Introduction to Medical Imaging Week 3: Introduction to Medical Imaging Week 3: CT – Reconstruction and uses Guy Gilboa Course

Reconstruction Collimators are used to keep the exposure to a slice. Image is built from multiple projections. Parallel rays are often assumed – simplifies the math. Preprocessing is done for fan-beam – conversion to parallel structure.

Radon transform Input space coordinates x, y Input function f(x, y) Output space coordinates r,  Output function F(r,  ) 2D Geometry

Filtered Back Projection The common fast and robust way to reconstruct

FBP illustration From [1] And another illustration

Sinograms

Sinogram example

Fan beam reconstruction Measurements are rearranged to form a parallel geometry representation.

Iterative Reconstruction A more sophisticated way to reconstruct. Can incorporate very accurate modelling of the physical projections. Estimates an initial solution and iteratively forward and backward projects until convergence. Slow, (considered state-of-the-art). Taken from

Helical CT Allows continuous gantry rotation – z axis is changing continuously, this is taken into account in the reconstruction.

Computed Tomography (CT) 3D imaging using X-ray radiography

Helical CT Taken from [1]

Multi Slice Detectors Taken from

Contrast agents Iodine is injected – increase attenuation coefficient of blood for a short time. liver vessels

Dual energy CT Can distinguish better between materials with different attenuation coefficients at different energies.

Dual energy CT – several methods Dual source (Siemens)

Dual energy CT kV Switching (GE)

Dual energy CT Dual layer (Philips)

Spectral CT by photon counting Future technology using direct conversion methods. In 2014, not a mature technology yet (works for animals, mummograph).

Spectral CT - example CT image of the thorax of a mouse injected with Au-HDL and iodine contrast agents. Conventional vs. photon-counting CT. Taken from di/Cormode-Fayad.wpd di/Cormode-Fayad.wpd

Uses of CT Used widely to scan almost every organ in the body, popular uses are: Cerebral scans – chronic and accute head and brain scans, internal bleeding, tissue oedema (swelling) and skull fracture. Also to diagnose and follow the progression of some brain tumors. Pulmonary disease – identify size and geometry of lesions. Calcification of nodules, increase in tissue attenuation – can be indicators for cancer.

CT Uses (cont’) Liver imaging - 3-phase liver scan: ◦ A pre-contrasted control scan is acquired as baseline ◦ Two scans following injection of contrast after ~35sec (arterial phase) and ~65sec (portal phase). ◦ Can detect hypervascular lessions, fatty infaltrations into the liver and other liver problems. Cardiac imaging ◦ Used primarily for assessing calcifications within the heart, particularly in coronary arteries. ◦ The presence of coronary calcifications is highly predictive of the future development of cardiac problems.

CT Uses (cont’) (Cardiac – cont’) ◦ Iodine contrast is used. High end machines are needed – with fast gantry rotation to “freeze” the cardiac motion. ◦ Can work with pacemakers, defibrillators, stents etc. where MRI cannot work. Trauma – as CT becomes available and fast – it is commonly used in trauma (ER) – a full body scan is often performed to diagnose internal bleeding and bone fractures.

Examples – cardiac scan Calcified and non-calcified plaque in coronary arteries.

Abdomen CT scan Numerous small heterogeneous hypodense lesions in the liver, some with central enhancement.

3D recon Postsurgical CT reconstruction. Image is of a young man who had suffered a serious motorcycle accident See also &ItemID= &ItemID=63867

New GE scanner (Dec 2013)

CT Summary Advantages: 3D data. High resolution. Very good anatomical info. Fast scanning compared to MRI / PET. Drawbacks: Ionizing radiation. Does not distinguish well between soft tissues. No functional info.