Radiation

The biggest potential problem with CTU performed using excretory-phase axial image acquisition is the incremental radiation that patients receive with multiphase CT. The amount of radiation to which patients are exposed is directly related to the mA and kVp, as well as to the number of phases acquired. Radiation dose can be estimated in a variety of ways. The amount of radiation emitted from the scanner during image acquisition that is absorbed by the patient can be calculated [previously in rad and more recently in grays (with 100 rad equaling 1 Gy)]. It is more accurate, however, to use the equivalent or effective patient radiation dose, which takes into account the toxicity of any absorbed dose of radiation. The equivalent effective dose was previously measured in rem, but is now measured in Sieverts [with 1 rad of absorbed radiation equaling 1 rem, and 1 Gy of absorbed radiation equaling 1 Sv)].

Herts estimated that CT exposes a patient to an estimated surface radiation dose of 2 rem [or 20 millisievert (mSv)] for each series of acquired images (21). According to this data, the absorbed surface radiation dose from standard

Figure 18 Ureteral kink. (A) Excretory-phase axial image demonstrates a tiny filling defect along the anteromedial aspect of the mid-right ureter (arrow). The left ureter also has an irregular contour. This is indicative of a small transitional cell carcinoma. (B) The coronal, excretory-phase, volume-rendered image reveals that the area of the filling defect (arrow), which was seen on this and other images, was merely the result of partial volume averaging in the region of a ureteral kink. Several kinks are also identified in the left ureter.

Figure 18 Ureteral kink. (A) Excretory-phase axial image demonstrates a tiny filling defect along the anteromedial aspect of the mid-right ureter (arrow). The left ureter also has an irregular contour. This is indicative of a small transitional cell carcinoma. (B) The coronal, excretory-phase, volume-rendered image reveals that the area of the filling defect (arrow), which was seen on this and other images, was merely the result of partial volume averaging in the region of a ureteral kink. Several kinks are also identified in the left ureter.

three-phase CTU protocols would be 6 rem (or 60 mSv). This is fairly similar to the radiation exposure these authors approximated for their 10 to 14 film excretory urograms of 5 to 7 rem (50-70 mSv). In another report, McTavish et al. estimated skin and total absorbed radiation doses resulting from their three-phase MDCTU

Figure 19 Prominent papillae. Excretory-phase axial image shows a prominent, concave impression on a left upper pole calyx due to a normal renal papilla (arrow). Note that a similar impression is also present on the more posteriorly located calyx. Nearly all of the other calices in both kidneys (seen on other axial images) showed the same appearance. The multiplicity of this finding and similarity of appearance across many calices indicate that this does not represent a true filling defect.

Figure 19 Prominent papillae. Excretory-phase axial image shows a prominent, concave impression on a left upper pole calyx due to a normal renal papilla (arrow). Note that a similar impression is also present on the more posteriorly located calyx. Nearly all of the other calices in both kidneys (seen on other axial images) showed the same appearance. The multiplicity of this finding and similarity of appearance across many calices indicate that this does not represent a true filling defect.

protocol to be 74.1 and 22.6 milligray (mGy) (13), as opposed to calculated doses of 81.2 and 11.4 mGy for EU. More recently, Nawful et al. estimated that the mean patient skin dose of their three-phase MDCTU protocol when calculated from phantom data and when measured with thermoluminescent dosimeter strips was 55 and 56 mGy, respectively (28). In this study, the mean effective dose for MDCTU was estimated to be 14.8 mSv, compared with 9.7 mSv for EU. Thus, the total effective dose resulting from three-phase MDCTU was estimated to exceed that of EU by a factor of at least 1.5 (28). Not surprisingly, Caoili et al. (12) calculated the total effective radiation dose for their four-phase protocol to be higher than that observed by McTavish et al. (13) and Nawful et al. (28). These authors estimated that an average-sized male studied using four-phase MDCTU (including two different excretory-phase image acquisitions, rather than one) received an effective total radiation dose of 25 to 35 mSv (12). This greatly exceeded the 5 to 10 mSv effective total dose for the 10 to 12 film EU that was routinely performed at the same institution.

Given the increased dose of three- or four-phase MDCTU (as assessed by the investigators referenced above), as previously mentioned, we and some others (17) have been reluctant to expand CTU indications to include all patients presenting with microscopic or gross hematuria. Instead, we have thus far reserved MDCTU for a selected group of patients in whom a high risk of urinary tract malignancy is believed to exist. This generally includes elderly patients with previously known urinary tract neoplasms, positive urine cytology, or persistent gross hematuria. Only rarely have we agreed to perform MDCTU in younger patients (usually those with intractable symptoms). As previously discussed, our restrictions can be contrasted with the policies of others (10,13,14), who, even now, advocate using MDCTU in any patient presenting with hematuria.

It must be remembered, however, that prior to the emergence of CTU, many patients with persistent, unexplained hematuria would have undergone imaging with

EU first, followed by CT if the EU did not identify any etiology. Thus, for many patients it is more appropriate to compare MDCTU radiation dose with that of EU and standard abdominal and pelvic CT combined. In such instances, the doses of these two imaging approaches are nearly comparable. Even when the data obtained in Caoili et al.'s series (utilizing four-phase MDCTU) are utilized (12), MDCTU exposed patients to only about 1.5 times as much radiation as EU and standard single-phase CT combined.

It must also be remembered that the probable carcinogenic risks of increased radiation from CTU must be balanced against the risks of not performing CTU and potentially missing malignant urinary tract pathology at an early stage when such pathology is more likely to be effectively treated.

There is one increasingly popular technical modification that allows for radiation dose from CTU to be reduced: the previously mentioned split-bolus technique. This technique, based upon a concept described by Chow and Sommer (10) but now utilized by many others (18,29), involves administering an initial intravenous bolus of contrast material. After a delay (allowing for excretion of the initial bolus into the renal collecting systems and ureters), additional contrast material is injected. Finally, a single series of thin-section contrast-enhanced scans is obtained after a further delay, allowing for the second bolus of contrast material to have enhanced the renal parenchyma homogeneously, while the first bolus has already been excreted into the renal collecting systems. In this fashion, nephrographic and excretory-phase images can be acquired simultaneously. As described in section "Recommended CTU Technique'', MDCTU can, therefore, be performed using only two series of images (one precontrast and one postcontrast).

Several authors have reported good success with the split-bolus technique (10,18), although there are a few potential drawbacks: use of a smaller volume of contrast material to opacify and distend the renal collecting systems and proximal ureters (the initial bolus), as well as a smaller volume of contrast material to enhance the renal parenchyma and remainder of the abdominal visceral organs (the second bolus). Additionally, it has been observed that, on occasion, excreted contrast material in the renal collecting systems and renal pelvis can create artifacts, which may interfere with the evaluation of the renal parenchyma (30). Although, in our experience, such artifact is rarely severe enough to interfere with one's ability to detect a renal mass, it can limit the accuracy of any subsequent measurements of regions of interest that are obtained of a detected lesion.

Although using the split-bolus technique eliminates one of the usual minimum of three CT series acquisitions, the savings in radiation is not as great as might initially be expected. At our institution, for example, employing the split-bolus approach allows us to avoid the second, nephrographic-phase only, acquisition, which is the least radiation-intense component of our MDCTU examination. This is because our nephrographic phase scans are performed only as far caudal as the lower poles of the kidneys (rather than to the symphysis pubis). They also utilize relatively thick sections (5 mm rather than 0.625 or 1.25 mm), permitting scan acquisition to be obtained with lower mAs than that used for the excretory-phase series. For these reasons, we estimate that using the split-bolus technique reduces MDCTU radiation to the point that the examination would expose patients to only about 1.2 times (rather than 1.5 times) that to which the patient is exposed for a 10 to 15 film excretory urogram.

It is also possible to reduce some of the technical parameters while performing CTU image acquisition (irrespective of whether two or three total series are obtained). mA can be reduced considerably on the initial precontrast series without interfering substantially with the ability to detect stones. A number of studies have demonstrated that radiation reduction can be accomplished during renal stone CT, by lowering mA settings, without sacrificing diagnostic accuracy (31,32).

Finally, additional CT modifications (dose modulation) may allow for further dose reductions (by allowing for reductions in mA and kV depending upon which body part is being imaged and which organs are closest to the X-ray beam as it enters the patient). Work in this area is still very preliminary.

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