Urinary tract calculus could be categorized by their anatomical locations such as the kidney, ureter, urinary bladder, and urethral calculus. Renal calyceal calculus shows high echogenic foci in renal calyx with acoustic shadowing on renal ultrasound. On plain radiographs of the kidney, ureter, and bladder (KUB), urinary tract calculus could be visualized as radiopaque calculus. On the other hand, urinary tract calculus not visible on KUB is called radiolucent calculus [1]. On excretory urography, radiolucent renal calyceal calculus could sometimes appear as a filling defect by its lower density than that of excreted contrast medium. On unenhanced computed tomography (CT), nearly all urinary tract calculi appear as high density foci regardless of its radiolucent or radiopaque appearances on KUB [1]. Nonetheless, when there is contrast opacification on the renal calyces, the detection of renal calyceal calculus becomes much more difficult. The use of a wide window setting could sometimes help in differentiating calculus from opacified contrast medium. However, if the density of the excretory contrast medium is identical to that of urinary calculus, they could not be distinguished from each other even in high window setting. Therefore, in detecting urinary tract calculus, such as renal calculus, unenhanced CT is most useful for its diagnosis.

Urinary calculus at the ureteropelvic junction (UPJ) could be a renal pelvis calculus extending to the UPJ or a discrete calculus at the UPJ area. Because the UPJ is an anatomical narrowed point, a UPJ calculus may be associated with variable degrees of urinary tract obstruction and thus resultant proximal hydronephrosis. Similar to urinary tract calculus of other locations, UPJ calculus is detectable on plain radiograph of the kidney, ureter, and bladder (KUB) if it is dense enough as the so-called “radiopaque” calculus. On excretory urography (EU), a UPJ calculus could be visualized as a radiopaque calculus before contrast medium administration or as a filling defect in UPJ after contrast medium excretion. However, in the absence of adequate opacification of the corresponding renal pelvis and UPJ, EU could fail to illustrate radiolucent UPJ calculus. Computed tomography (CT) could not only demonstrate the UPJ calculus itself but also the presence or absence of hydronephrosis regardless of excretory function of the corresponding kidney. Although magnetic resonance imaging (MRI) is much more expensive than CT, MRI has lower sensitivity than unenhanced CT for detecting urinary calculus. On MRI, urinary calculus usually appears as low signal intensity on both T1-weighted image (T1WI) and T2-weighted image (T2WI). Therefore, a UPJ calculus could be unrecognizable on T1WI because urine in the UPJ and renal pelvis shows identical low signal intensity as that of a calculus, which does not allow their differentiation. In contrast, urine in the UPJ and renal pelvis upon T2WI shows high signal intensity which highlights the presence of a low signal intensity lesion by a calculus. Nonetheless, the presence of low signal intensity lesion on MRI is not specific for diagnosis of urinary tract calculus because hematoma in the urinary tract could also appear as low signal intensity lesions on MRI. Generally speaking, unenhanced computed tomography is more sensitive and specific than other imaging modalities for diagnosing urinary tract calculus including UPJ calculus.

A renal staghorn stone means a renal calculus involving not only the renal pelvis but also one or more calyces at the same time. A staghorn stone is used to describe the picture of the stone simulating horns of a stag. If a renal staghorn stone involves the whole renal collecting system, complete renal staghorn stone is considered and incomplete staghorn stone if otherwise. Nearly all of renal staghorn stones are radiopaque on plain radiography of the kidney, ureter, and bladder (KUB). Therefore, KUB could readily detect and diagnose renal staghorn stones without difficulty. In spite of the large sizes of renal staghorn stones, obstruction and associated functional impairment of the corresponding kidneys are only shown in some of them. Excretory urography (EU), renal ultrasound (US), and computed tomography (CT) are useful in detecting hydronephrosis or hydrocalicosis due to obstructions by renal staghorn stones. Delayed or faint opacification of the ipsilateral renal collecting system on EU or enhanced CT of excretory phase suggests the presence of ipsilateral renal functional impairment. Furthermore, xanthogranulomatous pyelonephritis should be listed in the differential diagnosis if there are imaging findings of ipsilateral renal enlargement, hypodense areas surrounding the stone, and absence of excretion function of the ipsilateral kidney associated with a renal staghorn stone.

Ureteral calculus is best diagnosed by unenhanced helical computed tomography (CT) by high sensitivity, specificity, and accuracy shown in numerous studies of the literature. On unenhanced CT, ureteral calculus is diagnosed by direct and indirect signs [2, 3]. Direct sign on unenhanced CT means the recognition of a calculus with confirmation of its anatomical location being in the ureter [3]. Since nearly all ureteral calculus displays high density foci on unenhanced CT, there is no difficulty to recognize the calculus itself. Nonetheless, there are other alternative conditions (e.g., phlebolith, calcified lymph node, arterial calcification) which show similar calcification foci in the abdomen and pelvic cavity on unenhanced CT. Thus, it is necessary to confirm a calcified foci being within the ureteral lumen for diagnosis of a ureteral calculus. Tracing down the ureteral course from the ureteropelvic junction to the ureterovesical junction (UVJ) on unenhanced CT whenever possible is a must for clarifying the anatomical location of a ureteral calculus. On the other hand, the presence of associated indirect signs of a ureteral calculus on unenhanced CT adds confidence in making a diagnosis and provides information on the altered anatomical and functional status secondary to ureteral calculi [2]. Indirect signs of ureteral calculi on unenhanced helical CT include hydronephrosis, hydroureter, perinephric or periureteral stranding or fluid, tissue rim sign, ipsilateral nephromegaly, and ipsilateral decreased renal density [4]. Tissue rim sign is typically a soft tissue rim surrounding the whole calculus caused by ureteral wall edema, which is highly specific for diagnosing ureteral calculus. Patients with ureteral calculus usually present with acute flank pain, especially in emergency settings. Unenhanced helical CT provides alternative diagnoses other than ureteral calculus for patients presenting with acute flank pain in addition to its high accuracy of diagnosing ureteral calculus, which is thus a highly valuable tool for clinical decision making [2]. The detectability of ureteral calculus on plain radiographs of the kidney, ureter, and bladder (KUB) is affected by the size, location, and composition of the calculus itself as well as by the overlying anatomical structures and body habitus of the patient. Ureteral calculus located in the middle ureter has lower detectability than other locations on KUB because the overlying sacrum has similar density to that of a ureteral calculus [5]. Uric acid stones are usually radiolucent and thus undetectable on KUB. Renal US is usually used for detecting the presence or absence of hydronephrosis in a patient suspected of having a ureteral calculus instead of identifying the stone itself. Nonetheless, if hydronephrosis is shown on renal US in a patient showing a radiopaque calculus of ipsilateral ureter on KUB, a confidence diagnosis of a ureteral calculus with resultant hydronephrosis could be made by their association.

Ureterovesical calculus is diagnosed by the depiction of a calculus on the ureterovesical junction (UVJ). It is not uncommon for a UVJ calculus to be associated with urotract obstruction. Computed tomography (CT) is usually the best imaging modality for its location and depiction, especially before contrast medium opacification of the urotract.

Urinary bladder calculus has a variety of shapes, appearances, and densities. Bladder calculus may appear as laminated and oval in shape, has a round contour, and resembles a child’s toy jacks (so-called jackstone) [6]. Similar to urinary tract calculi at other anatomical locations, urinary bladder calculus may appear as dense radiopaque, faint radiopaque, or radiolucent (i.e., undetectable) stone on plain radiography of the kidney, ureter, and bladder (KUB). On unenhanced computed tomography, urinary bladder calculus appears as a high density focus which makes it readily detectable. On magnetic resonance imaging, urinary bladder calculus appears as a low signal intensity lesion on both T1-weighted and T2-weighted images.

Urethral calculus is more common in men than in women. It is because the male urethra has an anatomical narrowing point at the membranous urethra and is longer than the female urethra. On plain radiograph of the kidney, ureter, and bladder, urethral calculus is highly suggested if a dense focus is shown between the symphysis pubis. On excretory urography, stasis of contrast medium surrounding the calculus after voiding could further support the location of the calculus being in the urethra. On computed tomography, urethral calculus is diagnosed by the depiction of a calcified focus in the urethra with possible associated obstruction findings.

The urinary bladder base is normally above the symphysis pubis. Cystocele means abnormally low position of the urinary bladder below the inferior margin of the symphysis pubis. The underlying cause of cystocele is pelvic floor laxity. Therefore, cystocele could occur with prolapse of other pelvic organs including uterine prolapse, rectocele, or enterocele. Cystocele is therefore diagnosed on images by illustrating the urinary bladder base below the symphysis pubis. Nonetheless, the positioning of bladder base changes in different status. The bladder base descends in straining status as compared to that in resting status. Thus, mild cystoceles are only diagnosed in straining status and upright positioning. Dynamic magnetic resonance imaging (MRI) obtained at resting and straining status has recently become a popular imaging tool to assess pelvic floor dysfunction [7]. Hydronephrosis is associated with cystoceles due to bladder outlet obstruction or ureteral stretching effect.

Bladder herniation to the inguinal canal is an uncommon entity [8]. In severe bladder herniation to the inguinal canal, the urinary bladder shows a dumbbell appearance. The diagnosis of bladder herniation into the inguinal canal or even scrotum on images is based on the identification of the herniated part being contiguous to the main part of the urinary bladder regardless of contrast medium opacification or not. On the other hand, the opacification of the herniated part of the urinary bladder on excretory phase of computed tomography, magnetic resonance imaging, or on excretory urography and cystography helps in confirmation of the presence of bladder herniation. Patients with severe bladder herniation could suffer from incomplete emptying of the urinary bladder after voiding, due to the partial obstructive effect through the inguinal canal. Mild bladder herniation is usually detected as an incidental findings on images because it causes no symptom. Furthermore, some infants less than 1 year old have temporary bladder herniation to the inguinal canal due to incompetent inguinal ring, which is under development, usually called “bladder ear” rather than “bladder herniation.” If the child has persistent bladder herniation after 1 year of age, then existence of true bladder herniation is considered.