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Everything you need to know about Computed Tomography (CT) & CT Scanning


Kidney: Multidetector Computed Tomography Evaluation Of The Renal Artery

Harpreet K. Pannu, M.D. and Elliot K. Fishman, M.D.


Introduction

Multidetector helical computed tomography (MDCT) scanners with four rows of detectors represent the latest in CT technology. There are several advantages to using multidetector scanners for evaluating the kidney:


- narrow collimation can be used for better z axis resolution
- large volumes can be scanned without decreasing signal to noise ratio
- acquisition times are shorter due to a higher pitch (6-8 vs 2) allowing for better separation of arterial and venous phases
- thin and thick slices can be retrospectively reconstructed
- artifacts in three-dimensional rendering are decreased
- tube heating is decreased

Indications for renal CT arteriography include renal artery stenosis in native and transplanted kidneys, abdominal aortic aneurysm, aortic dissection and arteritis. Characterization of the vascular anatomy in potential renal donors and patients for partial nephrectomy are other indications.


Advantages of using MDCT to evaluate the renal artery

Volume of coverage in a single breath hold.

- Unilateral multiple renal arteries occur in 32% and bilateral multiple renal arteries occur in 12% of individuals.
- Accessory renal arteries can arise from the infrarenal aorta, lower thoracic aorta or iliac arteries.
- The scan distance to cover the main and potential accessory renal arteries is usually between 20 to 30 cm.
- With single detector scanners, 3 mm collimation and pitch of 2, 24 cm can be covered in approximately 40 seconds.
- With a multidetector scanner, 1 mm slice collimation and table feed of 6 mm/rotation, 20 cm can be covered in approximately 16 seconds.
- The combination of thin sections and large volume will likely help identify accessory vessels that were occasionally missed with single detector scanners.

Thin slices to evaluate patency.

- The proximal renal arteries are parallel or oblique to the axial plane.
- Single detector datasets are anisotropic because they are longer perpendicular to the axial plane than in the axial plane. Therefore, they are less accurate for vessels that are parallel to the axial plane.
- Isotropic datasets can be generated with multidetector scanners for superior z axis resolution to evaluate for stenosis.
- In an in vitro study, 2 mm collimation, pitch 2 and 1 mm overlapping reconstruction intervals were suggested as ideal parameters for a single detector scanner for accurately diagnosing and grading renal artery stenosis. 3 mm slices are usually done in clinical practice to cover the required volume. However, the error of the measured vessel diameter has been shown to be greater for 3 mm collimation compared with 1 mm. Another limitation of single detector scanners is broadening of the effective slice thickness as pitch is increased.
- With multidetector scanners, 1 mm thick slices can be routinely obtained as there is less of a compromise between slice collimation, scan time and volume of coverage.
- An adequate mA can also be generated to get adequate signal to noise ratio as tube heating is less of an issue.
- The ability to obtain thin sections will likely be helpful for evaluating disease intrinsic to the renal artery and the integrity of the renal arteries in aortic disease.


CT Protocol

Potential renal donor and renal artery stenosis evaluation

- Helical scan from above the kidneys through the bifurcation of the common iliac arteries
- 750 cc water orally and 120 cc of nonionic contrast IV at 3 cc/second
- Arterial phase delay of 25 seconds and venous phase delay of 50 seconds
- kV/mAs/gantry rotation time in seconds - 120/130/0.5
- Detector collimation 4X1 mm (Siemens)
- Slice collimation 1.25 mm
- Table feed 6 mm/rotation
- Reconstructed slice width 1 mm for arterial phase and 1 mm for the venous phase
- Volume rendering for 3D reconstruction

Abdominal aortic aneurysm

- Helical scan from the diaphragm to the symphysis pubis
- 750 cc water orally and 120 cc of nonionic contrast IV at 3 cc/second
- Scan delay of 25 seconds
- kV/mAs/gantry rotation time in seconds - 120/165/0.5
- Detector collimation 4X1 mm or 4X2.5 mm (Siemens)
- Slice collimation 1.25 mm or 3 mm
- Table feed 6 mm/rotation or 12.5 mm/rotation
- Reconstructed slice width 1 mm or 2 mm
- Volume rendering for 3D reconstruction


Summary

- Renal CT arteriography has diverse clinical indications such as establishing normal anatomy, evaluating for disease and preoperative planning for partial nephrectomy.
- With the four rows of detectors in multidetector CT (MDCT), z axis coverage and resolution are greater than with single detector scanners.
- This will likely be helpful for detecting multiple renal arteries, greater vessel opacification due to faster scanning, and diagnosing and grading renal artery stenosis due to thinner slice width during acquisition and reconstruction.

© 1999-2019 Elliot K. Fishman, MD, FACR. All rights reserved.