Post #37: Imaging for Flank Pain & Kidney stones – as simple as 1-2-3

There are 3 time-frames for stone imaging based on the patient’s clinical history:

1. Rule out a stone when one is suspected but not yet confirmed.

2. Follow up a diagnosed stone until it passes.

3. Follow-up imaging when stone requires invasive removal.

1. Suspected Stone  

Patients often present to a medical provider with sudden onset of flank pain.  The typical presentation is one of renal colic, involving extreme, writhing pain.  . This is a much different presentation than is seen with the inflammation and tenderness of cholecystitis, diverticulitis, or appendicitis. Because a suspected stone may be located in the kidney or ureter, initial imaging should always include a view of both the abdomen and pelvis.  For most patients, guidelines from urological organizations and the American College of Radiology suggest a single diagnostic study: CT abdomen and pelvis without contrast (74160).

There are three exceptions to performing this CT scan.  

A. Exception #1: Pregnant women.  To avoid radiation exposure, an ultrasound of the retroperitoneal area (76775) is the preferred initial study for these patients.  If Ultrasound fails to show signs of a kidney stone (either visualization of a stone or signs of hydronephrosis), MRI of the abdomen and pelvis is the secondary diagnostic study.  Because of possible effects of gadolinium on the fetus, the MRI recommended should be without contrast (74181 and 72195).

B. Exception #2: children with flank pain.  Most pediatric and radiologic organizations recommend withholding CT scans on children if there are alternatives, to avoid the long term effects of radiation.  As with pregnant women, children with suspected kidney stones should initially have an ultrasound of the abdomen and pelvis or a retroperitoneal ultrasound. If ultrasound is not diagnostic, MRI abdomen and pelvis (with and without contrast, 72195 and 72197) may be requested

C. Exception #3: patients who present with flank pain and hematuria.  These are usually middle aged or older patients for whom there is concern of a malignancy.  CT abdomen and pelvis without and with contrast (74178) (also called a CT Urogram) is the preferred imaging study, and will be approved if requested.  

2. Once the diagnostic CT is performed and has identified a stone, the choice of follow-up imaging depends upon patient symptoms and the location, size, and opacity of the stone.  If the stone is large enough to be seen on a plain film and is radio-opaque, an x-ray of the abdomen and/or pelvis may be sufficient to track the stone until it passes. If the stone has caused hydronephrosis, follow-up may be adequate by performing ultrasound.  The most common follow-up imaging study, however, (except in pregnant women and children), is a repeat CT abdomen and pelvis without contrast. To avoid additional radiation exposure, some guidelines suggest a more limited CT when the stone is located at the UV (utero-vesicular) junction.  Since stones do not generally ascend into the kidney once they have passed into the distal ureter, a follow-up CT pelvis without contrast (72192) is adequate to follow-up stones at this location.

3. Although many renal stones will eventually pass into the bladder and then be excreted, occasionally a stone will remain in the kidney, ureter, or bladder even when the flank pain has subsided. Many physicians choose to observe and follow the stone but not remove it invasively.   Nevertheless, if the patient continues to have flank pain and the kidney or ureteral stone has not passed, it may be necessary to physically remove it. There are many procedures available to remove stones, including:

• Post-shock wave lithotripsy

• Post-ureteroscopic extraction requiring fragmentation of the stone

• Post-ureteroscopic extraction with an intact stone

• Post-medical expulsive therapy

Recommendations for follow-up after these procedures depends upon the procedure used, the type of stone, the success of the removal procedure, and the presence of hydronephrosis and continued symptoms.  Many stones are followed with ultrasound, but if the stone or fragments have completely passed and patients are asymptomatic, no further imaging is necessary. For additional details on how to follow post-treatment renal stones, see this link (AB-4.3 Follow-up of treated Ureteral Stone - guideline page 24)

A word on ultrasound imaging for known stones:

Ultrasound is the most commonly performed imaging study to follow known large renal stones.  If the stone is identified on a CT scan and is large enough, periodic follow-up with sonography can avoid excessive radiation from repeated CT scans.  Once the stone is detected by ultrasound, the next decision is which scan to request.

• If the initially discovered stone is located in the kidney or upper ureter, it is necessary to image the entire GU tract, either with an ultrasound of the abdomen (76705) and pelvis (76857) or an ultrasound of the retroperitoneal space (76775).  Either of these scans will visualize the kidney, ureters, and bladder.  

• If the stone is initially identified in the distal ureter, urtero-vesicular junction or bladder, an ultrasound of the pelvis (76857) is sufficient.   Since the purpose of the follow-up scan is primarily to determine whether the stone has passed, a limited retroperitoneal ultrasound (76775) may be requested instead of a complete study.  The complete ultrasound includes assessment of the abdominal aorta, iliac arteries and inferior vena cava, (all of which are unnecessary when following a kidney stone.)

Lesson #37:  Nephrolithasis imaging can be divided into three diagnostic phases: diagnosis of a suspected stone, follow-up of a known stone, and follow-up after a stone is physically removed with an invasive procedure.  For a suspected stone, CT of the abdomen and pelvis without contrast is usually the recommended imaging study with three exceptions described above.