Retro-Grate Reflector for Linear Tomography
BACKGROUND
Computer Aided Tomography (CAT) is a process for generating 3D X-Ray images. A CAT Scanner
is a specialized and expensive piece of equipment. A CAT Scanner operates by sweeping an Xray
tube and detector along a circular arc. In this way, image data are collected around a 360 arc
enclosing the subject. Using a tomographic calculation, these image data are merged into a 3D
image.
Linear Tomography (also called Tomo Synthesis) is a Computer Aided Tomography process
for generating 3-D images using conventional X-ray equipment. See:
http://www.amershamhealth.com/medcyclopaedia/Volume%20I/tomography.asp
Figure 1: Siemens Iconos R 200 X-ray machine with Linear Tomography capability. (From:
http://www.siemens.com)
The Siemens Iconos R 200, see figure 1, is an example of a commercial Linear Tomography
system. See:
http://www.siemens.com/
Linear Tomography - page: 1 of 6
and search on Iconos R 200.
For Linear Tomography, X-ray images are collected while the X-ray tube moves through a
range of positions, generating images with a range of exposure angles. For example, a standard
doctor’s office or small clinic X-ray machine has a movable X-ray tube. Collecting images with
exposure angles spanning at least 60o with respect to the subject is desirable. The Iconos R 200
moving through a range of positions for Linear Tomography is illustrated in figure 2.
Figure 2: Iconos R 200 illustrating motion for Linear Tomography. (From:
http://www.siemens.com)
Because the arc swept out is much less than 360o, the 3D images produced by Linear
Tomography do not have the quality of CAT Scanner images, but the equipment may be much
less expensive and more generally available.
Linear Tomography - page: 2 of 6
The position of the X-ray tube must be known
To complete the tomographic calculation, the relative positions of the X-ray tube, imager and
subject must be precisely known. In the typical case the patient and imager are stationary and
only the X-ray tube moves, as seen in figure 2. The tomographic calculation depends on accurate
geometric information; the resolution of the Linear Tomography system can not be better then the
uncertainty in the relative X-ray tube / imager position.
Two approaches to determining the needed geometric information are possible: 1) the motion
of the X-ray tube can be precisely controlled by an automated precision motion system, or 2)
the motion of the X-ray tube can be precisely measured by a precision spatial measurement
system (such as RGR-6D). In spite of costs and safety concerns, existing Linear Tomography
machines employ the alternative of precisely controlling motion with motor-driven equipment.
In this approach, precise knowledge of X-ray tube position and orientation (Pose) is achieved
by building a precision machine and making one displacement measurement at each joint of the
machine.
Because of the cost of precisely controlling motion of the X-ray tube and subject, the Iconos R
200 is considerably more expensive than a manually moved X-ray machine (one GEMS manager
estimates $100K added for the cost of the high-precision X-ray tube motion system). In addition
to cost, the automated X-ray tube motion system adds considerable size, weight and complexity to
a Linear Tomography system, such as the Iconos R 200.
For Linear Tomography it is not necessary to control the position of X-ray tube precisely. It
is only necessary to know the position of the X-ray tube at the instant of each exposure. The
tomographic calculation can be equally well computed with pre-determined or variable X-ray tube
positions, long as the positions are precisely known and sweep out a suitable arc.
RGR/X-ray
By placing a Retro-Grate Reflector (RGR) target (c.f. U.S. Patents 5,936,722, 5,936,723 and
6,384,908 and several US and International patents pending) on or near the X-ray imager, the
bearing and range of the X-ray tube with respect to the X-ray imager can be precisely determined
from the X-ray image. No other camera, light source or attachment is required. The RGR target
will have to be engineered to operate with X-rays, rather than visible light (RGR/X-ray). The
RGR/X-ray target size will be limited only by the resolution of the digital X-ray imager. With
1 mm pixel size in the imager (current technology), the minimum RGR/X-ray target size will be
approximately 40x40 mm (1 5
8 ” 15
8 ”).
A possible configuration for RGR/X-ray is seen in figure 3. This illustration is not to scale.
Two RGR/X-ray targets are seen mounted to the digital X-ray imager. The X-ray tube is shown
Linear Tomography - page: 3 of 6
following a path with three exposures. If the exposure time quite short, it is not necessary for the
X-ray tube to stop at positions #1, #2 and #3; these are simply the positions corresponding to each
of three exposures. As illustrated in figure 3, the RGR/X-ray targets occupy a portion of the imager
surface, as small as 40x40 X-ray image pixels.
Bearing Vectors
Measured by RGR/X−ray
RGR/X−ray
Patient
X−ray Imager
X−ray Tube Position
#1
#2
#3
Motion RGR/X−ray
Figure 3: RGR/X-ray used to determine the position of the moving X-ray tube.
In each X-ray image, RGR/X-ray will reveal the bearing vector from the center of the target
to the center of the aperture of the X-ray tube. By incorporating 2 or more RGR/X-ray targets,
part per 10,000 accuracy can be achieved, far beyond the requirement for Linear Tomography. The
high potential accuracy will permit favorable design trade-offs, such as reducing the RGR/X-ray
target size or allowing unrestricted placement of the RGR/X-ray targets.
The RGR/X-ray targets will be completely passive (probably comprising metalization on glass)
and can be permanently attached to the imager or its frame. Permanent attachment will simplify
operation. Many other design optimizations are possible; for example, increasing the digital X-ray
imager resolution in the region corresponding to the RGR/X-ray target would permit the target size
to be further reduced.
Linear Tomography - page: 4 of 6
X-ray Tube Motion and Safety
In motion control systems. high precision is achieved with large correction forces applied by
motors. These large correction forces to rapidly correct for motion errors to provide high precision.
Thus, in addition to being large, heavy and rigid, existing Linear Tomography systems must
incorporate relatively high-power, high-force motion systems.
Patient safety is an important challenge facing the designer of any automated motion equipment
for medical imaging systems. This is because automatic motion systems inevitably possess failure
modes which can erroneously command the motor drives to full power. As an example, to
achieve precision, an automatic motion control system may apply large forces when responding to
measured position errors. But the position signal may not be correct, due, for example, to a wire
that has broken through repeated flexing due to motion. The control computer may command full
motor power to correct errors which do not exist. A well known example of this problem is the
sudden acceleration failures of automobile cruise controls.
In factory automation, comparable motion equipment very often incorporates mats or light
curtains to de-activate the motion equipment if a human is too near. An X-ray machine enjoys no
comparable luxury, because the patient is inevitably near to the equipment.
The inclusion of RGR/X-ray targets to measure X-ray tube position makes Linear Tomography
possible without a heavy, rigid and relatively high-power motion system; the motion system could
be a lower-precision, lower-power system. The reduced motor power level permitted by the relaxed
precision requirement will improve patient-safety.
However, RGR/X-ray makes possible Linear Tomography without any automatic motion
control system whatsoever. With RGR/X-ray to measure the X-ray tube / imager geometry
(assuming the patient is stationary), the X-ray tube could potentially be moved by hand as Xray
images are gathered. Hand-movement resolves the motion-control safety problem, but brings
the challenge of minimizing the technician exposure to X-rays.
An after-market, add-on solution to convert manual X-ray machines to Linear Tomography
systems without an automatic motion system
Directly measuring the X-ray tube / imager geometry makes possible a radical innovation for the
X-ray tube motion safety / precision / size / weight / cost challenge:
Goal: The motion system shouldn’t expose the X-ray technician to X-ray radiation, or have the
possibility of injuring the patient through any failure of the automatic control equipment.
Solution: For Linear Tomography, the motion system can be passive, comprised of springs and
dampers, similar to the lift gate on a minivan. The motion doesn’t need to be precise, it only
Linear Tomography - page: 5 of 6
needs to move through an approximate arc with suitable speed (describing the complete
arc in 1-2 seconds). RGR/X-ray will provide the needed precision (no separate camera or
equipment of any kind, except an RGR/X-ray target added to the existing X-ray imager).
The X-ray technician will manually ‘wind’ the passive motion system (similar to closing
a minivan lift gate). As today, the X-ray technician will move to a position shielded from
the X-ray radiation, and then release the motion system (similar to unlatching a minivan
lift gate). The combination of springs and dampers will passively carry the X-ray tube
through the needed range of motion. Various motions for various imaging protocols can
be mechanically implemented by various links or cams that will be selected and inserted by
the X-ray technician (according to pre-defined tables).
Such a passive motion system can be as safe as the lift gate on a minivan. For example, it can
be engineered to only move perpendicular to or away from the patient under spring force.
The patented RGR-6D is the fundamental enabling technology.
Additionally:
Such a passive motion system can be engineered to be sold as an after-market, add-on item,
to be installed on existing, manually operated X-ray systems. This will bring 3D imaging
(Tomography) to the huge base of installed X-ray equipment.
As an additional opportunity, digital X-ray imagers are already replacing film in dental
offices. The inclusion of RGR/X-ray targets to measure X-ray tube position and a passive
motion system (again, engineered to only move perpendicular to or away from the patient)
makes possible Linear Tomography with existing dental office X-ray equipment.
Linear Tomography - page: 6 of 6
RADIATION EXPOSURE IN THE CATH LAB – SAFETY AND PRECAUTIONS
Dr S M S Raza, MB BS, MD, MRCP,Dip.Card.(UK)
Specialist Registrar (Cardiology)Leeds Teaching Hospitals NHS Trust UK.
Ionising radiation is a workplace hazard that cannot be detected by the human senses. The cardiovascular laboratory or cath lab is one such place where ionising radiation is much in use. The cath lab is a closed atmosphere where the working staff (i.e. cardiologists, cardiac technicians, radiographers, nurses and trainees) is at a potential risk to radiation exposure almost on a daily basis. Compared to other departments (radiology, urology, operating rooms, etc.) that also use x-ray equipment, the cardiac cath lab is generally considered an area where exposure to radiation is particularly high. Factors such as the configuration of the of the x-ray equipment, the number of cases per day, and the often long period of screening required for a study, contribute to this relatively high level of exposure and monitoring results for staff members in the cath lab who wear single badges at the collar outside their lead aprons are generally amongst the highest in the hospital. Exposure rates exceeding 7.14 Gy/hr( i.e. 5 sievert/hr) in the cath lab have been reported (3) and interventional procedures such as percutaneous coronary intervention (PCI) and electrophysiological studies (EPS)/ pacing result in the highest radiation exposure to patients and staff (3).
Radiation in the cath lab is generated using two different modes: fluoroscopy or cine angiography (cine). Fluoroscopy is used for catheter placement and involves 95% of the total x-ray operation time but only causes 40% of the total radiation exposure to staff and patients. This is due to pulsed screening that reduces exposure dose. Cine is used to acquire diagnostic images and to generate a permanent record of the procedure and, although representing only 5% of the total x-ray tube operation time, 60% of the total radiation exposure to staff and patients occur during cine. This is primarily due to use of relatively high dose rapid sequence screening required to record onto film. Significant reductions in exposure can be realised by being aware of when cine is/will be used and applying radiation safety measures accordingly.
It is important to effectively measure radiation doses acquired by cath lab personnel but exact dosage quantities are difficult to derive due to the non-uniformity of irradiation and differences in X-ray intensity as well as the relatively low energies generated by modern equipment. Therefore the International Commission on Radiological Protection (ICRP) recommend the use of effective dose (E) to evaluate the effects of partial exposure and relate this to the risk of equivalent whole body exposure. It is expressed in Sievert units (Sv) ( 1Gray unit = 0.7 sievert unit). Modern cardiac interventional procedures (coronary angiography and PCI) produce effective doses of 4 to 21 mSv and 9 to 29 mSv respectively and are therefore relatively high (1 mSv is the equivalent of approximately 10 chest x-rays) (4). The intensity of the biological effect of X-rays is dependent on the absorbed dose (total radiation energy per unit mass) of sensitive tissue and is expressed in gray units (Gy). The average dose per procedure for the cardiologist is estimated as 0.05 mGy (6). To allow better comparison of patient and staff doses this value can be expressed as the dose area product (DAP). The DAP is calculated as the product of dose in air in a given plane and the area of the irradiating beam and is independent of the distance from the x-ray source. Coronary angiography and PCI produce mean-patient DAPs in the range 20 to 106 Gy.cm2 and 44 to 143 Gy.cm2 respectively (3).
Fluoroscopy
What is fluoroscopy?
Fluoroscopy is a study of moving body structures - similar to an x-ray "movie." A continuous x-ray beam is passed through the body part being examined, and is transmitted to a TV-like monitor so that the body part and its motion can be seen in detail.
Fluoroscopy, as an imaging tool, enables physicians to look at many body systems, including the skeletal, digestive, urinary, respiratory, and reproductive systems. Fluoroscopy may be performed to evaluate specific areas of the body, including the bones, muscles, and joints, as well as solid organs such as the heart, lung, or kidneys.
Fluoroscopy is used in many types of examinations and procedures, such as barium x-rays, cardiac catheterization, arthrography (visualization of a joint or joints), lumbar puncture, placement of intravenous (IV) catheters (hollow tubes inserted into veins or arteries), intravenous pyelogram, hysterosalpingogram, and biopsies.
Fluoroscopy may be used alone as a diagnostic procedure, or may be used in conjunction with other diagnostic or therapeutic media or procedures.
In barium x-rays, fluoroscopy used alone allows the physician to see the movement of the intestines as the barium moves through them. In cardiac catheterization, fluoroscopy is added to enable the physician to see the flow of blood through the coronary arteries in order to evaluate the presence of arterial blockages. For intravenous catheter insertion, fluoroscopy assists the physician in guiding the catheter into a specific location inside the body.
Other uses of fluoroscopy include, but are not limited to, the following:
locating foreign bodies
viscosupplementation injections of the knees - a procedure in which a liquid substance that acts as a cartilage replacement or supplement is injected into the knee joint
image-guided anesthetic injections into joints or the spine
percutaneous vertebroplasty - a minimally invasive procedure used to treat compression fractures of the vertebrae of the spine
