Essential parts of a medical imaging system

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2 Essential parts of a medical imaging system Acquisition = building the image = applying energy + sensing a response reflection, transmission

3 Medical imaging systems conventional X-ray, X-ray TV, Endosc., DSA, DR, (IR), NM tomographical (tomography) US tomographical (computed tomography) CT, MR, SPECT, PET, (EIT)

4 !

5 Projection X-ray (radiography) Different absorption chracteristics allow to distinguish different material (and provide contrast) in the image. X-ray attenuation is measured by the linear attenuation coefficient (μ). Projection X-rays (radiographs) are 2D projections of 3D data Převzato z

6 Conventional X-ray (projection radiography) Převzato z

7 Conventional X-ray (X-ray tube) Převzato z

8 Conventional X-ray (mamograph) Převzato z

9 Conventional X-ray TV (C arm with II) Převzato z

10 Conventional X-ray TV (image intensifier) Převzato z

11 Conventional X-ray TV (TV pick-up tube) Převzato z

12 Převzato z Ultrasound US imaging employs HF sound energy to image the interface between differing tissue types. When the sound wave strikes an interface, some energy moves across the interface and some energy is reflected backwards. The reflected energy is detected by a receiver and is used to form the image.

13 Conventional US ( tomography ) Převzato z

14 Nuclear medicine Radio-isotopes are introduced into the body to tag specifig physiologic functions. As the tracer accumulates in a particular anatomic location, it periodically emits a particle that can be observed and used to form an image. NM it can be used to form functional rather than structural images.

15 Conventional NM (Anger gama camera) Převzato z

16 Computed tomography (CT) CT is used to generate cross-sectional images (CT slices) from a set of projections images obtained at different angles. CT image pixels are reported in units called Hounsfield units (HU). The following reference points are useful to know:

17 CT Material CT number μ [cm -1 ] Bone Muscle Water Fat Air Převzato z

18 CT (history vs. present) Převzato z

19 Magnetic resonance imaging MRI images proton density by using a permanent magnet with a pulsed radio frequency (RF) field. The RF field changes the spin orientation of protons (tilting them and causing them to precess as they spin) within the body. We form an image by listening to a signal emitted as the protons relax back to their original orientation.

20 MRI We apply energy to perturb the system, but the signal itself is generated from the tissue sample under study! This places some fundamental limitations on MR image acquisition. Převzato z

21 Single photon emission computed tomography (SPECT) SPECT camera acquire multiple planar views of the radioactivity in an organ the data are then processed mathematically (iterative reconstruction) SPECT utilizes the single photon emitted by gama-emitting radionuclides such as 99m Tc, 67 Ga, 111 In, and 123 I this is in contrast to PET

22 SPECT Převzato z

23 Positron emission tomography (PET) PET cameras are designed to detect the paired 511-keV photons generated from the anihilation event of a positron and electron following emission, any positron travels only a short distance before coliding with electrons in surrounding matter the paired 511-keV annihilation photons travel in opposite directions (180 apart) along a line

24 PET Převzato z

25 Ionizing vs. non-ionizing radiation Ionizing radiation applied energy is sufficient to ionize atoms (ejects an electron from orbit, creating a positively charged ion). (e.g., X-ray, CT, PET, SPECT) Non-ionizing radiation insufficient energy to ionize atoms (MRI, US, optical)

26 Imaging structure and function Structure: tissue density, region size, shape, and orientation. Function: Activity (metabolic rate), perfusion, ventilation. Challenge: combining structural and functional information together in a synergistic presentation (ex. display blood flow distribution on top of a CT slice of lung).

27 Digital image processing What is an image? Formal definition: A digital image is a multidimensional signal that is sampled in space and/or time and quantized in amplitude. An image is often represented by a multidimensional matrix (array) of numbers.

28 Digital image processing The image may be 2-D (planar), 3-D (volumetric), or N-D. Image elements: an image is composed of: 2-D: pixels = picture x elements 3-D: voxels = volume x elements

29 Analog image to digital image conversion gradační greyscale stupnice test image TV TV camera kamera including včetně optické optical system soustavy CCD CCD detail detail CCD převod coversion snímané of 3D scény scene (optické (optical informace) information) nainto elektrický the electrical signál signal elektrický electrical signál signal (napětí, (voltage) elektrická informace) coversion převod elektrického of electrical signal signálu (voltage) (napětí) into na the číslo (číselnou number within the range informaci v rozsahu from od 0 0 do to A/D analog to digital converter součást as a tzv. part of frame "frame grabber grabberu" (karta (FG do PC počítače) card) A/Č A/D SW počítač PC recording paměťové (memory) medium médium

30 What do images represent? X-ray attenuation (density) X-ray, CT Water (proton) density, relaxation times MRI Acoustic impedance US Brightness TV Tracer uptake (distribution of radioactivity)nm Heat IR Electrical impedance EIT

31 Transfer properties of imaging Why? systems Analogy with 1D cases. Key point: spatial frequency, contrast transfer How to measure quality? Set of transfer functions

32 Impulse response of imaging system - PSF Předmět Object Image Obraz y Předmětová Object plane rovina y f( xy, ) x x Zobrazovací Imaging system soustava PSF hxy (, ) y Obrazová Image plane rovina y gxy (, ) x x f( xy, ) = δ ( x x, y y ) gxy (, ) = hx ( x, y y ) gxy (, ) = f( xy, ) hxy (, )

33 Transfer function of imaging system in frequency domain F { gxy (, )} = F{ f( xy, ) hxy (, )} Guv (, ) = Fuv (, ) Huv (, )

34 u Spatial frequency = 1 1, v X Y cy / mm Hz cy / s lp / mm cy / mrad r ua = = ru X Prostorová Spatial perioda X θ Úhlová Angle perioda r Distance Vzdálenost Place Místo of observation pozorování Prostorový Spatial frequency kmitočet: u = 1/ X [ cy / m] Prostorový Spatial angle úhlový frequency kmitočet: u = r / X [ cy / rad] a

35 Relationships among transfer functions j OTF F { h( x, y)} = H ( u, v) e φ ( uv, ) MTF H ( u, v) PTF Huv (, ) =φ( uv, )

36 Modulation transfer function PSF( x, y) OTF( u, v) Předmět Object Zobrazovací Imaging system soustava Obraz Image A A ac max min M = = A max + A min dc

37 Phase transfer function (PTF) OTF PTF

38 Effect of PTF on distortion 400 pixels 400 pixels Object Předmět Convolution Konvoluce * Image Obraz Convolution Konvoluční kernel jádro

39 Sensed scene Snímaná scéna Aliasing Reconstructed image - aliasing Rekonstruovaný obraz - aliasing Optický Optical antialiasingový antialising filter filtr Spatial sampling Prostorové vzorkování Rekonstrukční filter filtr Reconstruction Snímaná scéna Sensed scene Rekonstruovaný obraz bez aliasingu Reconstructed image without aliasing Brightness L profile Profil of jasu sensed snímané scene scény a and rekonstruovaného reconstructed obrazu image 1 1 0, 5 0 x Vzorkovací x Δx sampling Reconstructed Rekonstruovaný image Vzorky Samples of sensed period perioda without obraz bez aliasingu snímané scény scene Reconstructed image with aliasing Sensed Snímaná scéna scene Rekonstruovaný obraz s aliasingem

40 Useful www links H.html Materiály ke kurzu Získání a zpracování obrazu v mikroskopii [online]. Jiří Hozman, c Poslední změna [cit ]. URL: < Hozman, J., Roubík, K. Tomografické zobrazovací metody v lékařství - CT. Výukový videoprogram (VHS). Praha: AVTC ČVUT, Je k dispozici na a dále na

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