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Features of Diagnostic Abdominal Imaging PDF
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A detailed, pattern-based approach to abdominal imaging interpretationDiagnostic Abdominal Imaging provides a comprehensive review of abdominal diseases based on pattern recognition. Utilizing more than 2,300 images, the book includes discussions of the x-ray, sonographic, CT, MRI, and nuclear radiology features of abdominal diseases. Since accurate imaging diagnosis of diseases can only be achieved with the appropriate clinical history, the characteristic clinical presentations of abdominal diseases are discussed in conjunction with the image findings.Presented in fifteen organ-based chapters that highlight differentiation of disease on the basis of imaging patterns, Diagnostic Abdominal Imaging discusses the full spectrum of malignant and nonmalignant abdominal disorders. Each discussion begins with the most salient histologic, pathologic, and clinical features of the disorder under discussion. This is followed by a systematic review of the imaging features of the disease as seen by all modalities.Unlike most radiology texts which are organized by pathology, Diagnostic Abdominal Imaging is organized by imaging appearance—mimicking real-world practice. The book guides you through the process of imaging-based diagnosis and stresses the epidemiological, clinical, and imaging features that allow the most accurate prediction of disease.Features:More than 2,300 imagesClear, concise guidelines for determining a diagnosisImaging Notes emphasize the critical features of imaging interpretationDesigned to simulate the routine daily analysis that leads to a diagnosis
Editorial Reviews: From the Publisher Dr. Wallace Miller has been a member of the American Board of Radiology since 1991 and has held the sub-committe co-chair of thoracic radiology for the Radiiological Society of North America. He is a professor of Radiology at the University of Pensylvania School of Medicine and has been the recipient of many teaching awards. His teaching responsibilities include supervision of radiology residents, radiology fellows and medical students rotating through the Thoracic Imaging and Body CT Sections of the Radiology Department at the University of Pennsylvania. He has authored and co-authored many journal articles and is the author of two books. –This text refers to the hardcover edition. Excerpt. © Reprinted by permission. All rights reserved. Diagnostic Abdominal ImagingBy Wallace T. Miller Jr.McGraw-Hill Companies, Inc.Copyright © 2013The McGraw-Hill Companies, Inc.All right reserved.ISBN: 978-0-07-162353-7ContentsChapter OneFluid, Fat, Blood, Calcium, and Contrast: General Imaging Features Wallace T. Miller Jr., MD Sharyn I. Katz, MD, MTR I. IMAGING FEATURES OF FLUID II. IMAGING FEATURES OF FAT AND OTHER LIPIDS III. IMAGING CHARACTERISTICS OF HEMORRHAGE IV. IMAGING CHARACTERISTICS OF CALCIFICATION V. CONTRAST ENHANCEMENT Certain abnormalities can be seen in different organs but have a common imaging feature regardless of the organ involved. Before discussing abnormalities of specific organs, we will note some of these common features. IMAGING FEATURES OF FLUID Simple serous fluid has characteristic features on computed tomography (CT), ultrasonography (US), and magnetic resonance imaging (MRI). This fluid can exist freely in various potential anatomical spaces, for example, ascites, pleural effusion, and pericardial effusion, or can be loculated in cysts within solid organs. On CT images, simple physiologic fluid has an attenuation of 0 to 20 Hounsfield units (HUs). This measurement can be made during evaluation of the computer data at the imaging workstation. A visual estimate can also be made by comparing the abnormality to known structures. Simple fluid will usually have an attenuation between that of skeletal muscle and fat (see Figure 1-1). When the accumulated fluid is not a simple transudate of blood but rather becomes “complex,” admixed with proteinaceous material such as that derived from infection or hemorrhage, the attenuation will increase to greater than 20 HU (see Figure 1-2). One potential caveat that must be considered is that volume averaging within a reconstructed slice can confound Hounsfield unit density measurements and so it should be confirmed that the measurement of the tissue in question be made in a slice that has tissue in the slice above and below it so that volume averaging with adjacent distinct tissues does not occur. Simple fluid in US examinations is uniformly anechoic because there are no interfaces to reflect the sound, and therefore it appears uniformly black (see Figure 1-1B). Fluid-containing structures will also demonstrate increased through transmission of sound, which appears as a band of bright echoes behind the fluid structure (see Figure 1- 1B). Simple fluid has no structures to reflect sound, and therefore more sound passes through to the tissues deep to a fluid-containing structure. Therefore, compared to an ultrasound wave that passes through solid tissue, there is more sound to reflect back when it strikes an interface in the tissues deep to the fluid-containing structure. The US computer records this increased sound as brighter lines. So, the tissues deep to the fluid- containing structure appear brighter than adjacent tissues that are not deep to the fluid- containing structure. This artifact is known as “increased through transmission” of sound and is characteristic of any structure that contains fluid: cysts, gallbladder, urinary bladder, and pleural effusions. Complex fluid, which contains internal debris, can be observed in a number of scenarios such as hemorrhagic collections, abscesses, or central necrosis within a malignancy. US of these processes will often demonstrate multiple internal bright floating specks or “echoes.” These echoes represent the reflection of sound from floating particulate debris within the fluid. If plentiful in debris or clot and relatively low in water content, complex fluid can actually have a similar appearance to solid tissues on US. In these cases, the inherently higher than normal fluid content usually preserves the enhanced through transmission typical of fluid on US, a property characteristic of fluid structures (see Figure 1-3). In some cases, further imaging with MRI or CT will be necessary to confirm a diagnosis of a fluid-containing structure. In some instances, there will be septations within the fluid. These will appear as fine lines of increased echogenicity within the fluid-filled structure. US is often the most sensitive imaging test for the presence of septations. However, MRI can have an advantage in evaluating fluid-filled structures where US is limited because of large body habitus. For example, evaluation of complex renal cysts in an obese patient can be impaired by poor sound transmission through excess adipose tissue. MRI, on the other hand, is not limited in the setting of obesity and will accurately characterize the internal architecture of the cystic structure. CT is the least sensitive cross-sectional modality for detection of internal septations. Occasionally, CT can detect septations when there is enhancement of the septa by intravenous (IV) contrast, thereby increasing tissue contrast. Otherwise, septations will typically remain unrecognized by CT studies (see Figure 1-4). Fluid on an MRI examination appears uniformly high intensity (bright white) on T2-weighted images and appears a uniform intermediate intensity (gray) on T1-weighted images (see Figures 1-1 and 1-4). The bright signal intensity typical of fluid on T2-weighted images is a result of the slower loss of phase coherence of the proton nuclei in the transverse plane immediately following the imaging radiofrequency pulse. Interactions between adjacent proton nuclei results in loss of spin synchrony and causes measured MR signal in the horizontal plane. This process requires dissipation of energy, which is not efficient within water, thus leading to a prolonged or “brighter” T2 signal. As with other imaging modalities, an estimate of the composition of the structure of interest can be inferred from comparison to normal structures of known composition. For example, when considering the fluid nature of a cyst, the T2-weighted signal may be compared to the spinal fluid within the spinal canal, which is mostly composed of water. If the fluid being studied is “complex,” T2-weighted images may reveal low T2-weighted signal intensity debris or septae within otherwise high T2-weighted signal intensity water (see Figure 1-4). Cysts containing hemorrhage can have a variety of imaging appearances based on the MR signal characteristics of various phases of hemoglobin. However, in general, hemorrhagic cysts will be hyperdense on T1-weighted imaging. This phenomenon is discussed later in the section Imaging Characteristics of Hemorrhage. Simple fluid can collect as cysts in many organs. Besides the various characteristics of fluid in different modalities, there are several common characteristics of simple cysts: (1) smooth, thin, well-defined walls; (2) round or oval shape; and (3) internal characteristics of simple fluid within the cyst: attenuation between 0 and 20 HU on CT, anechoic, increased through transmission on US, and high T2-weighted signal on MRI (see Figure 1- 1). Furthermore, if IV contrast is administered, there is no enhancement of the structure. IMAGING FEATURES OF FAT AND OTHER LIPIDS Like fluid, fat has a unique appearance on cross-sectional imaging studies. Fat is less dense than other soft tissues, and because density is the major determinant of x-ray attenuation, pure fat will have a lower attenuation than other soft tissues on unenhanced CT exams. Moreover, because fat is also relatively hypovascular compared with other soft tissues, it will remain low attenuation on contrast-enhanced CT images. It has been shown that mature fat has an attenuation between -40 and -120 HU. The attenuation can be measured directly at the PACS workstation, but most often it is easiest to compare the attenuation of the structure in question with the attenuation of subcutaneous fat. If the attenuations are similar, then the structure contains macroscopic fat (see Figure 1-5). On US examinations, fat appears as regions of densely increased echogenicity. The numerous small fibrous septae that surround small fat globules act as innumerable specular reflectors, resulting in fine even echoes throughout the fat (see Figure 1- 5). The appearance of densely increased echogenicity is not unique to fat and can be seen with structures that contain psammomatous (numerous fine punctate) calcifications or with structures that are composed of a complex network of interfaces, such as hepatic hemangiomas (see Figure 1-6). On MRI, the majority of tissue contrast is derived from the intrinsic T1 time or spin-lattice relaxation time of a tissue. This T1 time refers to the time required for the radiofrequency energy of magnetization from the imaging pulse to dissipate through the infrastructure of the tissue, allowing the proton nuclear spin to return—or “relax”—to its original orientation within the magnetic field. This T1 time is a result of proton–proton nuclear interactions of different molecules within the tissue being imaged and generates relatively unique T1 times for different tissue compositions. By exploiting the relative tissue properties of T1 imaging, not only can the presence of fat be detected but also macroscopic fat can be differentiated from microscopic fat (ie, microscopic lipid admixed with fluid). These additional fatty characteristics can provide further information as to the nature of the tissue being studied. Macroscopic fat typically appears uniformly bright on T1-weighted sequences and intermediate to high signal intensity on T2-weighted sequences although typically not as bright as fluid. The bright T1-weighted signal of fat is a result of the fast dissipation of radiofrequency energy of magnetization within the tissue structure, resulting in a short T1 time leading to bright T1-weighted signal because the protons have relaxed, are synchronized, and are now generating signal in the plane of imaging signal acquisition (see Figure 1-5). As with CT, the easiest method of identifying fat in a structure is to compare the intensity of the structure in question with the intensity of subcutaneous fat. If the signal pattern on all sequences parallels that of subcutaneous fat, then the structure contains macroscopic fat. Note that it is important to compare all sequences. Some other substances, for example blood, can be very bright on T1-weighted sequences but will not follow subcutaneous fat on all sequences and are generally low in T2-weighted signal, with some exceptions. Macroscopic fat will also produce an artifact on opposed-phase images called the etching artifact. With this pulse sequence, voxels containing both lipid and water molecules will lose signal. At places where macroscopic fat is adjacent to nonfatty soft tissue, voxels will contain both lipid and water molecules, and a thin black line will be artifactually produced that appears to outline the abdominal organs (see Figure 1-7). Microscopic fat—lipid molecules admixed with water molecules—yields a signal intensity that is generally intermediate on T1- and T2-weighted imaging. However, microscopic lipid can be identified with the use of T1-weighted in-phase and opposed-phase imaging. As mentioned earlier, proton nuclei of lipids dissipate energy more efficiently and hence have a shorter T1 relaxation time than proton nuclei in water. In fact, the T1 time for water is approximately twice that of lipid. Therefore, in a voxel with approximately 50% lipid and 50% water, if the time-to-echo is timed at multiples of the T1 time of lipid, the signals between lipid and water will be opposed, termed opposed-phase, and thus cancel out at the odd multiples and be additive, termed in-phase, at even multiples of the lipid T1 time. When compared to in-phase T1-weighted images, the signal generated by a tissue where water and lipid are mixed within a voxel will result in a drop in signal in out-of-phase T1-weighted images, and the drop in signal will be maximal when the mix of molecules within that voxel is close to 50% water and 50% lipid. For this reason, macroscopic fat tissue, which has proportionally much more fat than water in any given voxel, will not exhibit a drop in signal from in- to out-of-phase T1-weighted imaging. This phenomenon is primarily used to identify the presence of cholesterol in adrenal adenomas but occasionally can be used for other purposes such as the detection of microscopic lipid in tumors of hepatic origin and in the differentiation of thymic hyperplasia, which normally contains diffuse microscopic lipid, from thymomas (see Figure 1-8). IMAGING CHARACTERISTICS OF HEMORRHAGE A hematoma undergoes various stages of evolution. Initially, extravasated blood forms a dense coagulated mass. With time, the hematoma undergoes degradation via the action of extracellular proteases and macrophage phagocytosis leading to liquification of the clot. Coincident with this process, hemoglobin undergoes a transition from oxygenated (oxy-hemoglobin) to deoxygenated (deoxyhemoglobin) to methylated hemoglobin (methemoglobin). Various characteristics of this transition process can be identified by imaging exams and can suggest the presence of extravasated blood. Computed tomographic examinations will often show acute hemorrhage as a high attenuation region relative to unenhanced tissues (see Figure 1-2). This high attenuation is a direct result of increased density of acute iron-containing clot relative to soft tissues. Although initially characterized for evaluating subarachnoid, subdural, epidural, intraparenchymal, and intraventricular hemorrhage in the brain, this phenomenon is now routinely used to identify acute hemorrhage throughout the body. It is important to note that extravasated blood is only high attenuation relative to surrounding vascularized tissues when those tissues are unenhanced with IV contrast. Use of IV contrast increases the CT attenuation of vascularized soft tissues such that acute hemorrhage will be relatively lower in attenuation. However, acute hemorrhage will always remain higher in CT attenuation than simple fluid. As a clot ages, it begins to degrade and becomes progressively less dense, and so the attenuation falls until it approaches or equals simple fluid attenuation (0-20 HU). In an intermediate phase, hemorrhage can equal the attenuation of unenhanced tissues and as a consequence can be difficult to identify. In many cases, an old liquefied hematoma will contain simple fluid in the nondependent portion of the hematoma and debris will settle by gravity into the dependent portion of the hematoma, generating a “fluid-debris level.” The presence of debris within a fluid collection will often indicate the presence of prior hemorrhage but can also be seen in abscesses and some neoplasms. On US examinations, acute hematoma will appear as an echogenic mass within the tissues. It is believed that the multiple fibrin strands act as specular reflectors and lead to many small echogenic foci. This can be confused with other echogenic masses such as fat-containing lesions or hemangiomas. As the hematoma liquefies, debris from within the clot will cause internal echoes within the fluid collection. This will appear as an echogenic cyst with increased through transmission. Similar to CT, the debris will often settle dependently within the collection, leading to a fluid-debris level. The complex cyst of a liquefying hematoma cannot be distinguished from other debris-filled cysts that can be seen with abscesses and some neoplasms. The imaging properties of blood products on MRI is dependent on location of the hemorrhage within the body, and hence surrounding tissues, and timing of imaging relative to time of hemorrhage. In the brain, the MRI appearance of a hemorrhage has been extensively studied and can yield very specific information on the age of the hemorrhagic products. In the hyperacute phase of intracranial hemorrhage (less than 12 hours from the hemorrhagic event), the predominant form of hemoglobin within the blood clot is oxyhemoglobin, which is isointense to brain parenchyma on T1-weighted signal and high intensity on T2-weighted signal. Over the initial 12 hours, the predominant blood product is oxyhemoglobin, but during the acute stage (12-72 hours posthemorrhage), the hemoglobin within the hematoma becomes increasingly deoxygenated and results in deoxyhemoglobin, which is a paramagnetic substance and leads to marked shortening (darkening) of the T2-weighted MR signal within the hematoma. During the early subacute phase of hemorrhage (3-7 d posthemorrhage), failure of extravasated red blood cells to maintain an intracellular functional reductase enzyme system results in oxidation of deoxyhemoglobin to methemoglobin, which is a strongly paramagnetic substance resulting in marked shortening of T1-weighted signal (signal brightening), the most dominant effect, and further shortening of T2-weighted signal (signal darkening) (see Figure 1-9). This is followed by release of methemoglobin into the extracellular environment, as the red blood cells subsequently degenerate and lyse, resulting in further T1-weighted signal shortening (brightening). In the chronic phase, which begins in the weeks following the hemorrhagic event, ongoing macrophage activity along the periphery of the evolving chronic intracranial hemorrhage will result in steady conversion of methemoglobin to hemosiderin, which results in marked T2 shortening (darkening) because of the strongly ferromagnetic properties of hemosiderin. This hemosiderin ring of markedly low T2-weighted signal around the intracranial hematoma increases in thickness with time, whereas the central scar contracts and ultimately collapses, usually resulting in a band-like hypointense ferromagnetic scar that sometimes persists indefinitely. (Continues…) Excerpted from Diagnostic Abdominal Imagingby Wallace T. Miller Jr. Copyright © 2013 by The McGraw-Hill Companies, Inc.. Excerpted by permission of McGraw-Hill Companies, Inc.. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site. –This text refers to the hardcover edition. About the Author Dr. Wallace Miller has been a member of the American Board of Radiology since 1991 and has held the sub-committe co-chair of thoracic radiology for the Radiiological Society of North America. He is a professor of Radiology at the University of Pensylvania School of Medicine and has been the recipient of many teaching awards. His teaching responsibilities include supervision of radiology residents, radiology fellows and medical students rotating through the Thoracic Imaging and Body CT Sections of the Radiology Department at the University of Pennsylvania. He has authored and co-authored many journal articles and is the author of two books. –This text refers to the hardcover edition.
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