Computed Tomography

Computed Tomography • CT Basics • Principle of Spiral CT • Scan Parameter & Image Quality • Optimizing Injection Protocols • Clinical Applications

X-Ray Discovery • X-ray was discovered by a German scientist Roentgen 100 years ago. • This made people for the first time be able to • view the anatomy structure of human body without operation • But it's superimposed • And we couldn't view soft tissue

History of Computed Tomography 1963 - Alan Cormack developed a mathematical method of reconstructing images from x-ray projections Early 1970s • My name is Godfrey Hounsfield • I work for the Central Research Labs. of EMI, Ltd in England • I developed the the first clinically useful CT scanner in 1971

CT Broke the Barrier For the first time we could view: - Tomographic or “Slice” anatomy- Density difference • But it's time consuming • And resolution needs to be improved

Concept of X-ray Attenuation SCATTERED X-RAYS An X-ray beam passing through the body is attenuated (loses its energy) by : • Absorption • Scattering Incident X-ray Transmitted ray BODY TISSUE Absorption by the tissue is proportional to the density More dense tissue Less dense tissue MORE ATTENUATION LESS ATTENUATION

X-ray generation Data acquisition Recon. & postpro. How does CT Work?

How does CT Work? X-ray goes through collimator therefore penetrate only an axial layer of the object, called "slice"

How does CT Work? • Patient is placed in the center of the measurement field • X-ray is passed through the patient’s slice from many direction along a 360° path • The transmitted beams are captured by the detectors which digitizes these signals • These digitized signals called raw data are sent to a computer which create the CT image

How is CT Image generated? The object slice is divided into small volume elements called voxels. Each voxel is assigned a value which is dependent on the average amount of attenuation

How is CT Image generated? The attenuation values are transferred to the computer where they are coded & used to create a slice image

CT Generations & Design “Generation” is used to label CT tube-detector designs 3rd Generation Design Rotating tube & detector 4th Generation Design Fixed ring detector

Slip-ring Technology Power is transmitted through parallel sets of conductive rings instead of electrical cables  Continuous Gantry Rotation  Prerequisite for Spiral CT Non Slip-ring Scanner Slip-ring Scanner

What is Spiral Scan? -- just 4“C” • Continuously rotating tube/detector system • Continuously generating X-ray • Continuously table feed • Continuously data acquisition

Continuous data acquisition Volume Data A B Reconstruction of arbitrary slices (either contiguous or overlapping) within the scanned volume Distance between the slices is called Increment

Contiguous Image Reconstruction  Slice Thickness • Increment = Slice Thickness • No Overlap • No Gaps  Increment

Overlapping Image Reconstruction   Slice Thickness Overlap  • Increment < Slice Thickness • Overlap of slices • Closer image interval • More images created  Increment

Image Reconstruction with Gaps  Slice Thickness • Increment > Slice Thickness • Gaps between slices • Images are further apart • Less images created  Increment

Shallow Inspiration Deep Inspiration Standard CT / Slice Imaging • Misregistration due to different respiratory levels between slices • Unable to resconstruct images at arbitrary position Partial Volume Effect • Slice imaging is slow

Spiral CT / Volume Imaging • Scan the whole region of interest in one breath hold • No gaps since radiation always • transmits the whole volume • Reconstruction of overlapping • images without additional dose • Retrospective reconstruction of slices in arbitrary position within the scanned volume

X-ray Tube Voltage (kVp) X-ray Tube Current (mA) Scan Time (s) Slice thickness or Collimation (mm) Scan Parameters • Table Speed (mm/rot) or Feed per 360 rotation • Pitch • Interpolation Process • Increment (mm)

Table Speed & Pitch Table Speed is defined as distance traveled in mm per 360º rotation Pitch => Table Feed per rotation Collimation

30s Pitch 2 covers 2x distance as Pitch 1 10mm P1 30s More Coverage in the same time with extended Pitch!! 10mm P2

Scan Range = 300mm 30s 15s 10mm P2 20 mm/s 10mm P1 10 mm/s Cover the same volume in shorter time with extended Pitch

Interpolation Algorithm • Converts volume data into slice images Interpolation To reduce artifacts due to table motion during spiral scanning, we use a special reconstruction process called INTERPOLATION

Slim Algorithm Wide Algorithm   2 (180+52) = 464° raw data 2 x 360° = 720° raw data Wide algorithm produces a broader image thickness Wide algorithm uses more raw data => less image noise

Pitch 2 scanning produces a broader image thickness Pitch 2 scanning does not increase image noise PITCH 1 PITCH 2 30% increase in image thickness with Pitch 2

Slice Sensitivity Profile ( SSP ) SSP describes the effective slice thickness of an image and to what extent anatomy within that slice contribute to the signal Image signal RESOLUTION Ideal SSP Collimation = width of x-ray beam =slice profile SSP All points within the slice contribute equally & points outside of the slice do not contribute to the image at all . Z-axis (mm)

Slice Profile (SP) • Effective slice thickness of an image Slice Profile Resolution • Factors influencing Slice Profile • Collimation • Pitch • Interpolation algorithm (360° or 180°)

Factors influencing SSP • Collimator width • collimation => SSP • Spiral CT • Table speed or Pitch • Interpolation Algorithm • => mathematical process required to reconstruct axial images from the spiral volume data set

Pitch & Slice Profile

Slim vs Wide – SSP Comparison Slice Profile Slim %Broaden Wide %Braden Pitch One 5.0 mm 0 6.3 mm 26 Pitch Two 6.5 mm 30 10.8 mm 116

SLIM 464 degree Less photons WIDE 720 degree More photons SSP Spatial resolution SSP Spatial resolution Smoother image Noisier image

Slim - Advantages • Improved Z – Resolution • Reduced partial volume artifacts • Slim + extended Pitch • Longer coverage • Same coverage with shorter scan time or thinner slices • Less radiation dose

Wide - Advantages • Noise Reduction • Smoother image • Useful for scanning huge patient Only for scanning at Pitch One

Slice Profile Comparison

Optimizing the Scanning Parameters SCAN RANGE = 150mm Lesion smaller than 1cm 10/10/10 (15s) 5/10/5 (15s) Slice Profile = 10mm Slice Profile = 6.5mm

Smallest Possible Effective Slice Thickness Scan Length (mm) Smallest Collimation (mm) Table Speed (mm/s) Scan Duration (s) Scan Duration Depends on the scan length & patient’s breath-hold compliance Table Speed Pitch Factor 1 < Pitch < 2  to cover the whole volume in one breath-hold

Injection Protocols Site Peripheral vein eg. antecubital vein 19-20 gauge needle or IV catheter Volume 80 - 150 ml  patients’ weight & region of interest Flow Rate 2 - 5 ml/s  cardiac output Scan Delay Delay between injection initiation & the start of the scan sequence Concentration 300 mg I/ml non-ionic contrast

Tailoring Scan & Injection Protocols Injection Duration must be equal to or greater than Scan Time Enhancement Curve of the Target Region HU HU 250 250 200 200 150 150 100 100 50 50 CONTRAST CONTRAST NaCl Time[s] Time[s] Bolus Duration < scan time Insufficient, inhomogeneousopacification Bolus Duration = scan time Uniform, maximum opacification

Contrast Bolus Timing Determines optimal scan delay for spiral CTA sequence HU Time-density curve of the target region 250 200 150 Early Late Optimal Window 100 50 CONTRAST NaCl Time[s]

Test Bolus Procedure • 10-20 ml of contrast is injected at the chosen rate for spiral • After a delay of 8-10s, low-dose, single-level axial images are acquired every 2s at the starting point of the imaging volume • Dynamic Evaluation to generate a Time-density curve Imaging Volume for spiral CTA Dynamic scans at this position

Dynamic Scans DynamicEvaluation • Time-density curve Scan Delay  Peak Enhancement Time • ROI placed in the Aorta

Dual Phase Liver Exam Liver Metastases • Arterial Phase • Venous Phase

Single Plane Imaging with Multiplanar Results Oblique recon. of Aorta 2D reconstruction based on a serial of axial images along a certain axis Sagittal Coronal

CTAngiography Max. Intensity Projection Surface Shaded Display (3D) Spine 3D image: AVM Femoral Arteries CT Angiogram

Computed Tomography