Introduction to Medical Imaging Introduction to Medical Imaging X-ray and CT Guy Gilboa Course

X-Ray First imaging modality (Discovered by Röntgen in 1895).

X-ray discovery Wilhelm Rontgen ◦ Discovered and detected X-rays ◦ Used it as a first medical imaging modality. Nobel 1901, first Noble prize in physics was awarded to him. First x-ray image. Hand of Anna Rontgen (wife)

X-ray machines Standard machine C-arm

X-ray tube

X-ray tube diagram Taken from

X-ray tube movie

X-ray on the spectrum

X-ray energy spectrum

Energy units

Standard optics constraints affect the quality of the X-ray image Taken from [1]

Popular for checking chest (lung problems) and bone fractures

Signal to Noise Ratio Poisson distribution Variance is SNR

X-Ray Summary Advantages: ◦ Cheap and simple ◦ Low radiation (compared to CT) Drawbacks: Does not give 3D info. Bones can occlude significant diagnostic data.

Digital Mummography Used to detect small tumors or microcalcifications in the breast. Very high spatial resolution can CNR are need for these type of pathologies (often <1mm in diameter) Low radiation is important – avoid tissue damage and allow frequent usage. Low energy (e.g. 26 keV) is used – high contrast, low radiation, low penetration.

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

History - Invention of CT Sir Godfrey Hounsfield (English Electrical Engineer), built first CT 1971, scanned head. Allan McLeod Cormack - math framework. Nobel prize for both in 1979 for the invention of CT – first full body scanner. Hounsfield sketch

CT scanner

CT – operation principle Taken from

Attenuation coefficients

Hounsfield unit Water: 0HU Air: -1000HU What is displayed in CT images? Typical medical scanner display: [-1024HU,+3071HU], Range: 12 bit per pixel is required in display.

Hounsfield units of tissues Taken from [1]

Rendering based on different HU thresholds Taken from

X-ray detectors Previously – film, analog radiography. Today – digital radiography ◦ Indirect conversion  X-ray to light using a scintillator (CsI:T1)  Light to voltage using photodiodes ◦ Direct conversion  Thin-Film-Transistors  Cadium-Teloride, Cadium-Zinc-Teloride – technology not mature yet.

Helical CT Taken from [1]

Gantry

Scanning Geometry of a CT System From [1]

Gantry rotation speed (times) Range ◦ Standard 0.4 – 0.6 sec/rot ◦ Cardiac 0.27 – 0.35 sec/rot Gantry rotation:

Multi Slice Detectors Taken from

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 , s Output function F( , s) 2D Geometry Taken from

Transform and Its Inverse Radon transform Inverse Radon transform There are several possible parameterizations of the transform, here is one:

Filtered Back Projection The common fast and robust way to reconstruct

FBP illustration From [1] And another illustration

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

Sinograms

Sinogram example

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

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

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 - 2-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.