Diagnostic Imaging CT Scan of the Renal Stone

Sunday, April 2, 2017

Although renal colic may be suspected based on the patient’s medical history and physical examination findings, diagnostic imaging is essential to confirm the size and location of urinary tract calculi or stones and assess obstruction. Several imaging modalities can be used: each has advantages and limitation as shown in the table below.

renal CT table
Imaging Modalities Used in the Diagnosis of Ureteral Calculi


renal ct table
Imaging Modalities Used in the Diagnosis of Ureteral Calculi

Plain Helical CT of Renal Stone

    Noncontrast Helical CT NCHCT was first describe in 1995 by smith et al. Since that time, NCHCT has gained widespread acceptance among radiologist, urologist, and emergency medicine physicians and has become the standard technique for evaluation of suspected renal colic. The advantage of NCHCT compared with all other technique is its diagnostic accuracy. More than 99% of stones including those that are radiolucent on plain film radiography- will be seen on NCHCT. The exceptions are the rarely occurring pure matrix and protease-inhibitor medicine stones.
    NCHCT can be rapidly performed and interpreted and does not require the administration of intravenous IV contrast material. NCHCT also provides most of the information required for the immediate management of ureteral calculi. A plain AP radiograph of the KUB is frequently used for follow-up purposes. In addition to demonstrating the size and site of the stone, measurement of stone density can also be useful. Stones of greater than 1000 Hounsfield Unit appear to respond less well to extracorporeal shock wave lithotripsy ESWL. Through a number of secondary CT signs, the presence of associated urinary tract obstruction can also be inferred from NCHCT.

Renal stone CT Scanning Procedure

    Renal stone CT scan examinations use continues data acquisition from the top of the kidneys to the base of the bladder. Collimation is typically 2.5mm to 3mm. Thin slices allow identification of small stones that may be overlooked with thicker slices.
    Renal stone CT protocols use a helical scan mode from the top of the kidneys to the base of the bladder and a slice thickness of 3mm or less. Thin slices allow identification of small stones that may be overlooked with thicker slices.

Noncontrast Helical CT on Renal Stone Radiation Dose

     The greatest drawback to the use of noncontract helical CT (NCHCT) is that delivers a relatively high radiation dose, particularly to the gonads. Heneghan et al. estimated the dose to the ovaries were 18mGy with a single detector row scanner and 23mGy with a multidetector row scanner, which is considerably higher than the dose from a standard IV urogram series 2.5mGy. This level of radiation exposure is of particular concern because many patients who have stone disease are young and have a tendency to experience repeat stone formation. Therefore, these patients have the potential of undergoing CT scan of the abdomen and pelvis many times during the course of their lives.
    Because of the aforementioned concerns, researchers have looked at ways to reduce the radiation dose while maintaining the diagnostic accuracy that sets NCHCT apart. The Heneghan concluded, In patients who weighed less than 200pounds or 90kg, unenhanced helical CT procedure performed at a reduced tube current of 100mA demonstrated a high accuracy when compared with the accuracy of the standard technique. This CT technique results in a concomitant decrease in radiation dose of 25% from multi detector row CT and 42% for single detector row CT. This technique has been incorporated into our routine protocol for detection of stones and, in our opinion, promises particular benefit to young patients who experience repeat stone formation.

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