Maximum intensity projection (MIP) Maximum intensity projection (MIP) images of multidetector CT (MDCT) images of multidetector CT (MDCT) enhance the visualization of urinary enhance the visualization of urinary

calculi in the presence of contrast media calculi in the presence of contrast media or ureteric stents.or ureteric stents.

Hazem A. Youssef*, Mohammad K. Omar*, Hossam A. Hazem A. Youssef*, Mohammad K. Omar*, Hossam A. Youssef*, and Hassan A. Abolella**Youssef*, and Hassan A. Abolella**

*Radiology Department, Faculty of Medicine, Assiut University.*Radiology Department, Faculty of Medicine, Assiut University.**Urology Department, Faculty of Medicine, Assiut University. **Urology Department, Faculty of Medicine, Assiut University.

With the introduction of multidetector technology, CT urography, to date, has emerged as the initial heir apparent to intravenous urography; many years of experience have now clearly demonstrated that CT is the test of choice for many urologic problems, including urolithiasis, renal masses, urinary tract infection, trauma, and obstructive uropathy.

INTRODUCTIONINTRODUCTION

CT urography provides a detailed anatomic depiction of each of the major portions of the urinary tract—the kidneys, intrarenal collecting systems, ureters, and bladder—and thus allows patients with hematuria to be evaluated comprehensively ©RSNA, 2009.

Because it provides fast results, unenhanced MDCT has largely supplanted intravenous urography, and, in the United States, MSCT generally prevails as the standard examination for detecting ureteral calculi (Wehrschuetz et. al., 2008).

The objective of the current study is highlighting the rule of MIP images among the various image processing techniques in the special situation when ureteric stents are implemented or when the urinary tract was unintentionally opacified by IV contrast.

OBJECTIVEOBJECTIVE

Study PopulationA total of 23 consecutive patients (18 males and 5 females, median age 41.2 years, range 2-65) with suspected urinary calculi were included in this retropective pilot study from April through September 2009. Twelve patients were referred for detection of urinary calculi after inconclusive intravenous uroengraphy (IVU) and 11 patients were referred after placement of ureteric stents. In the former group IV contrast was administered 1-24 hours prior to CT examination.

PATIENTS AND METHODSPATIENTS AND METHODS

Imaging protocolUnenhanced MDCT images were acquired with a 64 detector row CT scanner VCT GE, Milwaukee, WI (18 patients) or a 16 detector row CT scanner Brightspeed S GE, Milwaukee, WI (5 patients).Five-millimeter contiguous unenhanced axial CT images were obtained in a cephalocaudal direction from the diaphragm to the symphysis pubis. No oral or IV contrast was administered.

Scanning protocolFor the 64-slice scanner, scans were obtained with tube rotation time 0.8sec, pitch 0.984:1, table feed/gantry rotation 39.75mm, a tube voltage of 140 kVp and effective tube current-time product of 548mAs.For the 16-slice scanner, scans were obtained with tube rotation time 0.8sec, pitch 1.375:1, table feed/gantry rotation 27.5mm, a tube voltage of 120 kVp and effective tube current-time product of 248mAs.The images were then reconstructed at 0.625 mm thick slices.

Image processingAll datasets were sent to a commercially available

workstation (ADW4.4 or ADW4.3). In all patients 5mm MPR images were obtained in addition to the following:

1) Thick slab MIP.2) Volume rendering3) Curved reformats.

Image evaluationTwo independent radiologists and a urologist analyzed

the images. The observers were asked to separately determine whether pyelocalyceal and ureteral calculi were present. They were instructed to document the location and size of each calculus using standard measurement devices.

Urinary calculi were detected in 10 patients referred after inconclusive IVU and in 8 patients with ureteric stents. In 2 patients with inconclusive IVU ureteric stricture was the alternative diagnosis.

RESULTSRESULTS

No. of stones/ patientNo. of stones/ patient No. of patientsNo. of patients

00 55

11 22

22 33

33 33

44 11

55 33

66 11

77 22

7-157-15 22

>15>15 11

TotalTotal 2323

Distinction between the urinary calculi and/or contrast and ureteric stents was not possible in the standard window for viewing the abdomen. In the VR images calculi can be demonstrated against ureteric stent by their morphology despite of having the same density but they could not be demonstrated within the opacified collecting system. All the stones were clearly depicted against the contrasted lumina (by having higher density) or the ureteric stents (by having lower density) in thick slab MIP at bone window setting.

Bone windowing also facilitates visualization of stones against the pelvic bones or the spine. The major weakness of MIP images was the inability to measure the size of the stones. VR or MPR were then reviewed for that context.

REPRESENTATIVE CASESREPRESENTATIVE CASES

The use of maximum-intensity projection (MIP) and images in CT urography was initially described by McNicholas et al (1998). To our knowledge it was applied in the to date puplished literatures describing various techniques of CT urography. Since the majority of these studies were dedicated to detection of urothelial abnormalities they reported lower sensitivity of MIP images than thin section MPR images (Caoili et al. 2005), (Chow and Sommer 2001) in detection of similar lesions.

DISCUSSIONDISCUSSION

In our study, however, higher sensitivity of MIP images was evident. This is due to higher density of the stones compared with the low density uroepithelial lesions that may be masked by the contrast concentrated within the collecting system. Moreover the low contrast density in the collecting system caused by poor renal contrast concentration helped detection of the calculi.

The use of bone window for distinguishing stones from stent was reported by Tarnikut et al. (2004). In a series of a 3 patients they used bone window to distinguish stones from nephrostomy tubes in axial CT images after measuring the in vitro density values for stents.

The higher sensitivity of MIP images compared with that of VR images is related to the rendering technique as stated by Fishman et al (2006). VR images are formed by binary technique; voxels in volume are considered as either containing the specified densiInaccurate size measurement of the recorded abnormality at MIP images is explained by lack of depth (or Z-axis perception) in the 2D image that represent the MIP slab.

The CTU has become the radiologists most robust imaging tool for the evaluation of the kidneys, upper urinary tracts, and UB.Complete imaging of the kidneys and urinary tracts can be performed with MDCT. Studies are tailored to the clinical question and may be performed as noncontrast, combined non-contrast and post-contrast or post-contrast imaging studies only.

CONCLUSION AND RECOMMENDATIONSCONCLUSION AND RECOMMENDATIONS

Finally radiologists need to educate emergency room and referring physicians about the limitations of unenhanced CT scans, as it does not detect infarcts, pyelonephritis, small renal cell carcinomas, or small ureteral tumors.

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