CT scan introduction for surgical residents

CT basics

1. From CAT to CT Fundamentals of Computerized Tomography for Surgeon Rohit Sharma MD Surgical Oncology Marshfield Clinic

2. Objectives Tomography Radiograph vs CT Technology 4 generations of CT scanners CT Image Quality Pixels and Voxels Hounsfield Units Windows of View 4. Descriptive Terms 5. Planes Use of Contrast Artifacts

3. Objectives Tomography Radiography vs CT Technology CT scan generations CT image Quality Pixels and Voxels Hounsfield Units Windows of View Descriptive Terms Planes Use of Contrast Artifacts

4. Tomography I maging by sections or sectioning that uses any kind of penetrating wave. Itis derived from Ancient Greek τόμος tomos , "slice, section" and γράφω graphō , "to write" or, in this context as well, "to describe." Basic principle of tomography: superposition free tomographic cross sections S1 and S2 compared with the (not tomographic) projected image P

5. Radiography vs CT Superimposition vs. Cross- section Large beam area vs. collimated

6. Technology

7. Schematic representation of a CT scanner.

8. Advantages of Multi- Detector CT (MDCT) Speed Ability to obtain increased number of thin slices Increased spatial resolution Cover Large volumes in short scan times Useful if patient motion is a factor

9. CT image quality High contrast resolution (spatial resolution) Ability to differentiate small objects that are close together Affected by matrix size, pixel size, slice thickness Low contrast resolution Ability to tell apart 2 objects of similar density Affected by contrast scale Temporal resolution

10. Pixels and Voxels

11. Hounsfield Unit (HU) Tissue Density Hounsfield Unit Bone +400 +1000 Soft Tissue +40 +80 Water Fat -60 -100 Lung -400 -600 Air -1000

12. Windows HU Level (L) Window width (W) ( Maximum is 2000 = -1000 to +1000) HU left of window are displayed as black HU right of window are displayed as white W/L settings can be changed depending on area of interest Width/16 ( max shades of grey that human eye can differentiate) = range of HU for 1 shade of grey

13. CT window level and tissue range of Hounsfield units.

14. Descriptive Terminology White Hyperdense High attenuation Black Hypodense Decreased attenuation Location Remember the “worms eye view”

15. Planes Axial (Transverse) -First generation only axial scans- Hence CAT Coronal Sagittal Now multi-planar scans – Hence CT

16. Hypodense lesions (arrow) in a patient with hepatic metastasis from pancreatic cancer. ( B) Hyperdense lesions (arrow) in a patient with multifocal hepatocellular carcinoma .

17. Use of Contrast Intravenous Contrast Typically iodine based ( water- soluble) Ask patient about allergy, Hx of renal failure Gastrointestinal contrast Typically barium based ( not water soluble) Oral or delivered by enema

18. Recent Advances Helical CT scans Multidector CT Scan Virtual Reality Imaging and Advanced 3D CT Dual Energy CT scan

19. Helical/Spiral CT scans First/Second generation CT Scans imaged patient one slice at a time- Time consuming. Third generation CT scans- volume of tissue is scanned by moving the patient continuously through the gantry of the scanner while the x-ray tube and detectors rotate continuously for several rotations R esults in an extremely rapid acquisition of data that has no “gaps” between slices - allows for seamless reconstruction (reformatting ) of those images in almost any plane Resolution of the reconstructions is equivalent to the clarity of the original scan  Single breath hold scans entire anatomic regions

20. Multi-detector CT “Multi-Slice” can acquire up to 64 slices in a single gantry rotation Can cover about 40 mm in a single 0.4-second pass High-resolution scan of an organ such as the heart or brain can be acquired in about 5 seconds, while a whole-body scan can be performed in about 30 seconds.

21. Artifacts Volume Averaging Beam Hardening Motion Artifact Streak Artifact