Intra-abdominal fat. Part I. The images of the adipose tissue localized beyond organs.

Unaltered fat is a permanent component of the abdominal cavity, even in slim individuals. Visceral adiposity is one of the important factors contributing to diabetes, cardiovascular diseases and certain neoplasms. Moreover, the adipose tissue is an important endocrine and immune organ of complex function both when normal and pathological. Its role in plastic surgery, reconstruction and transplantology is a separate issue. The adipose tissue has recently drawn the attention of research institutes owing to being a rich source of stem cells. This review, however, does not include these issues. The identification of fat is relatively easy using computed tomography and magnetic resonance imaging. It can be more difficult in an ultrasound examination for several reasons. The aim of this paper is to present various problems associated with US imaging of unaltered intra-abdominal fat located beyond organs. Based on the literature and experience, it has been demonstrated that the adipose tissue in the abdominal cavity has variable echogenicity, which primarily depends on the amount of extracellular fluid and the number of connective tissue septa, i.e. elements that potentiate the number of areas that reflect and scatter ultrasonic waves. The normal adipose tissue presents itself on a broad gray scale: from a hyperechoic area, through numerous structures of lower reflection intensity, to nearly anechoic regions mimicking the presence of pathological fluid collections. The features that facilitate proper identification of this tissue are: sharp margins, homogeneous structure, high compressibility under transducer pressure, no signs of infiltration of the surrounding structures and no signs of vascularization when examined with the color and power Doppler. The accumulation of fat tissue in the abdominal cavity can be generalized, regional or focal. The identification of the adipose tissue in the abdominal cavity using ultrasonography is not always easy. When in doubt, the diagnostic process should be extended to include computed tomography or magnetic resonance imaging, or sometimes biopsy (preferably the core-needle one).

Gas, bones and fat are known obstacles for ultrasound wave propagation. It is commonly believed that a greater amount of fat in a given organ increases its echogenicity. Fatty liver or pancreas is a typical example(. The entirely different image of fat can be seen in sonomammography, where it is characterized by low echogenicity(. The adipose tissue deposited in the abdominal cavity can have various acoustic patterns, and the knowledge of them should prevent diagnostic pitfalls.

The aim of the paper is to present various problems associated with US imaging of unaltered intra-abdominal fat located beyond organs.

The adipose tissue is not only a coat-like structure that protects against heat loss or stores energy, but is also an important endocrine and immune organ, the function of which is complex both in normal and pathological conditions. Its role in plastic surgery, reconstruction and transplantology is a separate issue. Moreover, it draws the attention of research institutes owing to being a rich source of stem cells(. The common methods for the identification of the adipose tissue are computed tomography (CT) and magnetic resonance imaging (MRI). However, even the most modern versions of computed tomography are unable to distinguish between two types of fat in the human body, i.e. white and brown adipose tissue, which differ from each other in their cytological structures, degree of vascularization and thermogenic function. Such discrimination is possible with the use of a special technique of magnetic resonance imaging and positron emission tomography(. The brown adipose tissue is found in lower amounts and its stores decrease with age. It is mainly localized in the neck, in the subcutaneous tissue between the scapulae, in the mediastinum, axillary fossae, deep in the retroperitoneal space and even between the paraspinal muscles(. Ultrasound imaging does not enable these two types to be distinguished.

Irrespective of the type, the adipose tissue can be acoustically manifested in various ways, which makes its certain identification difficult. It was established very early that the varied acoustic profile of the adipose tissue mainly depends on two factors, namely the amount of fluid and the number of connective tissue septa(. The insolubility of fat in water is a commonly known fact. This way, fluid accumulated in spaces between adipocytes, together with the cells themselves, multiplies the number of areas that reflect and scatter ultrasonic waves. As a result, it leads to the formation of an overwhelming noise that destroys the image of the examined structure. A typical example of such a phenomenon, which has not been reported so far, is a serious problem with ultrasound imaging in patients in a grave clinical condition (with cardiovascular, renal or hepatic failure, or in acute pancreatitis). In these cases, the transudate accumulates between subcutaneous fat lobules and generates noise that renders the imaging of the structures located deeper impossible (Fig. 1). Numerous connective tissue septa that separate fat lobules cause slightly less troublesome acoustic effects – they increase the echogenicity of fat (Fig. 2 A, B). The proportion of these two elements, i.e. fluid and septa, determines the image of the adipose tissue in a given region. It must be assumed that the shades of the acoustic patterns of this tissue can have a very broad range in gray scale, i.e. from a hyperechoic area, through numerous structures of lower reflection intensity, to nearly anechoic regions mimicking the presence of pathological fluid collections (Fig. 3). The application of techniques that improve sonograms, i.e. various versions of harmonic and spatial imaging (with the same optimal enhancement parameters), have a lesser influence on fat echogenicity (Fig. 4). Other features of the normal adipose tissue are: its homogeneous structure, smooth outlines, no infiltration of the surrounding organs and no posterior enhancement(. However, the last feature is debatable since, on numerous occasions, we have observed posterior enhancement behind the extraperitoneal fat pad localized above the left liver lobe. In these cases, a slightly hyperechoic focus can be seen in the liver and is sometimes misinterpreted as a focal lesion (Fig. 5). Another important feature of the adipose tissue in US is its elasticity evaluated in a simple test consisting in applying pressure with a transducer. In our non-published studies, we have found a marked relationship between the compressibility of fat and its echogenicity. The lower the echogenicity, the greater the strain and vice versa: the greater the echogenicity, the lower the elasticity (Fig. 6 and 7). Moreover, the normal adipose tissue does not show any signs of vascularization in Doppler examinations of blood flow. Another feature that supposedly facilitates the assessment of the intra-abdominal fat is the symmetry of its distribution(. This is only relatively true since, due to considerable differences in the size between the liver and spleen, the left side holds considerably greater amounts of fat (Fig. 8). One should remember, however, that the unaltered greater omentum is sometimes localized between the diaphragm and the right liver lobe, particularly in men (Fig. 9)(. Moreover, the extraperitoneal fat localized above the left liver lobe, which coats the round ligament of the liver on its way to the umbilicus, is characterized by constant asymmetrical deposition; it only slightly crosses the midline to the left (Fig. 10)(. Sometimes, the aforementioned fat pad can be erroneously interpreted as a subphrenic abscess when accompanied by acute clinical signs in the epigastric region. This can be ruled out by observing a free visceral sliding movement induced by a deep inspiration against the immobile fat accumulation(. Moreover, individual predispositions to asymmetrical fat deposition, such as lipoatrophic-lipodystrophic syndrome, should be taken into account when discussing intra-abdominal fat distribution. This issue is rarely discussed in the literature(. Examples of this condition include: a 35-year-old slim woman followed-up in our center for four years in whom a fat pad of an unchanging size persists in the right iliac fossa at the cecum (Fig. 11) and a 56-year-old man with the same lesion localized in a similar area (Fig. 12). A tumor-like fat accumulation in other areas of the abdominal cavity is, however, rarely encountered (Fig. 13). In such cases, a US examination itself does not enable the differentiation of focal lipomatosis from a high-grade liposarcoma. The diagnostic process should therefore include all other means available to establish a correct diagnosis. These include CT, MRI, core-needle biopsy, surgical biopsy and sometimes a follow-up examination(. Pelvic lipomatosis is also noteworthy. In this case, the deposited adipose tissue, frequently hyperechoic fat, can cause urological or intestinal problems by compressing organs localized in this area, e.g. it can deform the urinary bladder to a cigar-like shape or it may even cause urinary retention in the upper urinary tract(.

Fig. 1

Acute pancreatitis with a fluid collection (C). On the left: the collection viewed through a fragment of the abdominal integuments without edema; on the right: the same collection viewed through a swollen fragment

Fig. 2 A

Two views of the renal capsule with a low number of connective tissue septa (arrows); the echogenicity is similar to that of the renal parenchyma

Fig. 2 B

Renal capsule in a different patient with numerous septa (arrows) that make this structure more echogenic than the renal parenchyma

Fig. 3

Above the fatty left liver lobe (L), there is an extraperitoneal fat pad of very low echogenicity which mimics a pathological fluid collection (F)

Fig. 4

The same view shows the same fat pad (F) captured with a different technique of harmonic imaging; no signs of marked differences in echogenicity

Fig. 5

An artefact that mimics a hyperechoic focus in the left liver lobe (A) by a biconvex fat pad (F)

Fig. 6

A 58-year-old patient treated with Encorton because of rheumatoid arthritis. Markedly hypoechoic greater omentum with high compressibility; it reduces 34% of its thickness upon compression

Fig. 7

A 64-year-old diabetic patient. The compression with a transducer induces an 11% reduction of the thickness of the slightly hyperechoic greater omentum

Fig. 8

A 54-year-old patient with no injury. Above the spleen (S), the deposited fat mimics a hematoma

Fig. 9

Above the right liver lobe (L), there is a slightly heterogeneous structure (O), which is the greater omentum

Fig. 10

A 51-year-old patient with left liver lobe agenesis. The fat pad is typically hypoechoic (F). The site of the left liver lobe is occupied by the markedly hyperechoic greater omentum (O)

Fig. 11

A very slim 35-year-old woman presents a fat pad in the right iliac fossa. Transducer compression reduces its thickness by 20%

Fig. 12

A 56-year-old man with normal weight. In the right iliac fossa, there is a highly elastic fat pad (thickness reduction upon compression by 36%)

Fig. 13

In the left hypochondriac region, there is a tumor-like fat accumulation (F) which should be differentiated from a highly mature liposarcoma

The adipose tissue plays an important role as a “filler” of missing organs and tissues following various surgeries associated with the removal of pathologically altered structures(. This fact is of particular significance in cancer patients in whom the appearance of any lesion in the tumor bed usually suggests a relapse. This is well-illustrated by a hypertrophied fragment of the adipose capsule after nephrectomy (Fig. 14 A, B, C). To conclude this part, our observation concerning incompetent kidneys should be mentioned. Multiple patients with renal failure present a vaguely circumscribed hypoechoic area in the adipose capsule. It is contiguous to the cortical layer and is not a fluid collection under the capsule (Fig. 15).

Fig. 14 A

A hypertrophied fibrolipid mass in the site after a removed kidney in a cancer patient. The compression test shows the compressibility of the lesion

Fig. 14 B

The same lesion. A color Doppler examination reveals that the structure is avascular

Fig. 14 C

A different patient after right nephrectomy also presents hypertrophied adipose tissue (F) but with different echotexture compared with Fig. 14 A. L – liver

Fig. 15

A 71-year-old doctor with kidney failure (creatinine 1.8 mg/dl, urea 80 mg/dl). US shows the kidneys of a normal size and normal thickness of the parenchyma. The only abnormality is decreased corticomedullary demarcation. The figure shows only the left kidney. An irregular hypoechoic area can be seen in the adipose capsule

The ability of ultrasonography to reflect a detailed structure of the adipose tissue (compared with CT or MRI) is a significant advantage of this modality. Despite this, any doubts concerning the nature of a lesion detected by ultrasound must be resolved in CT or MRI, or sometimes even by a biopsy, preferably the core-needle one.

Conclusion

Unaltered intra-abdominal fat is manifested by a broad echogenicity range, which makes the identification of its nature difficult. The signs that can aid the interpretation include: smooth outlines, homogeneous structure, no infiltration of adjacent tissues, relatively high elasticity and no vascularization. The deposited fat can present itself as diffuse, regional or tumor-like thickening. Any doubts must be verified in CT or MRI, or sometimes by a biopsy, preferably the core-needle one.