BODY CT PROTOCOL DESIGN

Introduction
 
Overview Advances in MDCT and MRI technology afford improvements in image quality only if protocols are optimized to utilize the hardware and software upgrades [1,2]. The purpose of this teaching module is to review the principles of protocol design in body CT and MRI, address the importance of radiation modulation in body CT, and discuss individual protocol design for a range of clinical applications, with representative cases to illustrate the role of protocol design in diagnostic efficacy.

 

Principles of MDCT Design Since the advent of 64-slice technology, isotropic or near-isotropic resolution has become the standard of care, afforded by sub-millimeter detectors. Spatial resolution is maximized by use of thin reconstruction sections and overlapping the reconstruction sections and interval. Narrow reconstruction sections carry the cost of increased noise, with the optimal balance between noise and resolution in the range of 3-5 mm for axial sections in most body CT protocols. The literature, however, supports use of thinner reconstruction sections to improve diagnostic efficacy for imaging suspected pulmonary embolism (<1 mm) [3], renal calculus [4], appendicitis [5] and endoleaks following endovascular stent repair [6].
 
Both contrast resolution and temporal resolution have been improved by the increase speed of newer generation MDCT scanners, with high pitch scanners able to scan the chest, abdomen and pelvis in under 5 seconds. For arterial phase imaging, high contrast infusion rates are required to match the contrast infusion duration to the acquisition time, which also results in imaging during peak arterial enhancement. Iodine delivery rate has been shown to correlate with arterial phase contrast enhancement level, as well as tumor visualization for hypervascular hepatocellular carcinoma [7,8]. Beyond contrast infusion rate, the iodine load is a critical parameter for certain applications, in particular hepatic imaging. Both hypervascular and hypovascular liver mass conspicuity relate to the iodine load relative to the patient's body size [9,10]. The iodine load is increased by use of a higher concentration contrast, preferable given the rapid acquisition time of current generation scanners, or a higher volume of a moderate concentration contrast [11], which is a reasonable alternative for venous phase hepatic imaging because of the 60 second delay.
 
The faster acquisition times facilitate imaging of the thoracic cardiopulmonary structures with prospective ECG gating, reducing motion artifact from the beating heart without the requirement for high radiation exposure. However, this technique cannot be performed in patients with large torso circumference. In specific applications, including coronary artery CT or suspected aortic root abnormality in a larger patient, retrospective gating is still required for optimal temporal resolution.
 
This is an overview of the basic principles of spatial, contrast and temporal resolution that dictate MDCT protocol design. Additional considerations pertain to reducing radiation exposure (both data acquisition and iterative reconstruction techniques) [12], reduced kVp techniques to improve contrast resolution [13], the role of dual energy CT in expanding diagnostic capabilities [14].

 

References 1. Prokop M. New challenges in MDCT. Eur Radiol. 2005 Dec;15 Suppl 5:E35-45.

2. Rogalla P, Kloeters C, Hein PA. CT technology overview: 64-slice and beyond.Radiol Clin North Am. 2009 Jan;47(1):1-11.

3. Jung JI, Kim KJ, Ahn MI, Kim HR, Park HJ, Jung S, Lim HW, Park SH. Detection of pulmonary embolism using 64-slice multidetector-row computed tomography: accuracy and reproducibility on different image reconstruction parameters. Acta Radiol. 2011 May 1;52(4):417-21.

4. Jin DH, Lamberton GR, Broome DR, Saaty H, Bhattacharya S, Lindler TU, Baldwin DD. Renal stone detection using unenhanced multidetector row computerized tomography--does section width matter? J Urol. 2009 Jun;181(6):2767-73.

5. Johnson PT, Horton KM, Kawamoto S, Eng J, Bean MJ, Shan SJ, Fishman EK MDCT for suspected appendicitis: effect of reconstruction section thickness on diagnostic accuracy, rate of appendiceal visualization, and reader confidence using axial images. AJR Am J Roentgenol. 2009 Apr;192(4):893-901. doi: 10.2214/AJR.08.1685.

6. Iezzi R, Cotroneo AR, Filippone A, Di Fabio F, Santoro M, Storto ML. MDCT angiography in abdominal aortic aneurysm treated with endovascular repair: diagnostic impact of slice thickness on detection of endoleaks. AJR Am J Roentgenol. 2007 Dec;189(6):1414-20.

7. Johnson PT, Fishman EK. IV contrast selection for MDCT: current thoughts and practice. AJR Am J Roentgenol. 2006 Feb;186(2):406-15. Johnson PT, Fishman EK. IV contrast selection for MDCT: current thoughts and practice. AJR Am J Roentgenol. 2006 Feb;186(2):406-15.

8. Shima W, Hammerstingl R, Catalano C, Marti-Bonmati L, Rummeny EJ, Montero FT, Dirisamer A, Westermayer B, Bellomi M, Brisbois D, Chevallier P, Dobritz M, Drouillard J, Fraioli F, Jesus Martinez M, Morassut S, Vogl TJ. Quadruple-phase MDCT of the liver in patients with suspected hepatocellular carcinoma: effect of contrast material flow rate. AJR Am J Roentgenol. 2006 Jun;186(6):1571-9.

9. Heiken JP, Brink JA, McClennan BL, Sagel SS, Crowe TM, Gaines MV. Dynamic incremental CT: effect of volume and concentration of contrast material and patient weight on hepatic enhancement. Radiology 1995;195 : 353-357

10. Yanaga Y, Awai K, Nakaura T, Namimoto T, Oda S, Funama Y, Yamashita Y. Optimal contrast dose for depiction of hypervascular hepatocellular carcinoma at dynamic CT using 64-MDCT. AJR Am J Roentgenol. 2008 Apr;190(4):1003-9. PMID: 18356448

11. Bae KT. Intravenous contrast medium administration and scan timing at CT:considerations and approaches. Radiology. 2010 Jul;256(1):32-61

12. Goldman, A.R., Maldjian, P.D. Reducing radiation dose in body CT: a practical approach to optimizing CT protocols. AJR Am J Roentgenol. 2013;200:748-754.

13. Chen CM, Chu SY, Hsu MY, Liao YL, Tsai HY. Low-tube-voltage (80 kVp) CT aortography using 320-row volume CT with adaptive iterative reconstruction: lower contrast medium and radiation dose. Eur Radiol. 2014 Feb;24(2):460-8

14. Petersilka M, Bruder H, Krauss B, Stierstorfer K, Flohr TG. Technical principles of dual source CT. Eur J Radiol. 2008 Dec;68(3):362-8. doi: 10.1016/j.ejrad.2008.08.013. Epub 2008 Oct 7.

 
BROUGHT TO YOU BY:          MY-LINH NGUYEN, MD            KRISTIN PORTER, MD, PHD          PAMELA JOHNSON, MD
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