Renal Imaging

The renal system is composed of 2 kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys, 2 ureters, a bladder, and a urethra. These structures function to filter blood and excrete urine, which contains waste products of metabolism. Varying conditions such as infections, cysts, solid masses, ischemia, and mechanical obstruction can affect the renal system. Evaluation of diseases rely on imaging methods such as radiography, ultrasonography, CT, and MRI. Some of these are also used to guide tissue sampling (e.g., renal biopsy).

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Introduction

Imaging methods

The common radiologic methods in use to evaluate the urinary tract Urinary tract The urinary tract is located in the abdomen and pelvis and consists of the kidneys, ureters, urinary bladder, and urethra. The structures permit the excretion of urine from the body. Urine flows from the kidneys through the ureters to the urinary bladder and out through the urethra. Urinary Tract are:

  • Radiography: kidney, ureters, and bladder
  • Ultrasonography
  • CT
  • MRI

Preparation and orientation

  • Prior to interpretation of any image, the physician should take certain preparatory steps. The same systematic approach should be followed every time.
    • Confirm name, date, and time on all images.
    • Obtain information on the individual’s medical history and physical examination.
    • Confirm appropriate exam and technique for desired pathology.
    • Compare any available images of the same area taken using the same method.
  • Determine orientation of image:
    • Right or left marker on X-ray
    • In the United States, standard exam views place a marker (dot) to patient’s right.
    • For CT/MRI: On axial view, the image is sliced and viewed from inferior to superior (as if you are looking from an individual’s feet up)

Radiography

Overview

  • Medical indications: 
    • Obtain a quick overview of the urinary tract Urinary tract The urinary tract is located in the abdomen and pelvis and consists of the kidneys, ureters, urinary bladder, and urethra. The structures permit the excretion of urine from the body. Urine flows from the kidneys through the ureters to the urinary bladder and out through the urethra. Urinary Tract
      • Kidney: renal outlines may be seen, spanning 3–4 lumbar vertebrae
      • Ureter: not visible, but stones can be detected
      • Bladder: if distended, may be seen
    • Examines the main areas “gas, mass, bones, stones”:
      • Detects free air in the abdomen
      • On rare occasions, gas, as noted in emphysematous pyelonephritis Pyelonephritis Pyelonephritis is infection affecting the renal pelvis and the renal parenchyma. This condition arises mostly as a complication of bladder infection that ascends to the upper urinary tract. Pyelonephritis can be acute or chronic (which results from persistent or chronic infections). Typical acute symptoms are flank pain, fever, and nausea with vomiting. T Pyelonephritis and Perinephric Abscess, is seen as a mottled collection of air in the kidney.
      • Rarely, soft-tissue masses may be seen.
      • Bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones lesions (sclerotic changes or lytic lesions) can be visualized.
      • Detects radiopaque calculi, but only 40%–60% of stones are detected radiographically.
  • Advantages:
    • Low cost 
    • Low radiation dose 
    • Ubiquitous availability
    • Fast
  • Disadvantages:
    • Poor resolution of soft tissue
    • Exposure to ionizing radiation
    • Individual must hold still for image

Exam technique

  • Positioning:
    • Rotation:
      • There should be no rotation.
      • Pedicles of the spine should be symmetric. 
    • Visualization:
      • Bowel gas should be completely radiolucent.
      • Peritoneal fat planes should be visible laterally.
  • Positioning for specific views:
    • Anteroposterior (AP):
      • Board is against the back. 
      • X-ray beams anterior → posterior direction through the individual
      • Can be completed supine or upright
      • Upright is better for evaluating bowel obstruction and free air.
    • Lateral decubitus (from side while laying down): 
      • Individual is supine.
      • Board is against side.
      • Usually left side down on table (allows better visualization of free air).
  • Penetration: degree to which radiation has passed through body, resulting in darker or lighter image
    • Underpenetration: Film appears whiter and features are less apparent.
    • Overpenetration: Film appears darker and features are less apparent.

Interpretation and evaluation

As the obtained image contains multiple organs and structures (and not just the urinary tract Urinary tract The urinary tract is located in the abdomen and pelvis and consists of the kidneys, ureters, urinary bladder, and urethra. The structures permit the excretion of urine from the body. Urine flows from the kidneys through the ureters to the urinary bladder and out through the urethra. Urinary Tract), an inside-out approach (central to peripheral) is taken to provide interpretation:

  • Look at bowel gas pattern.
  • Look for solid organ silhouettes ( liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver, spleen Spleen The spleen is the largest lymphoid organ in the body, located in the LUQ of the abdomen, superior to the left kidney and posterior to the stomach at the level of the 9th-11th ribs just below the diaphragm. The spleen is highly vascular and acts as an important blood filter, cleansing the blood of pathogens and damaged erythrocytes. Spleen, kidney).
  • Look for normal fat planes peripherally.
  • Look for evidence of free intraperitoneal air.
  • Evaluate soft tissues for abnormal calcifications (stones, masses).
  • Look at the lung bases for evidence of consolidation, effusion, and pneumothoraces.
  • Evaluate osseous structures (vertebral body height, iliac bones, femurs).
  • Once abnormality is noted, use pattern approach to come up with differential diagnoses.

Normal findings

AP view:

  • Bowel gas:
    • Should be present in the small and large bowel
    • Should not distend the small bowel > 3 cm
    • Should be uniform throughout the abdomen
  • Organs:
    • Liver is visible under the right hemidiaphragm; should be uniform with no free air.
    • Stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach and colon Colon The large intestines constitute the last portion of the digestive system. The large intestine consists of the cecum, appendix, colon (with ascending, transverse, descending, and sigmoid segments), rectum, and anal canal. The primary function of the colon is to remove water and compact the stool prior to expulsion from the body via the rectum and anal canal. Colon, Cecum, and Appendix are visible with air in lumen under the left hemidiaphragm. 
    • Renal outlines, if visible, should be in the RUQ and LUQ.
  • Fat planes: normal peripheral fat planes
  • Lung bases
    • Should be clear, with minimal lung markings
    • Costophrenic angles should be sharp.
  • Bones:
    • Pedicles should be present and symmetric bilaterally.
    • Vertebral body heights should be gradually enlarging as they move inferiorly.
    • Spinous processes midline
Kidneys, ureters, bladder x-ray showing no abnormalities

Kidneys, ureters, bladder X-ray showing no abnormalities

Image: “The kidney-ureter-bladder X-ray: no abnormal findings.” by Michalakis K, Moutzouris DA. License: CC BY 3.0

Ultrasonography

Overview

  • Medical indications:
    • Emergency care:
      • Trauma with concern for renal injury
      • Concern for urinary obstruction (e.g., calculi/stone)
    • Routine care:
      • Signs and symptoms of renal failure/ CKD CKD Chronic kidney disease (CKD) is kidney impairment that lasts for ≥ 3 months, implying that it is irreversible. Hypertension and diabetes are the most common causes; however, there are a multitude of other etiologies. In the early to moderate stages, CKD is usually asymptomatic and is primarily diagnosed by laboratory abnormalities. Chronic Kidney Disease
      • Screening for patients with known renal calculi
      • Screening for neonates with abnormalities noted on prenatal ultrasonography
    • Monitoring: known congenital disease with predisposition for renal cell carcinoma Renal cell carcinoma Renal cell carcinoma (RCC) is a tumor that arises from the lining of the renal tubular system within the renal cortex. Renal cell carcinoma is responsible for 80%-85% of all primary renal neoplasms. Most RCCs arise sporadically, but smoking, hypertension, and obesity are linked to its development. Renal Cell Carcinoma 
  • Advantages:
    • Low cost 
    • No radiation dose 
    • Widespread availability
    • Fast
  • Disadvantages:
    • Poor resolution 
    • Narrow field of view
    • Individual must hold still for image
    • Technician-dependent

Exam technique

  • Positioning:
    • Individual:
      • Access to the bilateral abdomen
      • Maximize contact between individual’s skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin and ultrasound probe
    • Visualization: The kidney should be most superficial to the probe without other organs/bowel between the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys and ultrasound probe.
  • Depth and gain: 
    • Determine the field of view and echogenicity characteristics of the tissue
      • Ideally, the entire kidney is visualized on the image without excess abdominal signal deep to the individual kidney.
      • Less depth → greater magnification
    • Gain (amplification of returning signals):
      • Should be adjusted such that the renal parenchyma is visualized 
      • Too much signal leads to loss of detail.

Interpretation and evaluation

Report on:

  • Kidney size:
    • Normal: 10–12 cm 
    • Renal cortex should be > 6 mm.
  • Echogenicity:
    • Renal cortex is homogeneous and usually slightly hypoechoic or isoechoic relative to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver.
    • Kidney with more echogenicity than the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver is abnormal.
  • Position: kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys are inferior to the left hemidiaphragm and liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver.
  • Associated structural abnormalities

Normal findings

Normal renal ultrasound:

  • Oval bean shape
  • Separated from the echogenic perirenal fat by a capsule (a thin, linear structure)
  • Parenchyma:
    • Hypoechoic 
    • Homogeneous
    • Renal medulla is hypoechoic (darker) compared to the renal cortex (and in some, the medulla is seen as cone-shaped central structures).
  • Renal hilum: hyperechoic
  • Ureter: 
    • Often not visualized owing to overlying tissue
    • Patency detected by Doppler when urine enters the bladder
  • Bladder: anechoic rounded organ in the pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis
Normal adult kidney

Normal adult kidney:
Measurement of kidney length on the ultrasound is illustrated by plus signs and a dashed line.
*: column of Bertin
**: pyramid
***: cortex
****: sinus

Image: “Normal adult kidney. Measurement of kidney length on the US image is illustrated by ‘+’ and a dashed line. * Column of Bertin; ** pyramid; *** cortex; **** sinus.” by MDPI, Basel, Switzerland. License: CC BY 4.0

CT

Overview

  • Medical indications:
    • Follow-up for suspicious ultrasound image:
      • Hypoechoic/hyperechoic lesions
      • Concern for vascular pathology
      • Contour deformities
      • Urolithiasis
      • Hydronephrosis Hydronephrosis Hydronephrosis is dilation of the renal collecting system as a result of the obstruction of urine outflow. Hydronephrosis can be unilateral or bilateral. Nephrolithiasis is the most common cause of hydronephrosis in young adults, while prostatic hyperplasia and neoplasm are seen in older patients. Hydronephrosis
    • Malignancy:
      • Evaluate initial staging Staging Cancer is the 2nd leading cause of death in the US after cardiovascular disease. Many malignancies are treatable or curable, but some may recur. Thus, all malignancies must be assigned a grade and stage in order to guide management and determine prognosis. Grading, Staging, and Metastasis of renal cell carcinoma Renal cell carcinoma Renal cell carcinoma (RCC) is a tumor that arises from the lining of the renal tubular system within the renal cortex. Renal cell carcinoma is responsible for 80%-85% of all primary renal neoplasms. Most RCCs arise sporadically, but smoking, hypertension, and obesity are linked to its development. Renal Cell Carcinoma
      • Surveillance for recurrence of disease
      • Tumor vein thrombosis
    • Major trauma:
      • Evaluation of renal parenchyma
      • Evaluation for renal hemorrhage +/– active extravasation on delayed images
      • Concern for subtle pathology not seen on ultrasound
  • Advantages:
    • Excellent resolution of anatomical detail 
    • Structures can be seen in 3 dimensions.
    • CT urography has replaced IV pyelography and is a test of choice in evaluating hematuria and the urothelium. 
  • Disadvantages: 
    • High radiation dose
    • Individual must hold still for exam.
    • Expensive

Exam technique

Standard CT scanning:

  • Individual lies supine on table: 
    • Table is moved in a CT scanner, which rotates around the individual.
    • Individual is instructed to hold breath and remain still for scan (for seconds).
  • Exams can be done with or without IV or oral contrast:
    • Without contrast: used in evaluation of renal or ureteral calculi
    • Likewise, noncontrast studies serve as a baseline in evaluating lesion enhancement (when contrast is administered)
    • Timing of IV contrast dye can help direct radiology inquiry of certain areas of pathology.
    • Contrast study: 
      • Nephrogenic phase: evaluates the kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys for any lesions
      • Excretory phase: evaluates the collecting system

Interpretation and evaluation

Interpretation should follow systematic and reproducible pattern.

  • History and examination are reviewed.
  • Ideal evaluation with soft-tissue window (W)/level (L): 400/50
  • Compare to available recent imaging of area of interest
  • Orient image:
    • Axial images are viewed as if looking from the individual’s feet up.
    • Sagittal and coronal 
  • Identify landmark anatomical structures.
  • Observe for “continuity” of parenchyma while scrolling through image slices.

Normal findings

  • Kidneys:
    • Oval bean shape
    • Retroperitoneal location, with the right kidney slightly lower than the left
    • Average length: 10–12 cm
    • Symmetric in size in general; difference of 2 cm suggests a pathology.
    • Homogeneous in noncontrast study
    • Divided into:
      • Upper pole
      • Interpolar region (parallel to the hilum)
      • Lower pole
  • Ureters:
    • Visualized as a 2–3-mm tubular structure coming from the renal pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis
    • Surrounded by retroperitoneal fat
    • With contrast studies, appear as contrast-containing round structures
  • Bladder: 
    • Rounded structure in the pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis
    • Both ureters and bladder are opacified with contrast studies.
Ct of the abdomen and pelvis (with contrast)

CT of the abdomen and pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis (with contrast):
From top left: sagittal, coronal plane, and axial plane, with 3-mm slice thickness. Images show normal anatomy.

Image: “CT of a normal abdomen and pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis, thumbnail” by Mikael Häggström. License: CC0 1.0

MRI

Overview

  • Medical indications:
    • Detailed evaluation of renal lesions
      • Hemangiomas
      • Cysts
      • Renal cell carcinoma
      • Indeterminate lesions noted incidentally on ultrasound and CT
    • Vascular disease (MRA for renal artery aneurysms)
  • Advantages:
    • Provides higher level of imaging and detail of fluid, enhancement, and soft tissue
    • Can be used for evaluation of pregnant individuals
    • Used as adjunct to previous test (ultrasound/CT)
  • Disadvantages:
    • ↑↑↑ Cost
    • Takes much longer to perform than chest X-ray, CT, or ultrasonography
    • Not suitable for all individuals:
      • Implants (particularly metal) distort image.
      • Requires individual to be in a loud, enclosed space
      • Individual must stay still for adequate images.

Exam technique

  • Positioning:
    • Supine on table
    • Table is advanced into scanner.
    • Individual is instructed to remain still for scan.
  • MRA:
    • To examine blood vessels
    • Exams can be done with or without IV contrast (gadolinium).
    • Advancements have allowed medicine to create both 2- and 3-dimensional image reconstructions.
  • Views:
    • T1-weighted images: 
      • Lesions with high fat content (e.g., angiomyolipoma) appear bright/white and compartments filled with water appear dark/black.
      • Postcontrast images are typically T1-weighted owing to the intrinsic properties of gadolinium. 
    • T2-weighted images: compartments filled with water appear bright/white.
    • Images oriented in 3-dimensional “slices”: 
      • Coronal 
      • Sagittal
      • Axial
Table: General principles of MRI
Tissue T1-weighted images T2-weighted images
Fluid (e.g.,CSF) Dark Bright
Fat Bright Bright
Inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation Dark Bright

Interpretation and evaluation

Interpretation should follow a systematic and reproducible pattern:

  • Review the individual’s history and physical examination information.
  • Compare to available recent imaging of the area of interest.
  • Orient image.
  • Identify landmark anatomical structures.
  • Observe for “continuity” of structures while scrolling through image slices.

Normal findings

  • Normal renal/kidney MRI appearance: 
    • Crescent-shaped structure
    • Smooth borders
    • Homogeneous cortex signal
    • Intensity: 
      • T1-weighted: slightly less intense than the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver
      • T2-weighted: more intense than the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver
    • Contrast:
      • Arterial phase: heterogeneous enhancement
      • Venous phase: homogeneous enhancement
      • Similar to CT pattern
  • Both ureters and bladder are also well visualized.

Abnormal Findings

Nephrolithiasis Nephrolithiasis Nephrolithiasis is the formation of a stone, or calculus, anywhere along the urinary tract caused by precipitations of solutes in the urine. The most common type of kidney stone is the calcium oxalate stone, but other types include calcium phosphate, struvite (ammonium magnesium phosphate), uric acid, and cystine stones. Nephrolithiasis

  • Radiography: radiopaque calculi are visualized (but some stones may be radiolucent).
  • Ultrasonography:
    • Nephrolithiasis Nephrolithiasis Nephrolithiasis is the formation of a stone, or calculus, anywhere along the urinary tract caused by precipitations of solutes in the urine. The most common type of kidney stone is the calcium oxalate stone, but other types include calcium phosphate, struvite (ammonium magnesium phosphate), uric acid, and cystine stones. Nephrolithiasis:
      • +/– hydronephrosis
      • Hyperechoic foci (stone) 
      • +/– posterior shadowing
    • Hydronephrosis Hydronephrosis Hydronephrosis is dilation of the renal collecting system as a result of the obstruction of urine outflow. Hydronephrosis can be unilateral or bilateral. Nephrolithiasis is the most common cause of hydronephrosis in young adults, while prostatic hyperplasia and neoplasm are seen in older patients. Hydronephrosis:
      • Anechoic fluid-filled interconnected space
      • Spectrum of dilation of the renal calyces
  • CT:
    • Method of choice for renal stones owing to:
      • High sensitivity and specificity
      • No contrast needed (best method: non-enhanced CT)
      • No patient preparation
      • Not operator-dependent
    • Features:
      • Hyperdense lesion (stone)
      • Hydronephrosis Hydronephrosis Hydronephrosis is dilation of the renal collecting system as a result of the obstruction of urine outflow. Hydronephrosis can be unilateral or bilateral. Nephrolithiasis is the most common cause of hydronephrosis in young adults, while prostatic hyperplasia and neoplasm are seen in older patients. Hydronephrosis (enlargement of the renal collecting system) noted if there is an obstruction
      • +/– hydroureter

Pyelonephritis

  • Infection affecting the renal pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis and the renal parenchyma
  • Can be diagnosed clinically, but no response in 48–72 hours prompts obtaining imaging studies
  • Ultrasonography:
    • Debris in the collecting system (internal echoes)
    • Reduced areas of cortical vascularity 
    • Loss of normal demarcation
    • Focal hypoechoic areas ( edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema)
    • Hyperechoic areas (hemorrhage)
    • +/– perinephric abscess (hypoechoic or mixed echogenicity fluid collection)
  • CT:
    • Method of choice (unenhanced images followed by contrast-enhanced study)
    • Features may include:
      • Swollen, edematous kidney (appearing less dense)
      • Fat stranding
      • Focal wedge-like regions with reduced enhancement compared with the normal portions of the kidney  
      • Perinephric abscess (soft-tissue or fluid attenuation within the perirenal space)

Renal infarction

  • Loss of renal tissue resulting from the disruption of renal blood flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure
  • Major causes are thromboemboli and in situ thrombosis.
  • Ultrasonography:
    • Complete or segmental blood flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure absence (color Doppler)
    • Contrast-enhanced ultrasonography can detect infarcts.
    • Infarcted area:
      • Hypoechoic  
      • Wedge-shaped region or patchy areas of reduced perfusion
      • Well defined 
  • CT:
    • Non-contrast CT is the initial test for the presentation of flank pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain (since kidney stones are more common).
    • But renal infarcts are most often identified with contrast-enhanced CT:
      • Classic finding: wedge-shaped lesion/perfusion defect
      • Hypodense
      • Well defined 
      • Arterial phase may show the thrombus/embolus or dissection.
  • MRI with gadolinium is an alternative to CT.

Renal cysts

  • Fluid-filled lesions, which can be:
    •  Simple cysts: benign and asymptomatic
    • Complex cysts:
      • Share features of malignancy
      • May require follow-up imaging or biopsy/surgery
  • Ultrasonography:
    • Simple cyst:
      • Anechoic, well-defined lesions
      • Round, with smooth margins
      • Strong posterior wall echo (good transmission through the cystic structure)
      • Thin wall, without septations or nodules
      • No complex cyst features
    • Complex cyst:
      • Internal echoes 
      • Calcifications
      • Thick wall
      • Solid component(s)
  • CT:
    • Contrast study is the method of choice in indeterminate renal lesions/cysts.
    • Simple cyst:
      • Sharp margins, with smooth, thin wall
      • Density measurement of water (–10 to +30 Hounsfield units) 
      • Non-enhancing (after contrast is administered), indicating an avascular structure
      • No complex cyst features
    • Complex cyst:
      • Septation
      • Calcification
      • Solid component
      • Enhancement (after contrast) indicating vascularity
      • Thick walls
      • Necrosis 
  • MRI:
    • Used when CT is contraindicated or if CT findings are equivocal
    • Simple cyst:
      • Fluid-filled lesion
      • Sharp margins
      • Non-enhancing
      • No complex cyst features
    • Complex cyst:
      • Septation (may be more apparent on MRI than on CT)
      • Calcification
      • Solid component
      • Enhancement
      • Thick walls
      • Necrosis

Renal cell carcinoma

  • Most common primary renal neoplasm
  • Malignancy originates from the cortex.
  • Ultrasonography (less sensitive than CT):
    • Able to distinguish cystic structure or solid tumor
    • Features:
      • Ill-defined margins 
      • Typically isoechoic (can be hypoechoic or hyperechoic)
    • +/– calcification
  • CT:
    • Method of choice (multiphasic CT scan of the abdomen and pelvis Pelvis The pelvis consists of the bony pelvic girdle, the muscular and ligamentous pelvic floor, and the pelvic cavity, which contains viscera, vessels, and multiple nerves and muscles. The pelvic girdle, composed of 2 "hip" bones and the sacrum, is a ring-like bony structure of the axial skeleton that links the vertebral column with the lower extremities. Pelvis)
    • Features:
      • May be solid masses or complex cystic masses
      • Indeterminate density 
      • Ill-defined border
      • Irregular enhancement
      • +/– calcification
      • +/– necrosis (hypodense)
      • +/– renal veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins and inferior vena cava Inferior vena cava The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels extension
  • MRI:
    • Alternative in cases of iodinated contrast allergy or if with inconclusive ultrasound/CT 
    • Features may be: 
      • Solid masses 
      • Complex cystic masses
      • Indeterminate density 
      • Ill-defined border
      • Irregular enhancement
      • +/– calcification
      • +/– necrosis
      • +/– renal veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins and inferior vena cava Inferior vena cava The venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs. Mediastinum and Great Vessels extension

Other renal abnormalities

  • Angiomyolipoma:
    • Most common benign kidney tumor
    • Made of varying amounts of fat, blood vessels, and smooth muscle 
    • Imaging: CT with and without contrast
      • Cortical lesion is well defined.
      • + Intralesional fat density
  • Renal agenesis: 
    • Developmental renal defect, which can be unilateral or bilateral
    • Prenatal diagnosis relies on ultrasonography, which shows:
      • Empty renal fossa
      • Large contralateral kidney
      • Congenital elongation of the ipsilateral adrenal gland
      • Color flow Flow Blood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls. Vascular Resistance, Flow, and Mean Arterial Pressure Doppler will fail to show renal vessel(s).
  • Horseshoe kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys:
    • Involves abnormal migration of both kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys
    • Fusion of both kidneys Kidneys The kidneys are a pair of bean-shaped organs located retroperitoneally against the posterior wall of the abdomen on either side of the spine. As part of the urinary tract, the kidneys are responsible for blood filtration and excretion of water-soluble waste in the urine. Kidneys at their inferior poles
    • Often located below the inferior mesenteric artery
    • Presentation is often without symptoms, with the anomaly detected with antenatal ultrasonography.
    • In others, imaging workup is directed toward the presenting symptoms, including: 
      • Pain
      • Hematuria
      • Urinary tract infection 
      • Obstruction
  • Crossed kidney ectopia: 
    • Kidney and ureter cross the midline, reaching the other side (to the contralateral kidney), with or without fusion.
    • The contralateral kidney may be in its normal position or positioned lower.
    • More often an incidental finding on either ultrasonography or CT.
      • No kidney in the renal fossa
      • Kidney found in the other side, with the other kidney

References

  1. Faubel, S., Patel, N. U., Lockhart, M. E., Cadnapaphornchai, M. A. (2014). Renal relevant radiology: use of ultrasonography in patients with AKI. Clin J Am Soc Nephrol 9:382–394. https://doi.org/10.2215/CJN.04840513
  2. Gash J.R., Noe J (2011). Radiology of the urinary tract. Chapter 9 of Chen M.M., Pope T.L., Ott D.J. (Eds.), Basic Radiology, 2nd ed. McGraw Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=360&sectionid=39669018
  3. Hansen, K.L., Nielsen, M.B., Ewertsen, C. (2015). Ultrasonography of the kidney: a pictorial review. Diagnostics 6(1):2. https://doi.org/10.3390/diagnostics6010002
  4. Hiorns, M.P. (2011). Imaging of the urinary tract: the role of CT and MRI. Pediatr Nephrol 26:59–68. https://doi.org/10.1007/s00467-010-1645-4
  5. Kruskal, J., Richie, J. (2021) Simple and complex kidney cysts in adults. UpToDate. Retrieved December 15, 2021, from https://www.uptodate.com/contents/renal-ectopic-and-fusion-anomalies
  6. Rosenblum, N. (2021) Renal ectopic and fusion anomalies. UpToDate. Retrieved Dec 14, 2021 from https://www.uptodate.com/contents/renal-ectopic-and-fusion-anomalies
  7. Salem, U., Matta, E.J., Youssef, A., Elsayes, K.M. (2014). Genitourinary system. In: Elsayes, K.M., Oldham, S.A. (Eds.),  Introduction to Diagnostic Radiology. McGraw-Hill. https://accessmedicine.mhmedical.com/content.aspx?bookid=1562&sectionid=95877752

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