Patient preparation is helpful to facilitate an efficient exam. Fasting decreases intestinal contents and can allow for a more anterior window for imaging the kidney. This can avoid the thicker flank musculature and take advantage of the liver as a window to the right kidney. In the urology office, however, renal imaging is often performed on short notice to investigate an acute problem during the appointment. Thus, it is usually impractical to have the patient fast several hours prior to the exam. For planned examinations, such as upper tract imaging for a hematuria workup or screening for postoperative hydronephrosis after endoscopic stone extraction, the patient should fast for 6 h prior to the examination.

The exam room should have the capability to dim room lights and reduce glare from natural lights. The ultrasound image detail on the screen is dramatically improved by reducing ambient light.

Patient positioning will depend upon the indications for the exam. A focused unilateral follow-up exam such as for a post-ESWL hematoma or for renal trauma can be done efficiently with the patient in supine position. However, larger patients with more truncal obesity are likely best imaged in the lateral position since this allows for both coronal and sagittal (lateral and posterior) imaging of the particular renal unit. A bilateral examination will likely then involve moving the patient from supine or right lateral to left lateral position to image the left kidney. Ultrasound for urolithiasis will involve imaging from the high posterior flank coursing inferiorly to the lower anterior abdominal wall. Ultimately evaluation of the ureterovesical junction (UVJ) will include attempts to locate a distal stone and document ureteral jets.

The ultrasound machine should be positioned so that the screen is easily visible to the operator and that the keyboard and patient can be reached comfortably. Extreme care should be taken to avoid falls caused by rolling patients from side to side on a standard examination table. Typical exam tables are narrow and have no side rails. The physician’s attention will likely be focused on the ultrasound monitor so that ancillary staff should observe unsteady patients.

Ultrasound evaluation of each renal unit is influenced by the orientation of the kidney in the retroperitoneum. Figures 5.2 and 5.3 demonstrate the anatomic position of the right and left renal unit. The lower pole of the right kidney is 15° lateral to the upper pole in the coronal plane. The renal hilum is rotated approximately 30° posterior to the horizontal coronal plane when examined transversely. The psoas muscle displaces each kidney such that the lower pole is more anterior than the upper pole. For longitudinal views, the orientation of the ultrasound probe should match the long axis of the kidney as depicted in Fig. 5.3. This will allow identification of the true midsagittal plane of the kidney.

The renal contours should be smooth and of an ovoid reniform shape. Any deviation from a smooth uniform contour should be noted and a representative image saved. All surfaces of the kidney should be scanned from the top to the bottom and from anterior to posterior.

The kidney measurements should be recorded in both transverse and longitudinal planes. The transverse view of the mid kidney should be measured and a representative image saved. These measurements can include parenchymal thickness , cortical thickness, and possibly renal width from lateral capsule to medial extent of renal sinus. An image of the midline longitudinal length should be measured, including anterior to posterior height as well as length, and saved. Care should be taken to ensure that an accurate midline longitudinal view of the kidney is obtained for measurement of the parenchymal length. It is quite easy to obtain an oblique image that appears to represent the full length of the kidney anatomically with a resultant shorter measured length. Figure 5.8 demonstrates that there may be a substantial difference in the measurement of the long axis of the kidney if the renal unit is measured in a parasagittal plane versus a true midsagittal plane. The true midsagittal plane is characterized by a medial discontinuity of the renal parenchyma representing the hiatus through which the renal vessels and collecting system enter and exit.

Normal adult kidneys measure 9–12 cm longitudinally, 5–7 cm in transverse width and 3 cm in anteroposterior dimension [7]. Han and coworkers also provided information regarding normal renal measurements in pediatric patients [8]. Although there is a normal variation in the anatomy of each renal unit in an individual patient, a difference in length of more than 1.5 cm is considered abnormal [9].

Renal cortical thickness should be uniform throughout the kidney. Any bulges or indentations in the cortex should be noted on the report and captured as an image. Parenchymal and cortical thickness should be noted. The cortical thickness is measured from renal capsule to the base of the triangular medullary pyramids. The parenchymal thickness is measured from the renal capsule to the edge of the renal sinus. In adults, the medullary pyramids are often indistinct on ultrasound imaging making the measurement of cortical thickness inaccurate. Therefore, parenchymal thickness is often easier to measure. A renal parenchymal thickness under 1 cm is considered abnormal [9]. Figure 5.9 demonstrates cortical versus parenchymal thickness measurements.

The echotexture of the kidney should be described. The normal adult renal cortex should appear hypoechoic or isoechoic relative to the normal liver or spleen. The normal cortex should appear homogeneous in echogenicity. The medullary pyramids should be hypoechoic to the renal cortex. This distinction is much more pronounced in children compared to adults.

The renal sinus is hyperechoic compared to the renal parenchyma in normal renal units. The renal sinus is highly echogenic due to its many different reflectors including blood vessels, sinus fat, and the collecting system.

Renal ultrasound by urologists is usually performed for a specific clinical indication and is focused on the kidneys. Adjacent organs will be in the field of view and any remarkable findings of those organs should be noted.

The adrenal gland in adults is not normally visualized on ultrasound. However, specific attention to the tissue anterior and medial to the upper pole of each kidney can reveal the normal adrenal gland and represents a potentially useful tool for examining adrenal lesions [10]. A prominent adrenal gland seen on ultrasound should be considered potentially abnormal and further imaging with CT or MRI can be considered.

It is not the intention of a focused urologic examination to provide complete imaging of the liver, gallbladder, spleen, or aorta. However, any abnormalities noted in these organs during the renal exam should be documented and further testing pursued as appropriate. Often the urologist has an ongoing relationship with the patient and may be able to provide valuable information regarding ancillary findings to the patient.

The importance of a detailed and accurate ultrasound report cannot be overstated. The generation of the ultrasound documentation is a critical component of all renal ultrasound examinations and is routinely audited by other entities for clinical information, appropriateness of the examination, and reimbursement. The report has four components: indications, equipment, findings, and impression.

The clinical history and reason for performing the study should be documented in the indication section.

Each ultrasound report should identify the make and model of the machine that is used for the exam. Additionally, the type of probe, frequency, and any pertinent modifications of US power, Doppler and elastography (if applicable) should be mentioned.

The findings section should include a succinct description of the normal findings as well as any notable abnormalities. Specific aspects of the report can also be reviewed in the AIUM report on urologic ultrasound guidelines [4]. Formal ultrasound terminology should be used with the findings described without an associated diagnosis. The diagnosis should be reserved for the impression section of the report. For example, one should report, “a 9 mm brightly hyperechoic focus in the lower pole of the right kidney with strong posterior acoustic shadowing” in the findings and report “consistent with the appearance of a stone” in the impression.

The impression portion of the report should include an interpretation of the sonographic findings based on clinical history and associated physical findings. When the urologist is the ordering, performing, and interpreting physician, the impression may also include a plan for subsequent imaging or intervention.

Representative images of the exam should be captured and saved in the permanent patient record. These can be digital or hard copy images. It may also be possible to capture the actual cine loop of an examination. This approach has been found to improve the subsequent review of the exam by other providers [11]. Minimum image documentation will vary based on the clinical indication for the study but for a full examination of both renal units captured images should include:

Color and power Doppler are useful tools in renal imaging. The ability to confirm normal blood flow in the kidney can be valuable in the follow-up of trauma, partial nephrectomy, and obstruction. The ability to characterize hypoechogenic structures in the kidney relative to blood flow can influence the diagnosis and future intervention. The application of Doppler mode can quickly distinguish between vessels and hydronephrosis (Fig. 5.10). The use of Doppler to demonstrate renal artery stenosis is generally beyond the scope of office urologic practice.

Resistive index is a calculated metric that represents the degree of downstream resistance to blood flow measured at a specific point in a blood vessel. In the kidney, it is calculated by quantifying the velocity of blood flow from the Doppler waveform of intrarenal arteries. The arcuate arteries running along the junction of the cortex and medulla or the interlobar arteries running between the medullary pyramids are targeted. Color or power Doppler can be used to identify a suitable vessel. The Doppler angle indicator is aligned with the long axis of the blood vessel being interrogated and the gate set at no greater than 75 % of the inner diameter of the vessel. The angle of insonation (Ø) should always be ≤60°. The resultant waveform is analyzed (Fig. 5.11). The peak diastolic and end systolic velocities are marked. The formula to calculate resistive index (RI) is peak systolic velocity (PSV) minus the end-diastolic velocity (EDV) divided by the PSV (PSV − EDV/PSV). A value of <0.7 is generally accepted as normal. Higher RIs can be seen normally in children and the elderly. Comparing side-to-side RIs, called resistive ratio, may be useful. A resistive ratio is the ratio between the RI of the affected kidney and the contralateral normal kidney. However, a difference in the RI of >0.1 on the symptomatic side compared to the asymptomatic side may be more important than a resistive ratio clinically [12–14]. Studies have shown acceptable sensitivity and specificity for RI in evaluating obstruction [5]. Ureteral obstruction causes elevated arterial resistance even preceding the development of significant hydronephrosis [15–17]. Nonobstructive causes of renal pelvis dilation tend to demonstrate normal RIs. For example, pregnant women have been demonstrated to have normal RIs despite the development of hydronephrosis of pregnancy [18–20]. These studies suggest that ultrasound calculation of RI can be useful in distinguishing physiologic hydronephrosis of pregnancy from renal colic in pregnant woman while avoiding ionizing radiation .

The RI seems to rise reliably only in complete obstruction. Studies which included patients with partial obstruction demonstrated a reduced ability to discriminate between anatomic obstruction from functional dilation [21, 22]. The resistive index is not reliable as an isolated finding to prove or disprove a clinically significant obstruction. Rather, it can be another useful finding of a point-of-service ultrasound examination to support or refute a diagnosis during a clinical patient encounter.