The Future of Continuing Education in Diagnostic Imaging

Common Pathology & Imaging of the Kidney

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Author: Deirdre O'Donnell, M.Sc.

Abstract: Kidney pathology results in damage to the tissues of this organ, and affects its ability to perform its vital functions. This damage, if allowed to progress untreated, reduces kidney function over time. This is known as chronic kidney disease, and may be associated with a number of risk factors. Chronic kidney damage increases the risk of kidney failure. This must be treated with life-changing procedures, the most well-known of which are transplantation and dialysis. Transplantation may increase the risk of other forms of kidney disease in turn. The conditions and diseases that may contribute to chronic kidney disease must be diagnosed and monitored regularly. Imaging can play an important role in both. This article will outline various types of kidney pathology and how they are detected by various types of imaging.

Introduction

 

The kidneys are organs found bilaterally in the lower abdomen. Their essential functions are to filter liquids in the body for useful nutrients, minerals and water prior to allowing the release of the same as urine. The normal kidney is roughly bean-shaped (hence the common name of such a plant product) and reddish-brown in colour. In cross-section, a normal adult kidney has an outer 'shell' called the cortex. This forms the general shape of the organ, and surrounds the deeper medulla. At this level, several vaguely triangular structures, called renal pyramids, interface with collecting 'ducts' near the centre of the kidney. These amalgamate at the calyces (singular: calyx) which ultimately form the renal pelvis. At this point, a narrow tube called the ureter connects the kidney to the bladder. This opening into the kidney also allows major blood vessels to pass into the organ. From here, they subdivide and form a complex structure of capillaries, which bring blood containing both waste products and the useful molecules mentioned above into the kidney, and drain blood free of waste out of it and back into circulation. Other functions of the kidney include acid/base homeostasis, and the balance of electrolytes in the body. Renal regulation of these, and other molecules such as salts, also contributes to normal blood pressure. Therefore, kidney damage may lead to the increased risk of hypertension in some cases.

 

Renal pathology, in the form of various infectious conditions, cancers and other anomalies of the kidney, is relatively common. These may be relatively easy to detect, through overt symptoms such as hematuria (blood in the urine). Other abnormalities concerning urine, including unusual odors or colors, urinary volumes (i.e. atypical increases or reductions) and pain while attempting to urinate, may also indicate renal pathology. In addition, pain in the lower abdomen, back or side may be indicative of renal dysfunction. On the other hand, fatal or severe forms of disease may not result in any of these symptoms, or in abnormalities (including hematuria) at very low levels that may even require diagnostic microscopy. Alternatively, conditions that affect the kidney may be detected through objective differences in the size, shape and color of the organ compared to the expected anatomical norm. These include growths such as cysts, which may be clearly visible on the surface of the kidney. Other superficial anomalies include scarring, which may result in a kidney with an irregular or granular appearance as opposed to the smooth outer shell of a healthy kidney.

 

Renal ischemia is often associated with sections of kidney tissue that are considerably paler than the rest of the organ. This indicates tissue necrosis due to the loss of blood supply, and may also be accompanied by visible crowding (usually through striations of darker surface color) of the surrounding cortex and medulla as they take on the load of the damaged pale tissue. A swelling (or hydronephrosis) in renal tissue may indicate an obstruction to the normal flow of fluids. These obstructions are often related to the formation of kidney stones, a condition known as urolithiasis. These stones are usually small and may be passed through the ureter, but can also become large enough to become lodged within the kidney on a more permanent basis. This may cause the discoloration or even destruction of surrounding cortical tissue. Hydronephrosis is also often accompanied by dilations of the calyces or renal pelvis. Some advanced forms of cancer may be visible as severe malformations of the kidney, with superficial color changes and circumscription. They may be palpable masses in the abdomen to a practitioner performing a routine assessment.

 

However, all of these diagnostic methods (with the exception of one) require invasive procedures (the least extensive of which is a biopsy or intravenous urography) with all the attendant potential risks and side-effects of such measures. These may be the most direct and possibly effective routes to the diagnosis of kidney pathology, but may not apply uniformly in all cases. For example, some renal cancers, even those of advanced and invasive stages, may remain within the normal dimensions of the organ, or remain undetected regardless of size due to the relatively large space to extend within the lower abdomen. Cancers of the bladder or kidney may avoid detection by biopsy, unless their cells lose differentiation and show considerable abnormality compared to normal cells. Some cases may not present common symptoms such as lower flank pain or hematuria. These include renal vein thrombosis, which may persist in the long term while avoiding observation or detection through symptoms such as pain. As the reader may have guessed, some of these anomalies, including kidney stones, also require transverse sectioning of the bladder for adequate visual detection. Therefore, non-invasive diagnostic techniques may have a distinct advantage in the study of renal pathology.

 

It can be argued that urine analysis may satisfy the requirements for non-invasive diagnostics. This branch of diagnostics assesses the levels of renal disease markers present in the urine (which is an appreciable source of kidney by-products in any case). This analysis may detect microscopic concentrations of blood, as well as white and red blood cell counts, certain protein markers, pH changes (abnormally high or low pH may indicate reduced kidney function) and high levels of molecules that are normally retained rather than excreted. Some blood tests, such as those for differences in serum creatinine levels, may also detect renal disease [1]. However, imaging can take the form of a virtual alternative to slicing the kidney into sections, thus allowing the potential to visualize tumors and other anomalies deep to the cortex or even the medulla.

 

Imaging is often integrated into the routine assessment of patients who have received a renal transplantation. Kidney transplants - or the removal of a dysfunctional organ and its replacement with a healthy, donated kidney are common treatments for the final stages of many diseases and conditions outlined in this article. In

addition, transplantation is also a risk factor for some pathologies described here. This procedure is a major surgical undertaking, and yet is more cost-effective and confers improved life quality and survival in comparison with other long-term management strategies for end-stage kidney disease and failure (i.e. dialysis) [2]. On the other hand, it is associated with many serious risks and potential complications. These include infection, which can also be detected using imaging. Non-invasive assessment is therefore a preferable tool in determining outcomes and recovery in these patients. Imaging may also detect renal scarring, obstruction and infection.

 

 

Non contrast CT kidney scarring

Fig. 1. Non-contrast axial CT of one kidney (circled) showing severe scarring and malformation compared to the other. Case courtesy of Dr Roberto Schubert, Radiopaedia.org. From the case rID: 16345.

 

 .

Gross imaging of the kidney may also provide a general picture of kidney failure. Long-term kidney function is largely determined by the proportion of kidney tissue that is active and productive to that which is necrotic or otherwise inactive. Therefore, images indicating widespread kidney damage or tissue death may be related to the onset of kidney failure. Conversely, images indicating a relatively functional kidney may avoid surgery or other treatments for this condition. Imaging procedures are often a standard component of assessment for patients complaining of pain in the areas outlined above, hematuria or other symptoms likely to be related to kidney pathology.

 

This, then, suggests the role of a registered technologist (R.T.) in the diagnosis of renal pathology. Patients presenting with complaints of stomach, flank or back pain may be candidates for renal imaging. The R.T. is ideally prepared to deliver the equipment, techniques and expertise required to produce images that will assist in diagnosing pathologies of the kidney. This requires an understanding of these diseases or conditions and their treatment. The R.T. should also be aware of the choice of techniques involved in the effective detection of individual conditions.

 

 

Imaging Modalities/Techniques & Their Applications in Renal Imaging

 

 

Computed Tomography (CT)

 

CT is a long-established imaging modality capable of delivering highly-detailed, multi-section images. This may be well-suited to the needs of renal images. Contrast-enhanced CT is a standard indication for the assessment of renal infections (or nephritis) [3]. However, this is discouraged in younger patients and other patient groups in which repeated exposure to radiation or contrast material may be a concern [3]. Contrast materials are associated with the increase of nephrotoxicity (toxic kidney damage), which may adversely affect older patients with pre-existing kidney damage [4]. CT may also highlight the presence of severe pyelonephritis (see below) as singular or multiple areas of reduced attenuation [5]. These represent foci of infection [5]. Standard CT may also detect kidney tissue calcifications, renal hemorrhage and inflammation [5]. CT is regarded as highly specific and selective for many types of renal pathology. However, a study using an animal model of renal infection found that the specificity and sensitivity of magnetic resonance imaging (MRI) was comparable to that of CT [6]. On the other hand, a CT scan of the kidney may be much less time-consuming (and thus potentially more cost-effective) than a full renal MRI [3]. There are other variations on standard CT that are also useful or efficacious in renal pathology. These include:

 

  • Single Photon Emission Computerized Tomography (SPECT): This development was compared to CT and MRI in an animal study of nephritis, incorporating 70 kidneys and over 100 foci of infection. This found that the specificity and sensitivity of SPECT were 93.8% and 92.1% respectively, compared to 87.5% and 86.8% for CT and 87.5% and 89.5% for MRI [6].

 

 

Magnetic Resonance Imaging (MRI)

 

MRI is a modality that depends on the resonance of water or other molecules, enhanced with contrast materials that are often chelates of gadolinium. As the kidneys are rich in water-based fluids, they should be amenable to imaging with MRI. On the other hand, some paramagnetic contrast materials are associated with nephrotoxicity. This is a disadvantage of MRI in longitudinal studies of kidney disease and/or failure [4]. Traditional T1- and T2-weighted MR-generated images may detect renal pathologies such as those associated with infection (i.e. nephritis) [3]. These techniques may be used to confirm a diagnosis of nephritis, and to assess patients at increased risk and who are affected by complications such as renal abscesses [3]. However, its role in the routine assessment of this condition is relatively diminished [3]. In a study of different techniques in the detection of nephritis, the median visibility scores associated with T1-weighted imaging were significantly higher than those associated with T2-weighted imaging [3].

 

  • Diffusion weighted imaging (DWI) is a form of MRI in which the measurement of the Brownian motion of water within a single voxel is measured. This may lead to the generation of 3D and/or color-coded images based on the direction of diffusion. The analysis of DWI can also generate images based on apparent diffusion coefficient (ADC). These ADC maps may differentiate between normal tissue and pathological sites, and between different types of pathology. For example, local reductions in ADC may indicate swelling. An initial study into DWI as a diagnostic application in the assessment of chronic kidney disease took DW-generated images of the coronal section of the kidneys of 43 patients and 72 healthy controls. This study found that the ADC values of patients were significantly lower than those of volunteers [7]. These values were significantly reduced at all stages of CKD, with the exception of the first [7]. ADC values and serum creatinine levels were also significantly correlated [7]. A later study investigating the application of DWI to CKD pathology recruited 71 patients and 12 healthy controls [1]. DWI-generated ADC maps were compared to pathology scores derived from biopsy results and the severity of pathology. This resulted in ADC values that were significantly (and negatively) correlated to pathology score [1]. ADC mapping was also capable of significant discrimination between controls and patients with mild, moderate and severe kidney damage [1]. However, ADC values of different pathology type were not significantly different [1]. Therefore, ADC mapping may be an effective technique in determining the severity, but not the type, of pathology in chronic kidney disease. It should be noted that some DW-generated images have been reported as inefficient in discriminating between the cortex and medulla, however [7].

 

Reduced ADC values on DW-generated images may also be associated with nephritis [3]. A study recruited 21 patients and 60 healthy controls to compare the efficacy of T1-, T2- and DWI-generated images in the assessment of nephritis. This found that DWI had a higher specificity for foci of infection compared to T1- or T2-weighted imaging [3]. The potential of DWI in infectious conditions has not yet been fully explored, yet the technique appears to perform well in detecting kidney infections [3]. This may be due to differences in diffusion caused by the pathological signs of nephritis (see 'Nephritis' section).

 

 

Ultrasound Technology

 

Ultrasound, also known as ultrasonography, US or UST, is another well-established imaging technique. This modality can generate coronal, transverse and longitudinal images of the kidney [6]. Ultrasonography is also often indicated as a standard assessment of patients presenting with common symptoms such as lower abdominal pain [3]. US can evaluate basic markers of kidney function, such as kidney size, the condition of the ureter, renal pelvis and calyces, and vascular function (of both major and minor vessels in and around the kidney) [2]. Spectral and color Doppler imaging can assess small vessels within the kidney, kidney function and signs of diseases such as infection [2]. However, this modality may be outstripped in some dimensions of kidney imaging by more modern options such as MRI and SPECT. A comparative study incorporating 70 animal kidneys with experimentally-induced nephritis found that the specificity and sensitivity of power Doppler US in detecting pyelonephritis (see below) was 56.7% and 74.3% respectively, compared to 93.8% and 92.1% for SPECT, 87.5% and 86.8% for conventional CT and 87.5% and 89.5% for MRI [6]. These values were higher when detecting pyelonephritis in zones (or foci) of infection, however. US achieved a specificity and sensitivity of 81.4% and 56.6% respectively, compared to 95.4% and 94.1% for SPECT, 93.5% and 88.2% for CT and 92.6% and 91.2% for MRI [6]. Therefore, US is currently being phased out in favor of newer forms of MRI and CT [4]. On the other hand, modern developments in US, such as contrast-enhanced US, may show improved competition with other modalities in similar tests. A study of the use of contrast-enhanced US in the detection of acute pyelonephritis with contrast-enhanced MRI as a reference found that this form of US demonstrated a specificity of 100% and a sensitivity of 95% for this pathology [4].

 

Ultrasonography is also a standard procedure in the post-operative monitoring of transplant patients. This is due to its low radiographic profile, which may be preferable for regular routine procedures. Some US equipment is more portable (i.e. not in need of a full-body scanner) and therefore more amenable to patients for whom movement is restricted as they recover from surgery [2]. Spectral and color Doppler US is effective in detecting renal vessel function, in terms of flow velocity, homogeneity and quantification [2]. US data can be analyzed to deliver quantitative data such as pulsatility index (PI), resistivity index (RI) and systolic/diastolic ratio. These can be expressed as both US images and waveforms [2]. Images can represent kidney shape and differences in tissues (i.e. to differentiate between the cortex and medulla) as well as vascular function [2]. Waveforms correlate to renal vein or artery function [2]. US can assess function, acute rejection and chronic rejection in the course of routine monitoring for post-transplant patients [2].

 

US of a kidney post-transplant

Fig. 2a. US of a kidney post-transplant. (a) Gray scale ultrasound showing that this kidney is normal, demonstrating good contrast resolution between cortex and medulla, although there is a slight dilation of the collecting system (arrow). (b) Color Doppler US demonstrating that the renal artery and vein of the transplant kidney are functioning normally. (c) Color Doppler US showing normal homogeneous blood flow. Courtesy: Kolofousi, C., et al., Ultrasonographic features of kidney transplants and their complications: an imaging review. ISRN Radiol, 2013. 2013: p. 480862.

 

 

 

 

..

Spectral Doppler US showing normal renal vein waveform

Fig. 2b. (d) Spectral Doppler US showing normal renal vein waveform. (e) Spectral Doppler US showing normal intrarenal artery waveform, with appreciable systolic and diastolic flow. Resistive index is normal (RI = 0.71). (f) Spectral Doppler US showing normal waveform of the renal artery. Courtesy: Kolofousi, C., et al., Ultrasonographic features of kidney transplants and their complications: an imaging review. ISRN Radiol, 2013. 2013: p. 480862.

 

As mentioned above, US can detect chronic rejection (the progressive loss of function in the transplanted kidney) and acute rejection (the more abrupt failure of the transplant, which may be accompanied by localized tenderness, flu-like symptoms and pyrexia.) As rejection may or may not be accompanied by diagnostic signs such as size increases, swelling and changes in echogenicity, changes in vascular functions as detected by US may be a more robust marker of these events [2].

 

Most forms of US are associated with reduced costs and nephrotoxicity compared to other imaging modalities listed here [2].

 

 

Common Pathologies of the Kidney

 

 

Renal Obstruction

 

Obstructions often arise in the kidney as a result of the formation of kidney stones, as outlined above. These stones are most often large crystals of calcium oxalate [8]. These form on the renal papillae (the 'tips' of renal pyramids) and may be associated with dysfunctions in the gastrointestinal mechanisms controlling the uptake of calcium from dietary sources, rather than allowing it to pass into the kidney for processing. Other, less common, forms of kidney stones include uric acid crystals and crystallizations of the amino acid cysteine [9]. Several risk factors affect the probability of stone formation, including chronic kidney infection, body mass, dehydration, genetic and dietary factors [10, 11]. The majority of these stones (or calculi) are small (approximately 1-3mm in diameter) and pass without complication through the ureter, into the bladder, through the urethra and thus pass out of the body. However, they may be associated with acute pain when passing through these tubes and organs.

 

Larger stones may cause a more chronic obstruction, with more extensive tissue damage and structural anomalies. Many cases of obstruction with larger calculi occur in the renal calyces or pelvis. In these cases, they may result in the dilation of these structures and a type of extensive swelling known as hydronephrosis. The location of hydronephrosis is often an indicator of the site of obstruction; for example, hydronephrosis of the renal pelvis indicates an obstruction at the start of the ureter (or the uretero-pelvic junction). Some calculi may invade an entire calyx and take on its shape, thus rendering it extremely difficult to dislodge from the kidney. These are often referred to as 'staghorn' stones, due to the shape taken on by such a calculus. This may result in other pathological signs, including pyelonephritis (inflammation of the renal pelvis) and changes in the structure and volume of the cortex. Advanced hydronephrosis may be a risk factor for chronic infection, and even kidney failure if it is allowed to persist untreated. Other symptoms of renal obstruction may include lower abdominal pain, cloudy or dark urine and hematuria [12].

 

 

Pyelonephritis of the left kidney imaged by three separate variants of MRI

Fig. 3. CT showing right kidney hydronephrosis due to obstruction. Case courtesy of Dr Sajoscha Sorrentino, Radiopaedia.org. From the case rID: 15679

 

 

Less common forms of renal obstruction are occlusions of the renal artery or vein. Renal vein thrombosis is relatively difficult to detect, even with direct visualization, as outlined above. This form of obstruction may arise as a complication of transplant surgeries [2]. Other risk factors for renal vein thrombosis may include hypervolemia, pre-existing renal tumors and venous damage or compression [13]. Renal vein thromboses may also cause the abrupt loss of ability to urinate. Renal artery occlusions may also arise as a result of thromboses. Alternatively, they may be due to arterial narrowing (stenosis) due to atherosclerosis. The risk factors for stenosis may include dietary and genetic factors, and may also occur due to pre-existing atherosclerotic disease in the portion of artery included as part of a donated kidney for transplant [2]. Arterial occlusions may lead to serious complications. These can include renal infarcts leading to ischemia and necrosis, (see above) renal atrophy due to interruptions in blood supply, possible hypertension and kidney failure [2].

 

Renal infarct imaged with standard US

Fig. 4. A renal infarct imaged with standard US, (a) power Doppler US showing defects in blood flow (b) and contrast-enhanced US (c) showing the absence of perfusion (red arrow). Courtesy: Kolofousi, C., et al., Ultrasonographic features of kidney transplants and their complications: an imaging review. ISRN Radiol, 2013. 2013: p. 480862.

 

Visualization of Renal Obstruction

 

Renal obstruction may be challenging to visualize, but the imaging of this pathology may be preferable to other diagnostic techniques such as urography, particularly in the early stages. A study investigated the application of DWI to the detection of early-stage obstructions. 26 patients with acute dilation of the renal pelvis or calyces underwent DW-MRI that incorporated echo-planar (i.e. EPI-DWI) sequences [14]. This resulted in images with reduced, but not significantly different, ADC values compared to the expected norm [14]. Standard CT can detect obstructions in or near the ureter [5]. Calculi can also be visualized using standard US, on which they appear as a small focus of echogenicity [2]. Thromboses may be also be visualized using US. They are associated with echogenesis in the vein, enlargement, loss of differentiation between cortex and medulla, and loss of echogenesis in the affected kidney [2]. Partial thrombosis may be associated with increases in RI [2]. Renal artery stenosis may be detected through aliasing on a color US image [2]. It may also be associated with abnormal arterial waveforms such as the tardus-parvus pattern [2]. This pattern, also known as 'parvus et tardus', results from a reduced arterial pulse with a delayed systolic peak [2].

 

 

Grayscale US showing a calculus developing in a calyx

Fig. 5. Grayscale US showing a calculus (arrow) developing in a calyx. Courtesy: Kolofousi, C., et al., Ultrasonographic features of kidney transplants and their complications: an imaging review. ISRN Radiol, 2013. 2013: p. 480862.

 

 

 

 

Maximum-intensity projection and volume-rendered CT imaging of severe renal arterial stenosis

Fig. 6. Maximum-intensity projection and volume-rendered CT imaging of severe renal arterial stenosis. Courtesy: Iezzi, R., et al., Interventional Radiological Treatment of Renal Transplant Complications: A Pictorial Review. Korean Journal of Radiology, 2015. 16(3): p. 593-603.


 

 

Treatment of Kidney Obstruction

Treatments for this condition include strategies that result in the same removal of the kidney stone in question. Alternatively, there are several methods of disrupting or breaking up the stone. This may alleviate obstruction, and again allow for the safe expulsion of the stone from the body. Procedures that achieve this include shock-wave lithotripsy. As the name suggests, this involves the use of a probe that emits sound-waves strong enough to disrupt the structural integrity of the stone [15]. Shock-wave lithotripsy is a safe, effective method of treatment that may be delivered outside the body (i.e. it is non-invasive) [16]. However, it may be an unsuitable option for some patients, particularly those with a higher body mass index [17]. In these cases, an ureterorenoscopy to enhance visualization during stone removal or disruption may be necessary [17]. A more traditional option in the treatment of obstruction is surgery to remove or destroy stones. These procedures are typically recommended in the cases of larger stones [18]. These procedures include retrograde intrarenal surgery (RIRS), which involves the insertion an endoscope through the urethra and its extension through the bladder toward the stone [19]. The stone is then disrupted using a device such as a laser, or excised with a thin forceps. Alternatively, this stage can be performed percutaneously, or through the skin above the kidney or ureter. This is done through a microsurgical incision, through which a needle is passed and extended into the renal pelvis to locate the stone. This is known as percutaneous nephrolithotomy (PCNL), and has been developed over recent years to reduce invasion [20]. On the other hand, surgery is also often contraindicated for obese or morbidly obese patients.

As mentioned above, passing a stone naturally is another option for patients with smaller and more mobile calculi. Some small, asymptomatic stones may be passed without awareness of their existence on the part of a patient or their healthcare professionals [21]. A patient choosing to wait for spontaneous passage may be advised to drink plenty of fluids. Pharmacotherapy such as adrenoceptor-alpha antagonists to expedite passage may be an option in some cases [22]. Some patients may wait months or even years to pass a stone [23]. This requires longitudinal imaging studies that assess stone location and movement over time, the probability of passage and stone growth [23]. This confers disadvantages such as the increased probability of obstruction and the necessity of a procedure as above to correct it, enduring ongoing symptoms such as pain, and the possible need of treatment such as analgesics to manage said symptoms [22, 24]. A study of over 100 patients found that respondents who had experienced the passage of a larger stone were more likely to choose an alternative treatment in the event of developing another [23].

Treatment for thromboses may include courses of anticoagulants, which may also be administered prophylactically to transplant recipients at increased risk of this condition [13]. Alternatively, thromboses can be broken up with minimally-invasive procedures using a catheter inserted into the vessel in question to disrupt it and thus remove the obstruction [25]. Larger thromboses may also be removed surgically (i.e. thrombectomy) [25]. Renal arterial stenosis may be corrected using percutaneous transluminal angioplasty or various forms of anastomosis. These are surgical procedures in which the functional tracts of the renal artery are surgically connected to another nearby major vessel (most often the iliac artery) [25]. This recovers renal blood supply, preserves kidney function and also controls blood pressure if applicable. The location and type of anastomosis depends on the location of stenosis, donor type (i.e. living or dead) and presence of donated renal artery if applicable [25].

 

 

Nephritis

 

Nephritis, as outlined above, is kidney damage related to infection. Generally, this term refers to the infection of more peripheral tissues of the kidney (i.e. the cortex and/or outer medulla, or the parenchyma of the kidney). However, nephritis may also extend into the renal pelvis, in which case it resembles pyelonephritis without hydronephrosis. Staghorn stones are associated with chronic infection, and may also cause pyelonephritis [11]. Nephritis may lead to complications such as renal abscess [3]. This condition may be accompanied by symptoms including pain in the flank and fever [5]. Untreated nephritis may lead to consequences such as renal scarring, hypertension and even chronic kidney failure [5]. Kidney infections arise in response to a number of risk factors, which include prior transplantation procedures, hydronephritis and prolonged obstruction (see above) [3].

 

Another form of nephritis is associated with the autoimmune condition lupus erythromatosus [26]. This is known as proliferative lupus nephritis, and results in the severe inflammation of many subcomponents or tissues of the kidney (glomeruli or tuberointerstitial tissues) due to autoimmune attack mediated by the body's own immunoglobulins [26]. This often develops in childhood, and may lead to chronic kidney disease and kidney failure over time [27].

 

 

Visualization of Nephritis

 

Low-dose CT is a standard indication in cases of nephritis in which the presence of calculi is suspected [3]. A recent study suggests that DWI can also image nephritis with high specificity, and can also detect complications such as abscess formation [3]. This study also showed that the ADC values derived from DWI were significantly lower for patients with nephritis compared to controls [3]. Nephritis appears as wedge-shaped, striated multiple foci of reduced ADC on DW-generated images [3]. This condition appears to be associated with significantly reduced diffusion, which is illustrated well by ADC maps derived from DWI [3]. These changes in diffusion are thought to be related to increases in the local leukocyte count of the infected area, leading to increases in cell density [3]. Abscesses may be detected through reduced diffusion related to accumulated pus [3]. CT and US are also regarded as effective in terms of abscess detection [3].

 

 

Multiple foci of infection clearly visible on a DWI-generated image

Fig. 7 CT of nephritis. This modality may show a loss of differentiation between cortices and medulla, as in this case, and peripheral abscesses (arrows). Case courtesy of Dr Hani Al Salam, Radiopaedia.org. From the case rID: 18306

 

 

 

Treatment of Nephritis

 

Nephritis may be resolved through the administration of antibiotics [3]. DWI has demonstrated potential in illustrating the response to this treatment [3]. Severe or advanced forms of nephritis may require surgery to remove damaged portions of renal tissue [28]. Lupus nephritis may be managed with immunosuppressants such as mycophenolate mofetil or azathioprine, or with glucocorticoids (steroid-based drugs) which also act to suppress immune responses [29].

 

 

Renal Cancer

 

The incidence of renal cancers is on the increase. In the 2000s, it was the fifth most common cancer type in men [30]. Cancers of the kidney mostly arise in the form of renal cell carcinoma. Cancers of the urothelium, or a specialized cell layer found in nearly all reaches of the urinary system, are also relevant to kidney pathology. This extends into the renal pelvis and calyces; therefore, urotheliomas may metastatize inwards or outwards in relation to the kidney. Tumors within the kidney may occur singly or in small groups ('local carcinoma') or spread throughout the organ. Renal cancers may be associated with common symptoms including lower abdominal pain and hematuria. They may also be palpable through the skin (as above). However, some cancers may remain undetected for a considerable length of time, due to the factors listed in the 'Introduction' section of this article. Biopsy analysis may detect neoplasms, but only if cell samples exhibit signs such as increased cell density and the loss of differentiation. Routine imaging may detect these masses, however.

 

Metastatic renal tumors may invade the rest of the abdomen (i.e. visceral metastasis) and pervade other organs (such as the liver) or other structures such as the adrenal gland [30]. Patients with kidney transplants may be at a higher risk of cancer development, due to the need of treatment with immunosuppressants to enhance graft acceptance. The prevalence of neoplastic growth in these patients is reported to be approximately 6% [31].

 

 

Visualization of Renal Cancers

 

Carcinomas may appear as changes in the size and/or shape of the kidney compared to the anatomical norm. Neoplasms or small tumors may be more resistant to detection, however. CT imaging is a standard procedure in the detection of renal cell carcinoma [30]. This technique may efficiently distinguish between benign lesions and malignant tumors [30]. Benign renal growths typically appear as areas of reduced density (compared to normal cortex) [30]. However, the imaging of malignant tumors using CT is rarely straightforward. Renal tumors may appear as areas of either reduced or increased densities [30]. However, CT may also show more specific signs of cancer development, including significant increases in contrast and necrosis within tumor masses [30]. Renal cell carcinoma also tends to exhibit characteristics such as well-defined margins, homogenous masses and calcifications [32]. Some types of this cancer tend to be distinguished by cells with clear cytoplasm (that part of the cell outside the nucleus, i.e. clear-cell carcinoma) [33]. This is apparent only on microscopic images of samples taken from a biopsy, however [33]. CT can also help estimate tumor diameter, with a margin of error of approximately 0.5cm [30]. CT appears to have limited success in detecting metastases, however, with a false positive rate of up to 43% [30]. Both CT and MRI seem to have limited efficacy in detecting multifocal lesions [30].

 

CT of the abdomen with a patent tumor mass on the right kidney

Fig. 8. Contrast-enhanced CT image showing renal cell carcinoma in left kidney (arrows). Case courtesy of Dr Frank Gaillard, Radiopaedia.org. From the case rID: 3046

 

 

Treatment of Renal Cancers

 

Renal tumors are often treated through resection, or surgical removal. This is regarded as an effective treatment for localized masses within the organ [34]. However, it is less successful in cases of metastasis [34]. Similarly, multifocal tumors may be unsuitable for resection [30].Some patients respond to cytokine therapy, which modulate immune responses to target cancer cells [34]. Other forms of immunomodulatory therapy for the treatment of renal cancer include drugs that inhibit proteins involved in immune system regulation (e.g. mTOR) [34]. Radiofrequency ablation (RFA) is an increasingly popular treatment for renal cancers, with the ability to reduce tumor size and to alleviate symptoms such as hematuria [13]. DWI may also have some validity in the detection of renal cell carcinoma [35]. A review of 16 studies incorporating over 1400 patients found that the ADC values derived from images of patients with malignant tumors were significantly decreased compared to those derived from images of people with benign growths [35].

 

 

Discussion

 

This article supplies information relevant to the detection of renal pathology through various modalities of imaging. This may suggest the role of this imaging in the routine assessment of conditions that affect the kidney. Imaging is linked to certain advantages. These include non-invasion and thus aggravation of some risks and side-effects, such as those associated with transplantation surgeries. On the other hand, many radiographic contrast materials are also nephrotoxic. This may limit the application of imaging in repeat assessments of patients with conditions such as renal infections, lupus and chronic kidney disease. This is due to the potential of further kidney dysfunction. On the other hand, initial or emergency assessment of patients presenting with symptoms such as pain or hematuria may be enhanced through imaging.

 

Some modalities are well-established in the routine assessment of these abdominal symptoms. These are often used to image the kidneys to eliminate the possibility of events such as renal obstruction, nephritis or stone passage. However, some techniques may be less efficient than others. Comparative studies using kidney disease models suggest that SPECT and MRI appear to be more selective and sensitive in detecting both general and finer details of pyelonephritis [6]. Some researchers suggest that DWI is an attractive and advantageous replacement for CT in the routine assessment of kidney pathology. This is connected to recommendations that call for minimal radiation exposure, particularly in the case of pediatric patients [3]. Effective doses in children can be as much as 24% higher than those received by adults, although they absorb up to 44% less energy per scan [36].

 

In some cases, the potential of some modalities in this branch of medicine is still emerging or at the proposal stages. The applications of DWI in renal imaging have been investigated in relatively few studies, despite its apparent advantages over CT. This form of MRI have shown significant efficacy in discriminating between healthy and pathological renal tissues, and between pathologies of different severity [3]. However, the failure of DWI to discriminate between the different tissues of the kidney as expected may ultimately limit its applications in the detection of renal disease.

 

 

Conclusion

 

Imaging is useful in the assessment of patients in whom kidney pathology is suspected due to common symptoms such as lower abdominal pain and urinary abnormalities. However, these symptoms may be common to many diseases and conditions. Imaging can distinguish one from the other, despite the relative complexity and heterogeneity of the tissues that make up the kidney. When imaging is appropriate in the routine assessment of chronic kidney conditions, an R.T. may need to take factors such as dose, energy exposure and cost into account when selecting imaging techniques for this.

 

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