Mohammad Baharvandy & Sina Fazelpour

Transcription

1 Mohammad Baharvandy & Sina Fazelpour

2 Ultrasound Basics Data acquisition in 3D Reconstruction of 2D images 3D Ultrasound Modeling Medical applications 4D Ultrasound 2

3 Ultrasound consist of sound waves of frequencies above the range of human hearing (15 Hz to 20 KHz). Frequencies between 2 to 10 MHz is used in medical diagnosis ultrasound. the original idea comes from RADAR systems Naval doctor G. Ludwig reported the first measurements of sound speed in body( ave1540m/s). Dr. J.J. Wild English surgeon and J. Reid an electrical engineer attempted to use 15 MHZ radar to detect cancer in stomach. During 1960 s s several companies developed ultrasound systems suitable for Imaging fetus. R. Solender of Siemens designed the first real-time mechanical ultrasound scanner in Dr. I McDonald and DR. T.G. Brown developed the first commercially successful diagnostic Ultrasound imaging system. 3

4 Transducer: piezoelectric crystals in electrical current produce sound waves. (transmission) Sound waves hits crystals and produce electric current. (reception) 4

5 C is assumed to be 1540 m/s in human tissue. If we use an electrical circuit model Z (sound impedance) is analogous to R (resistance) and P (pressure wave) is analogous to V (potential difference), which makes I (intensity) to have similar behavior to power. 5

6 Advantages: Flexibility Real-Time imaging Non-ionizing radiation No known bioeffects Portable Relative low cost Disadvantages: Resolution Interpretation of results 6

7 First experiments occurred over 20 years ago, with CT scan. Research for 3D ultrasound started around the same time. Real-time frame rates, make 3D ultrasound reconstruction much harder. 7

8 Data Acquisition Image Recording Reconstruction Display 8

9 Free-hand Acquisition Acoustic Articulated arm Electromagnetic positioner Mechanical Localizers Linear scanning Fan scanning Rotational scanning 2D Transducers arrays Movements of probe should be fast and precise. Position and orientation of probe must be known all the time. 9

10 -Acoustic- Most common method in free-hand acquisition. Position and angle are obtained from three sound-emitting devices mounted on the transducer and is detected by three microphones above the patient. -Electromagnetic- Position and orientation are detected by magnetic field sensor. Continuous of ultrasound transducers using 100 Hz field measurements. -Acoustic- -Articulated Arm- Transducer on a mechanical arm with movable joint. Less movement, more precision. -Electromagnetic- 10

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12 Mechanical probe moves in a precise manner. Use the knowledge about anatomy of body. Small, expensive localizers or large localizers than can be assembled on 2D device. Take slices in predefine intervals. Linear distance angle Linear scanning Fan Scanning Rotational Scanning 12

13 Linear scanning Fan scanning Free-hand offers more flexibility where mechanical localizers are more precise in most cases. 13

14 No need to move the probe. Generates 3D data in real-time. (4D) Mostly in research process, because of constraints such as: Large number of transducer in a small area. Higher probability of false detection. 14

15 3D image reconstruction refers to the generation of a 3-D 3 representation of the examined structures from the acquired set of 2D images. There are two distinct ways to implement the reconstruction process: First, 3-dimensional surface model,, Segmenting series of 2D images to extract the desired features before the 3D image is reconstructed. Second, Acquiring series of 2D images to build a 3D voxel based Cartesian volume. 15

16 Segmenting Approach: The boundaries in between tissues should be outlined either manually or automatically. Accurate segmentation is required which is a particularly difficult problem. 3D surface model is developed from the boundary descriptions using different techniques. Provides images with increased contrast between segmented structures which increases the image artifacts. This method reduces the amount of information allowing for efficient 3D rendering. 16

17 Voxel Based Approach: No information is lost during the 3D reconstruction. Allows a variety of rendering techniques such as: - Texture mapping - Ray-casting Requires no user intervention and is easily automated. However this approach results in large data files 17

18 Plays an important and at times, dominant role in verifying information to the operator. Four classes of displaying 3D images: Surface-based Multi-planar Combined surface and multi-planar rendering Volume based 18

19 This technique is based on visualization of surfaces of structures or organs A segmentation or classification step is required which can be done either automatically by an algorithm analysis or manually by a skilled operator There are two basic methods available for viewing: - Wire-frames - Surface Rendering 19

20 A Wire-frame image of heart Surface rendered image of fetus It is not an ultrasound image Though! 20

21 Requires that a 3D voxel- based image be first reconstructed Two common techniques are used to view the image: - Using computer user-interface tools to select planes from the volume for viewing as reformatted 2D images. - Multi-planar visualization with texture mapping (the 3D image appears as a polyhedron). 21

22 An example of Texture mapping: Three-dimensional color Doppler US image of the carotid artery shows the sharply jagged irregularity of the vessel wall and the color pattern caused by slight variations in the beat-to to-beat movement of the artery. The image was acquired with cardiac gating, which improves the quality of the 3D image but increases total imaging time. 22

23 Presents to the viewer a display of entire 3D image after it has been projected onto a 2D plane. Density-Weighted The most common approach which is the ray-casting techniques. Maximum intensity projection Another common approach to display only the maximum intensity voxels along each ray 23

24 Cout = Cin (1-α(k))(k)) + c(k). ).α(k) in (1 α(k) is opacity value c(k) ) is shade value P(r) is opacity and color of pixel r, k is the kth voxel along the rth ray 24

25 Subtle surface features can be enhanced by using gradient Subtle can be enhanced by using gradient information from the volume to highlight interfaces. (by using some s type of gradient shading) One method to get the gradient is to use Mark-Hildreth operator which is the convolution of a Gaussian with a Laplacian: 25

26 Exponential depth shading is a useful technique to enhance the perspective of depth in a volume rendered image In this approach the density of voxel is a function of voxel depth in the volume which is reduced by an amount based on an exponential function: β(k) : attenuation factor at kth location along rth ray 26

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28 Fetal Imaging: -3D US has shown most promise in obstetric imaging - Reported to make anomalies easier to recognize - Better understanding of fetal abnormalities *A study of 204 patients with anomalies: 62%:3D more advantageous than 2D 36%: both equal 2%: 3D disadvantageous * Study was performed by Merz et al. 28

29 Images of embryos, demonstrating the degree of detail that can be shown using 3D Ultrasound techniques Images courtesy of Drs. Benoit and Bonilla- Musolles 29

30 Gynecology: One of the most important applications is characterization of uterine anomalies - Viewing transverse plane through uterus More Accurate measurements of endometrial volume Useful in evaluating infertility patients C-plane view of uterus 30

31 Prostate Imaging: 3D US gives the possibility of more accurate repeated measurement than 2D which can be very useful when accurate volume assessment is needed for dosimetry planning or for estimating prostate- specific antigen levels. 3D US image shows hypoechoic tumor invading the seminal vesicle 31

32 There several other applications of 3D US such as: Cardiology Musculoskeletal Breast imaging Biopsy related imaging Dermatology Surgical applications 32

33 4DUS is the main goal of research in ultrasound field. Why is it more difficult to get a 4D ultrasound? Limits in acquisition methods Limits in hardware and software computational devices Limits in present image displaying techniques. Very helpful in diagnosis of dynamic organs, such as heart or fetus. PLAY THE MOVIE! And we are done THANK YOU! 33

34 Fenster,, Aaron, B. Downey, Donal. 3-D D Ultrasound Imaging: A Review, IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE /1996. Lees, William, Ultrasound Imaging in Three and Four Dimensions, Seminars in Ultrasound, CT, and MRI, Vol 22, No 1 (February), 2001; pp Nelson, Thomas R., Elvins,, T. Todd, Visualization of 3D Ultrasound Data. IEEE Computer Graphics & Application Nelson, Thomas R., Pretorius,, Dolores H., THREE-DIMENSIONAL ULTRASOUND IMAGING, Ultrasound in Med. & Biol., Vol.24, No 9, pp , Welch, Jacqueline, Johnson, Jeremy A., Bax,, Michael R., Badr, Rana, Shahidi, Ramin, A A Real-Time Freehand 3D Ultrasound System For Image-Guided Surgery, 2000 IEEE Ultrasonics Symposium. Harvey, Christopher A., Pilcher,, James A., Eckersley,, Robert J., Blomley,, Martin J K., Cosgrove, David O. Advances in Ultrasound,, 2002 The Royal College of Radiologists. intro.html#lt4 ultrasound.net/history-3d.html 34