(1)
Department of Radiology, Stony Brook Medicine, Stony Brook, NY, USA
Imaging Summary Facts
Non-contrast CT is the most rapid and accurate technique for evaluating kidney stones.
In pregnant patients with flank pain, renal ultrasound (renal US) is the best initial study.
An X-ray of the Kidneys, Ureters and Bladder (KUB) is a follow-up method to see if the stone has passed or advanced. A KUB can be used in combination with an US to determine if the kidney stones are radiopaque vs. radiolucent.
MAG-3 Renal Scan can be used to evaluate real function.
MRI has a limited role in the evaluation of kidney stones and in general is not used for this purpose.
Introduction
Urolithiasis is the condition where kidney stones are formed anywhere along the course of the urinary tract. Diagnostic imaging is used in patients with symptoms suggestive of renal stones to confirm that Urolithiasis is the source of the patient’s pain and to identify the location, size and possible complications of the kidney stones. It will also dictate the most appropriate treatment to follow [1].
There are different ways to work up kidney stones; some of these methods involve the use of x-rays such as CT scan, KUB (an X-Ray of the Kidneys, Ureters and Bladder), and Intravenous Pyelogram (IVP) [2]; others use sound waves, such as Ultrasonography [3]; MRI is another method, which uses a big magnet in order measure the water density in tissues and produce an image.
After a stone is found, depending on it’s specific characteristics, it may be treated medically with Shock Wave Lithotripsy (SWL), that uses shock waves to break a kidney stone into small pieces that can more easily travel through the urinary tract and pass from the body, or removed by Percutaneous Nephrolithotomy (PCNL), where the surgeon makes a small incision in your back to remove kidney stones. He or she then puts a hollow tube into your kidney and a probe through the tube.
Initially patients are followed with KUB and ultrasound. The KUB can detect larger stones but clinically significant fragments may not be detected and a non-contrast CT will find them [4]. If the physician is worried about recurrent or residual stones as stated earlier a non- contrast CT followed by a second set of images obtained after intravenous dye (contrast) may be performed to determine the best treatment.
The diagnosis and initial management of Urolithiasis have undergone considerable evolution in recent years. Technological advancements have greatly facilitated the diagnosis of stone disease. The purpose of this chapter is to summarize the different types of imaging available to diagnose stone disease, including the advantages and disadvantages of each of them.
Computed Tomography
Computed tomography (CT scan), is a technology that combines a series of X-ray views taken from many different angles and computer processing to create cross-sectional images of the bones and soft tissues inside your body. The images can be viewed as a horizontal cut (i.e., axial); from front to back (i.e., coronal); or from the side (i.e., sagittal) view (see Fig. 16.1). The resulting images appear as virtual slices that can be compared to looking down at single slices of bread from a loaf.
Fig. 16.1
Multiple non-contrast renal protocol CT scan images of a patient with a ureteral stone in the proximal (upper) third of the left ureter (yellow arrows) in the (a) axial, (b) coronal and (c) sagittal views
Non-contrast CT is the preferred initial study in patients with flank pain with a clinical suspicion of a ureteral calculus. Other indications include a Patient with ureteral calculus who is successfully treated with medical expulsion therapy and continues to have hydronephrosis (swollen kidney) by ultrasound. A third indication is for patient’s who have ultrasound evidence of hydronephrosis and who have recently passed a kidney stone [5].
A CT with IV dye (contrast) is performed after a stone has been removed surgically and the patient still has pain. It is also performed in patients who have an abnormal ultrasound (hydronephrosis) at the follow up after stone removal, even if the patient does not have pain [5]. The contrast dye CT is only performed if the patient’s overall kidney function is near normal and if the patient is not allergic to the dye itself.
The advantage of non contrast stone protocol CT is that it is very accurate [6] and it does not take long to perform or interpret. The patient will receive the appropriate treatment in a timely manner, ultimately reducing cost [2].
The disadvantage of non-contrast CT scan is the radiation exposure that although relatively low, can accumulate in patients who are recurrent stone formers. A low dose CT in an adult has an effective dose of approximately 4 mSv [5] and a standard non contrast CT has an effective dose of 10–30 mSv [7]. It is not ideal to use this method in young children and pregnant woman because there is ionizing radiation, which is a concern [1, 5]. CT without contrast will detect the most common types of stones, although kidney stones are composed of different substances and the X-ray does not detect a minority of these.
Renal Ultrasound
Renal ultrasound – Renal US, is an oscillating sound pressure wave produced by a probe with a frequency greater than the upper limit of the human hearing range. A transducer collects the sounds that bounce back and a computer generates images.
Renal ultrasonography is the preferred initial imaging modality for children with suspected renal stones, pregnant women and recurrent stone formers.
The advantage of ultrasound is that it does not use ionizing radiation, which is ideal for imaging pregnant women, children and recurrent stone formers. The sound waves detect kidney stones and indirect findings such as a swollen kidney, called hydronephrosis (Fig. 16.2), that will let the doctor know if there is obstruction or not.
Fig. 16.2
Longitudinal gray scale (renal ultrasound) image of the left kidney shows hydronephrosis (yellow arrow) within the kidney (+)
The ultrasound can let us know if a stone is obstructing the flow of urine even if the stone is not seen, by detecting the urine flow within the urinary bladder, referred to as ureteral jets (Fig. 16.3). For example, if there is strong flow from the right kidney but only minimal flow from the left kidney, there is likely a stone on the left [3].
Fig. 16.3
Urinary bladder ultrasound image demonstrating left UVJ stone (yellow arrow) in the (a) transverse and (b) longitudinal view. (c) Ureteral jet (urine flow) demonstrated in red from the right ureter (yellow arrow) and the absence of a jet from the left ureter (white arrow)
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