What is DICOM?

DICOM stands for Digital Imaging and COmunications in Medicine. DICOM is a standard that defines two things about medical (image) data:

1. What information is save in DICOM data and how they are organized?
2. What kind process can be performed on DICOM data?

 ...to be continued

C-arm Cone-beam CT

Reference:

• Current Products and Practice, Three-dimensional cone beam computerized tomography in orthodontic,Journal of Orthodontics, Vol. 32, No. 4, 282-293, December 2005
• C-arm Cone-beam CT: General Principles and Technical Considerations for Use in Interventional Radiology, J Vasc Interv Radiol 2008; 19:814–821

Calibration for C-arm Reconstruction

Due to their open design and mechanical instability, C-arm imaging systems acquire projections along non-ideal scan trajectories. Volume reconstruction from C-arm projections requires a very precise knowledge of the imaging geometry. These knowledge can be gained by calibration.

"Fortunately, the C-arm motion is highly reproducible over a period of six months (Fahrig and Holdsworth 2000), and so the calibration can be done off-line with updating only needed about twice per year in clinical environments."

Calibration is a process to determine the transformation between 3D world coordinate and 2D image coordinate.

xw = P * xi

Here xw and xi homogenous coordincate of world system and image system. P is the projection matrix that need to be computed by calibration.

The calibration method can be off-line or on-line:

• On-line calibration can real-time compute the projection matrix, but it requires more complicated image processing algorithm. Till now, online calibration is still under research in the academic community.
• Off-line calibration is an standard approach in industry and clinic practices. A calibration phantoms of known geometry is used. 

"The measured 2D location of the spheres in the cone-beam projection together with their known 3D positions then allows us to obtain expressions for all calibration parameters."

Reference:

• http://www5.informatik.uni-erlangen.de/Forschung/Publikationen/2007/Hoppe07-GCO.pdf
• http://deposit.d-nb.de/cgi-bin/dokserv?idn=994417551&dok_var=d1&dok_ext=pdf&filename=994417551.pdf
• http://www.cs.huji.ac.il/~josko/papers/cas-02-miccai-calibration.pdf
• http://spie.org/x648.html?product_id=479945

2D-3D Registration

The typical application of 2D-3D registration is to align the real-time 2D fluoroscope frames with pre-operative 3D CT volume. The knowledge of 3D positioning is valuable information during surgical intervention.

Generally speaking, we need to estimate 4 quaternions parameters for rotation (with 3 degree of freedom)  3D pose estimation. Given a orientation, we compute the Digitally Reconstructed Radiograph (DRR) according to the rule of ray-casting and compare with Digital Video Fluoroscopy (DVF) by measure the similarity index. Several similarity indices are applied, for example, mutual information, normalized cross-correlation. The optimal parameter set is the one producing the largest similarity index. 

References:

• http://www.hindawi.com/journals/asp/2010/806094.html
• http://people.csail.mit.edu/lzollei/research/papers/ms_thesis.pdf

Difference between SPECT and PET

 It's all about the isotopes they use.  PET uses positron emitters (F-18, C-11,...), this positron annihilates with an electron in the body of a patient and thereby creates two gamma-rays under an angle of 180°C.  The beautiful thing of PET are these two gamma-rays.  When two detectors (in the ring) detect a ray at the same line, you can draw a virtual line between them (180°C!!).  On the intersection of all the different "lines" lies your source of radiation.  This gives PET a good sensitivity and resolution and quantification possibilities.

A SPECT isotope (Tc-99m) emits one gamma ray, so reconstruction is more difficult, only rays that fall straight on the detector are detected (otherwise you won't know from which direction it came), so the camera has to rotate to make a full image.
I hope a made it more clear to you...

Reference:

• http://www.spineuniverse.com/exams-tests/nuclear-imaging-pet-spect-scans
• http://www.chemicalforums.com/index.php?topic=19916.0
• http://www.macalester.edu/psychology/whathap/UBNRP/Imaging/pet.html

Coronary Angiography

• http://www.nlm.nih.gov/medlineplus/ency/article/003876.htm
• http://www.medmovie.com/mmdatabase/MediaPlayer.aspx?ClientID=65&TopicID=556

Fluoroscopy and Image Intensifiers

If we want to define fluoroscopy in one sentence, we could say,

Fluoroscopy is a series of x-ray images recording along the time, namely x-ray video.

 Compared to the conventional x-ray imaging, fluoroscopy doesn't use film but a CCD video camera to record the projection. As a result, the "live" x-ray images can be played on a monitor.



The story about fluoroscopy has not yet been finished, we also need image intensifiers.

Due to the limited light produced from the fluorescent screens, early radiologists were required to sit in a darkened room, in which the procedure was to be performed, accustomizing their eyes to the dark and thereby increasing their sensitivity to the light.

Image intensifiers refer to a special component of the machine, which amplifies low intensity x-rays. This technique allow a radiologist to view the images on the monitor, not in darkened room any more.

Reference:

• http://en.wikipedia.org/wiki/Fluoroscopy
• http://en.wikipedia.org/wiki/X-ray_image_intensifier

Perfusion CT

 Perfusion CT is a kind CT imaging for observation of micro-circulation of tissue, i.e, to see what's going on with blood flow in the organ. Prefusion CT imaging is a quatitative measurement with the unit ml/min/100g, i.e. ml blood per min flow through 100g tissue. Clearly perfusion CT measures the speed of blood flow among tissues. For example, normal cerebral perfusion is approximately 50 ml/min/100g to 60 ml/min/100. When perfusion falls below a threshold of approximately 20 ml/min/100g, cerebral metabolism is reversibly impaired.

Technically, perfusion CT is somehow similar like Digital Subtraction Angiography (DSA).

"All the techniques are based on the intravenous administration of iodine based CT contrast material and the fact that the change in attenuation due to this contrast material is directly proportional to the concentration of the contrast material."

In a plain text, physician take a baseline volume without contrast material at the begin. Then, they inject the contrast materiel, wait for a given time and scan the same location by a rapid CT again. Usually, people substrate the this volume with the baseline volume to enhance the change of attenuation caused by flow of contrast material. A rapid CT scanning technique is a precondition. Perfusion CT has been widely used for acute stroke and oncology.

Reference

• Perfusion CT: a worthwhile enhancement? British Journal of Radiology (2003) 76, 220-231, 2003 British Institute of Radiology

Major CT Vendors

• Siemens Healthcare
• GE Healthcare
• Philips (Brilliance series)
• Toshiba Medical System

Gating for CT

First of all, we must know a basic assumption of 3D-reconstruction: The algorithm assumes that the object should be always static during scanning. However, the human body is not a stone, even patients don't move, you cannot stop breathing or heart beating. If we directly use these ungated projection data, the motion artefact will exist in reconstructed volume.

ECG gating and respiratory gating is used to remove the artefact caused by cardiac and respiratory motions.

Take ECG gating as example. All projects are synchronized with ECG signals and classified into several groups according to different ECG phases. Then we reconstruct volume 1 by projections in phases 1, volume 2 by projections in phase 2, and so on. In a word, each volume is reconstructed by the project in the same phase and motion artefact is almost removed. Someone call gated it also 4-D CT, namely, volume1, volume2,...volume n, because they not only record spatial information, but also present the motion of organ along the time-axis.

Resolution Tool

Resolution depicts the ability of a system to image details of a given pattern. For example, a test bar phantom or resolution tool, as in figure, is used for CT measurement. The phantom composed of groups of parallel lines of different width. Each group is characterized by density of such line. The measurement unit is line pair per millimeter (lp/cc) or line pair per centimeter (lp/cm).

Helical CT and Multislice CT

Today, almost all CT scanners are helical CT. Different from previous conventional CT machines, a helical CT has x-ray source and detectors which are continuously rotate in a ring. The patient table slides the patient through the source-detector ring. The path of source and detector, respect to the patient, is a helix. And some people call it spiral CT .

The major improvement of helical CT, compared to old CT, is the rapid volumetric data acquisition. Later, the speed was further increased by multislice CT, which has a "thick" fan-beam and multiple parallel rows of detectors, typically, 16-slices or 64-slices.

Note: Enhanced 3-D-reconstruction algorithm for C-arm systems suitable for interventional procedures

Two major challenges of 3D reconstruction of C-Arm system:

• low number of projections
• irregular sampling patterns caused by the mechanical instability

Two groups algorithms were applied to solve these problems:

• algebraic method
• analytical method (Feldkamp alg. like)

From 2D fan beam CT, it is known that a partial rotation over 180 degree is sufficient for reconstruction.

Reference:

• Enhanced 3-D-reconstruction algorithm for C-arm systems suitable for interventional procedures. IEEE Trans Med Imaging. 2000 May;19(5):391-403.