医疗城 · 设备实验室 · 第 46 台，共 69 台
双能 X 线骨密度仪 (DEXA)
DEXA Scanner
BMEssentia 医疗城设备实验室中的双能 X 线骨密度仪 (DEXA)。以可交互 3D 打开设备，逐个部件拆解，了解其工作原理，然后在 BMEssentia 平台上系统学习，并向 Essentia 提问。
本页的设备实验室内容为英文，与医疗城中的内容一致。
- 10 个部件
- 设备实验室中有 2 种配置
- 又称: Dxa, Bone Densitometer, Bone Density Scanner
设备实验室
DEXA Scanner
A low table with a C-arm inside it: the tube in the base below the tabletop, a single detector on an arm above the patient, and the two joined so they move as one object. The arm travels the length of the table and the detector head travels across it, so the beam visits the region point by point on a raster. A circular aperture a millimetre across defines the beam.
配置
设备实验室中的各个版本。
Pencil-beam raster scanner with a single detector
A low table with a C-arm inside it: the tube in the base below the tabletop, a single detector on an arm above the patient, and the two joined so they move as one object. The arm travels the length of the table and the detector head travels across it, so the beam visits the region point by point on a raster. A circular aperture a millimetre across defines the beam.
Narrow fan-beam scanner with a linear detector array
The same low table and the same C-arm, with a slit instead of a pinhole and a linear array of a hundred and twenty-eight elements instead of one detector. The arm travels the length of the table once and the fan covers the width, so the region is acquired in a single sweep. The array carries its own channel gain and geometric calibration, because a hundred and twenty-eight detectors that do not agree with each other draw stripes.
工作原理
在设备实验室中探索。
The determinant, and what happens as it shrinks
Harden the high-energy pulse and watch four numbers move together: the two effective energies, the bone-to-soft ratio at each of them, the determinant, and the standard deviation of the areal density at a fixed number of counted photons.
Where the independence actually comes from
Compute the determinant twice: once with the real attenuation coefficients, and once with only the Compton part of each of them.
The third unknown there is no equation for
Set the fat fraction the analysis assumes and the fat fraction the patient actually has to different values, and watch the areal bone density move while nothing about the bone changes.
A density that is not a density
Take one bone, scale it up by a fifth in every dimension without changing what it is made of, and watch what the machine reports.
The smallest change that means anything
Set the counts per measurement, read the counting floor the physics gives, then set the department’s own measured precision error beside it and read the least significant change.
部件
逐个部件讲解。
X-ray tube, stationary anode and oil bath
A small cylinder lying on its side in the bottom of the table, under the patient, with its port facing upwards. It is a fraction of the size of the tube in a radiography room and it runs at a couple of milliamperes rather than hundreds, because a densitometry scan is minutes of continuous output rather than a flash of milliseconds.
Switched high-tension supply and potential control
A small oil-filled tank and two circuit boards in the base of the table, well under a kilowatt in total. The part that makes this a dual-energy machine rather than an X-ray machine is one of those boards: it alternates the accelerating potential between two values, many times a second, for the whole length of the scan.
Beam filtration and energy-separation assembly
A metal disc a hundred and ten millimetres across sitting directly over the tube port, turning in step with the potential switch. Four stations are cut into it: a light absorber for the low pulse, a much heavier one for the high pulse, a reference absorber of known attenuation, and one station left open.
Low-attenuation scanning tabletop and pad
A flat panel a little over half a metre above the floor with a thin pad on it, both of them directly in the beam on every single measurement. The table is low because patients who come for this examination are often elderly and have to get onto it unaided, and the panel is carbon-fibre composite rather than the aluminium a radiographic table can afford.
Positioning aids: knee block, foot separators and arm rests
Three foam blocks and a pair of radiolucent supports on a rack on the side of the machine. They look like the least technical objects on the device and they are worth more to the precision of the result than any electronic component on it.
C-arm scanning carriage and longitudinal drive
A rigid C lying on its side inside and above the table, with the tube in its lower limb below the tabletop and the detector in its upper limb above the patient. The whole C travels the length of the table on a rail, driven by a belt and a motor, and the fact that it is one object rather than two is the most important thing about it.
Position encoders and the scan-step datum
A fine scale down the length of the base with a read head on the carriage, a datum flag near one end, and on the raster machine a second encoder on the screw that drives the detector head across the table. Small, cheap, and on one of the two configurations it is the component that supplies the denominator of the answer.
Detector signal chain and energy discrimination
A small crate of boards behind the column: a preamplifier, a shaping stage, a discriminator and a counter, in that order. What it produces is not a current and not an image.
Workstation, bone-edge detection and the analysis region
A computer on a desk beside the table with two screens: a portrait one showing the scan and a landscape one showing the report. It is where the two integers per measurement become a number somebody acts on, and it is responsible for more of the variation in that number than anything electrical on the machine.
Calibration phantom and the daily quality-control record
A block of tissue-equivalent plastic on a bracket on the side of the machine with steps of bone-equivalent material of known mineral content set into it. Every result the department produces has its scale established by this object, and by nothing else.