- A 3D machine uses different probes
- The probe captures images in the 3 planes simultaneously
- Scanning may be done by free hand technique
- Automatic 3 plane scan by most machines
- 1980's first 3D rendering pictures of fetus
- 1990's section reconstruction as the orthogonal planes
- 1991 volume rendering as translucent display
- Computers to obtain images
- 1993 3D with defocusing lens without computer
- 1994 first surface rendered fetal images
- 1996 real time ultrasound beam tracing without a computer.
BASICS OF 3D ULTRASOUND
- 3D is volume visualization
- Aim is to represent morphology of organ of interest in 3D space.
DEFOCUSING LENS METHOD (Flow chart 1.1)
- Volume imaging or thick slice 3D imaging
- Simple and lens expensive
- Convex or linear probe with a defocusing lens on its surface
- The lens diverges the beam
- Resolution is poorer
- All echoes within the diverged ultrasound beam are displayed
- Hence a volume image is obtained
- With probe solution a good visual perception of 3D structure is possible
- Sometimes high quality surface rendered images can also be possible
- Real time observation of fetus possible
- Difficult in obese patients and with less liquor
- This is also sometimes referred as “Pseudo 3D” and alsmost all 2D machines can have this feature with joint one defocused lens mounted on a 2D probe.
VOLUME VISUALIZATION AND COMPUTER PROCESSING
- Presently this is the method of choice for 3D and 4D imaging
- It involves:
- 3D data acquisition
- 3D data sets construction
- 3D data display on a 2D screen.
Acquisition of 3D Data
- Movement of ultrasound probe
- Large number of consecutive tomograms
- Parallel fan like scanning
- Free surface scanning and rotation
- Automatic 3 D probes having a built in convex/sector probe which brings in a fan like manner
- Tomogram is digitized
- Fed into computer.
Construction of 3D Data Sets
- Data as many tomograms
- These data are reconstructed into 3D data sets
- 3D data sets are fed into computer
- Interpolation and filtering to improve data quality
- Now data is as volume elements or voxels
- Each voxel is assigned a gray value or brightness value.
Display of 3D Data on a 2D Plane (Fig. 1.1)
- Presented on a 2D display (Monitor)
- Section reconstruction
- Surface rendering
- Volume rendering.
- Normally a 3D set is multiplanar representation
- Information from all three planes that cut the voxel
- Orthogonal to each other
- These planes can be shifted and rotated
- Like anatomic cross section pictures
- Does not produce a 3D impression of that object.
- Objects are extracted from voxel
- Surface rendered display
- 3D data set can be rotated to view or change direction
- Image is shaded to give a 3D appearance (done by dept cueing) (near components bright distant image components darker).
- 3D data projected in 2D plane directly
- Volume ray tracing method
- A variety of images may be obtained
- A still image may not have a 3D appearance
- 3D appearance is seen by rotation.
REAL TIME 3D OR 4D IMAGING
- Real time 3D is now possible with good frame rate
- 3D volume can be imaged live with visualization of fetal movements and expressing (Yawning, etc.)
- Also helpful in fetal echocardiography
- 3D data acquisition and calculation and projection is done simultaneously
- With real time 3D (4D) the viewing direction limited to that of the probe.
- Kretz technique Austria were the first to make available of the voluson 3D systems
- Rapid developments have occurred in the last few years in hardware and software
- Voluson 730 Pro with harmonics, stic, inversion, etc. features are new available commercially as state of the art ultrasound machine.
HOW TO DO 3D STEP BY STEP?
- Proper equipment
- Adequate training
- Patience and sound knowledge
- Select transducer for organ of interest (transabdominal, transvaginal, transrectal)
- Liberal use of gel
- Get a proper window by a 2D scan with the 3D probe. Ask patient to hold breath for 3D volume acquisition
- If mechanical probe hold at area of interest 4 to 10 second
- Free hand scanning with defocusing lens probe
- Move probe slowly with small step size
- Use smooth sweep with uniform velocity without tilling or twisting
- Automatic probe to be hold with the target in view till the machine completes all three planes sweep
- View planar slices (orthogonal planes)
- Reorient anatomy by rotation (x, y or z)
- Identify area of interest, review volume data
- View the rendered image, free from artefacts
- Ray costing, acquire nest volume if desired
- Surface rendering
- Threshold settings, use inversion, stic or whatever needed
- Adjust view, use electronic scalpel, vocal if needed
- Save and store
- Save in rotation mode
- Save as volume or surface rendered image
- Print hard copy and archive images with volume
- Save as AVI file on hard disk or as CD Rom or on attached VCR
- View as 4D for fetal and same AVI 4D.