersecreting and present earlier and at a smaller size when compared with nonhypersecreting tumors. The most common hormone produced is cortisol, which manifests as Cushing's syndrome. The signal intensity of adrenal cortical carcinomas is variable and they generally are heterogeneous on T1- and T2-weighted sequences. This is secondary to necrosis and hemorrhagic components that are common within these lesions. After the administration of gadolinium, the viable portion of the tumor enhances. Because this neoplasm originates from the adrenal cortex, it is not surprising that intracellular lipid may be present in a portion of the mass. It is possible that some of the lesion loses signal on the out-of-phase T1-weighted images, similar to an adenoma [39]. In most cases, however, differentiation from an adenoma should not be difficult. For an adenoma, the entire lesion should lose signal on the out-of-phase images as compared with an adrenal carcinoma, in which only a portion of the lesion drops out in signal. In addition, adrenal cortical carcinomas are typically larger (>5 cm) than adenomas; are frequently necrotic; and may be poorly marginated. In those cases in which radiologic differentiation is difficult, however, laparoscopic adrenalectomy can be performed [40]. Adrenal cortical carcinomas may grow to be very large and may directly invade adjacent organs including the kidney, liver, spleen, pancreas, and diaphragm. At times, it may be difficult to determine the exact organ of origin, especially when a normal adrenal gland cannot be identified. Imaging in the coronal or sagittal plane is very helpful in showing the relationship of the tumor to its surrounding structures and demonstrating the suprarenal location of an adrenal mass. Furthermore, adrenal cortical carcinoma has a predilection to invade the adrenal veins, grow into the renal vein and inferior vena cava, and extend cephalad toward the heart. Gadolinium-enhanced MR imaging can clearly demonstrate the venous extension of an adrenal cortical carcinoma (Fig. 12). It is critical to include a pheochromocytoma in the differential diagnosis of cortical carcinoma, because their imaging features may be identical and failure to do so may result in catastrophic consequences in the operating room. Pheochromocytoma Pheochromocytomas are neoplasms of the adrenal medulla that produce catecholamines. They occur with equal frequency in men and women, and most commonly occur during the third and fourth decades of life. Pheochromocytomas are extra-adrenal, bilateral, or malignant 10% of the time. Although most commonly sporadic, pheochromocytomas may be associated with other syndromes including multiple endocrine neoplasia, von Hippel-Lindau disease, and neurofibromatosis [41]. Although patients may be symptomatic, the symptoms are nonspecific, and include palpitations, headache, sweating, and hypertension [42]. Even though hypertension is one of the more common presentations, pheochromocytomas account for the cause of hypertension in less than 1% of patients [42], [43]. The MR imaging appearance of pheochromocytoma has classically been described as markedly hyperintense on T2-weighted sequences [44], [45]. Subsequently, it has been demonstrated that pheochromocytomas may have variable signal on T2-weighted sequences, especially when they are greater than 5 cm [45]. MR imaging is more useful in identifying an adrenal mass in a patient who is clinically thought to have a pheochromocytoma than in characterizing an adrenal mass as a pheochromocytoma. Furthermore, MR imaging is useful in identifying extra-adrenal pheochromocytomas (paragangliomas) in the retroperitoneum along the paraspinal muscles. Confirmation with nuclear medicine studies is useful in equivocal cases. Myelolipoma Adrenal myelolipoma is a rare nonfunctioning benign neoplasm that contains a variable amount of hematopoietic tissue and fat. Calcification can be seen in approximately 20% of cases [46]. In general, they are asymptomatic and are incidental findings at ultrasound, CT, or MR imaging, but may cause pain if they hemorrhage or are large enough to exert mass effect on the adjacent organs. The diagnosis of myelolipoma rests on the demonstration of macroscopic fat within an adrenal mass. With MR imaging, the fatty portion of the lesion is hyperintense on T1-weighted images. This is nonspecific and can be seen in any lesion that contains hemorrhage. Just as in diagnosing a renal angiomyolipoma, it is necessary to perform a frequency-selective fat-suppressed T1-weighted sequence and compare it with the non–fat-suppressed T1-weighted sequence. The fatty portion of the lesion should lose signal on the fat-suppressed sequence, and is diagnostic of a myelolipoma (Fig. 13). Myelolipomas may also be diagnosed with chemical-shift imaging by identifying the India ink artifact at the interface of the bulk fat and soft tissue components of the lesion. When a predominately fatty adrenal myelolipoma becomes large and exerts mass effect on the adjacent organs, it may become difficult to ascertain that it arises from the adrenal gland. In this instance, a myelolipoma may be confused with a well-differentiated retroperitoneal liposarcoma or even an exophytic renal angiomyolipoma. By obtaining images in the coronal or sagittal planes, however, it is usually possible to demonstrate that the lesion has a smooth interface with the kidney and that there is no defect in the renal cortex. This finding is highly suggestive that the lesion does not arise from the kidney, excluding an angiomyolipoma. Furthermore, a liposarcoma is expected to engulf or displace the adrenal gland. If a normal adrenal gland is identified, a myelolipoma may be excluded. An adrenal gland that contains a large myelolipoma is expected to be stretched around the periphery of the tumor, or if the tumor is large enough, not be seen at all. Adrenal cysts, pseudocysts, and hematomas Cysts and pseudocysts of the adrenal gland are rare and are usually incidentally discovered on cross-sectional imaging. Patients with these lesions are usually asymptomatic unless they are large enough to produce mass effect on adjacent organs (Fig. 14). Adrenal cysts have been subdivided into four main categories: (1) endothelial (angiomatous or lymphangiectatic); (2) epithelial; (3) pseudocysts; and (4) parasitic. Pseudocysts may be posttraumatic or postinfectious. At MR imaging, simple adrenal cysts are usually hypointense on T1-weighted and hyperintense on T2-weighted images. Some pseudocysts may contain hemorrhage, however, and their signal intensity on different pulse sequences can vary. The wall of an adrenal cyst should be thin, without nodular or enhancing components. In addition, calcification may occur within the wall, which is depicted better with CT than with MR imaging. Use of a long echo-time (>15 milliseconds) and short flip angle gradient-echo sequence, however, usually can identify susceptibility artifact from calcium or hemosiderin. 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