Ultrasound of the Kidneys and Retroperitoneal Space: Measurement Protocol
Normative Basis
The practical standard AIUM Practice Parameter for the Performance of an Ultrasound Examination of the Abdomen and/or Retroperitoneum, 2021 describes which structures and images should be obtained during an ultrasound of the abdomen and/or retroperitoneum. For the request 'kidneys and retroperitoneal space,' it is crucial not to mix a full abdominal protocol, a full retroperitoneal protocol, and a limited study: the composition of the protocol depends on the clinical task and should be clearly stated in the conclusion.
Study Classes by AIUM
AIUM divides studies into complete and limited. This is not a formality for billing but a way to show which anatomical areas are truly assessed. If a doctor refers for 'kidney ultrasound,' but only the kidneys and bladder are actually examined, the protocol should not appear as a full abdominal examination.
| Study Class | What It Means for the Protocol | Practical Record |
|---|---|---|
| Complete Retroperitoneal Examination | Structures of the retroperitoneal space are assessed according to the task: kidneys, vascular structures, identified masses; upon urological request — the bladder. | 'Retroperitoneal ultrasound, complete protocol' |
| Limited Retroperitoneal Examination | A specific organ, area, or clinical problem is assessed. | 'Limited ultrasound: kidneys and bladder' |
| Complete Abdominal Examination | Includes abdominal organs and retroperitoneal structures according to AIUM standards. | Does not substitute the targeted urological protocol if the task is only kidneys. |
| Limited Abdominal Examination | A specific area of interest is documented. | For example, monitoring a known mass or pain in a specified area. |
Mandatory Kidney Slices
For each kidney, AIUM requires longitudinal and transverse images. In practical protocol, this means that the right and left kidneys should be shown so that the upper and lower poles, sinus, parenchyma, and contour are visible. Transverse images are needed not only for a 'formal' frame: they help identify focal changes, parenchymal asymmetry, dilation of the calyceal system, and volumetric formations.
Minimum documentation for each kidney: side, position, contours, parenchymal echogenicity, sinus and collecting system condition, presence of calculi, cystic or solid formations, perinephric fluid. If visualization is limited, this is recorded in the protocol as a diagnostic limitation.
Mandatory Kidney Measurements
The key mandatory measurement by AIUM is the maximum length of each kidney. It is obtained in the longitudinal plane at the greatest distance between the poles. It is important not to shorten the organ with an oblique slice: if in doubt, several longitudinal accesses are performed, and the largest correct value is recorded.
AIUM 2021 does not establish a numerical norm for kidney length for adults or children and does not set a threshold for pathological reduction. Therefore, in a standardized protocol, it is more correct to write the actual value and describe asymmetry, parenchymal changes, and clinical context, rather than refer to an unspecified universal threshold in the document.
| Structure | Mandatory Visualization | Measurement by AIUM | Protocol Comment |
|---|---|---|---|
| Right Kidney | Longitudinal and transverse planes | Maximum length | Compare echogenicity with the liver, if visually accessible. |
| Left Kidney | Longitudinal and transverse planes | Maximum length | Compare echogenicity with the spleen, if visually accessible. |
| Collecting System | Assessment of calyces, pelvis, visible parts of the ureter | No numerical threshold set | Describe the presence or absence of dilation and level, if traceable. |
| Focal Formation | In two mutually oriented planes | Sizes according to the identified finding | Indicate localization, echostructure, contours, blood flow if necessary. |
Echogenicity and Parenchyma
AIUM recommends assessing the renal parenchyma, including echogenicity. In practice, the right kidney is usually compared with the liver, the left with the spleen, if these organs are accessible in the acoustic window. In the protocol, it is useful to separate the description of the cortical layer, medullary substance, and sinus, but numerical norms for cortical thickness should not be substituted unless taken from an applicable local standard or another agreed document.
Formulations should be reproducible: 'parenchymal echogenicity is not increased,' 'diffusely increased,' 'cortico-medullary differentiation preserved/reduced.' In chronic kidney disease, the ultrasound picture is nonspecific, so the conclusion should not replace laboratory assessment of kidney function.
Calyceal System and Ureters
The collecting system is assessed in both kidneys. If there is dilation, it is necessary to describe whether it is limited to the pelvis, extends to the calyces, whether the dilated ureter is visible, and to what level it is traceable. AIUM does not set a numerical scale for hydronephrosis in this parameter, so anatomical description is appropriate in the protocol, rather than assigning an external classification without indicating its source.
The ureter is often not visualized along its entire length in normal conditions. If it is dilated, a calculus, mass, or extrarenal cause of obstruction is found, this should be reflected as a significant finding of the retroperitoneal study.
Bladder in the Retroperitoneal Protocol
For a urological task, AIUM includes the bladder in the urinary system examination. Filling, wall, lumen, intravesical formations, calculi, clots, diverticula are assessed if visualized. Residual urine or bladder volume is measured when clinically required.
Important: AIUM 2021 does not provide a universal numerical threshold for pathological residual volume. Therefore, the conclusion should indicate the measured volume and conditions of measurement — before urination, after urination, degree of filling, visualization limitations.
Aorta, IVC, and Other Retroperitoneal Structures
If the study is declared as retroperitoneal and the clinical task is not limited to the urinary system only, it is necessary to assess the main retroperitoneal vascular structures and identified masses. For the aorta, accessible sections, diameter, and presence of local expansion, thrombotic masses, or other visible changes are documented. IVC is described if visualization is available; if limited by gas or constitutional factors, this is indicated.
Any identified retroperitoneal mass should be described by localization, size, echostructure, relation to the kidney, aorta, IVC, and ureter. If the formation cannot be reliably attributed to an organ, the conclusion should be descriptive and recommend correlation with CT/MRI based on clinical indications.
Minimum Conclusion Template
A structured protocol reduces the risk of missing mandatory elements. Practical order: type of study; quality of visualization; right kidney; left kidney; collecting systems and visible ureters; bladder; retroperitoneal vessels and additional findings; final conclusion.
- Type: complete or limited retroperitoneal ultrasound.
- Kidneys: position, contours, maximum length, parenchyma, sinus, calyceal system, focal changes.
- Ureters: not dilated or describe visible dilation/cause.
- Bladder: filling, wall, lumen, volumes when indicated.
- Retroperitoneal Space: aorta, IVC, lymph nodes/masses if visualized.
- Limitations: gas, obesity, pain, postoperative changes, incomplete bladder filling.
Common Protocol Errors
The first error is the absence of the length of one of the kidneys in a formally 'normal' description. The second is the conclusion of a complete retroperitoneal examination without documented assessment of the bladder upon urological request. The third is the use of numerical norms not present in the chosen AIUM standard without reference to another validated source. The fourth is describing hydronephrosis only with the word 'present' without indicating the side, anatomical level, and visible state of the ureter.
The optimal protocol by AIUM is brief but verifiable: each mandatory structure has an image, key measurement, and clear conclusion, and each visualization limitation is explicitly named.
Frequently asked questions
Is it always necessary to measure kidney parenchymal thickness?
AIUM 2021 requires recording the maximum length of each kidney for the basic protocol. Parenchymal thickness can be useful, but a universal numerical standard is not set in this document.
Can a study be called complete if only the kidneys are examined?
No. If only the kidneys or kidneys with the bladder were assessed, it should be designated as a limited retroperitoneal or targeted urinary system study, not a full abdominal protocol.
Is residual urine calculation mandatory?
Calculation of bladder volume and residual urine is performed when clinically indicated. AIUM 2021 does not set a single threshold for pathological residual volume, so the protocol should indicate the actual volume and conditions of measurement.