In our previous blog, we outlined the importance of vacuum technology in a number of medical procedures and treatments. In part three, we discuss how vacuum technology is fundamental for medical analysis through MRI, Mass Spectrometers and Electron Microscopes.
Since the 1980s, MRI (magnetic resonance imaging) has become a powerful tool in diagnostics complementing the CT (computed tomography). While the latter uses X-rays for imaging, the MRI uses spin echoes from the H2O molecules in tissue which doesn't expose the body to radiation.
The patient must be brought into an external magnetic field. Almost all these magnets are superconducting, cooled at 4.2 K (- 269 C). These ultralow temperatures only can be maintained with vacuum insulation. That means each MRI magnet evacuated below 10-05 mbar before cooling down to cryogenic temperatures. This is done by TMP pumping stations. Due to the magnetic stray field the pumps cannot be operated while the magnetic field is on, but the large cold surfaces act as a cryopump and maintain the vacuum if there are no leaks. Maintenance of the MRI magnet and the cryocooler is even done under full magnetic field with non-magnetic tools without re-evacuation.
Image: 1 MRI scanner
Mass spectrometers are common tools used in the research and analysis of proteomics and genomics, forensics, drug development, food and water safety and homeland security. This family of different instruments is gaining in prominence now with the growing demand on medical research in proteomics in the field of virology. These instruments are found in the research lab, analytic labs and hospitals.
The various types of mass spectrometers ionize samples and analyse the compounds in one or several quadrupoles in series. The functional principle of the quadrupoles is the same as in residual gas analysers ('RGAs').
The ionisation of the molecules comes from the sample. Spray ionisation occurs in most cases in the 1 - 5 mbar region. A single stage rotary vane pump is enough for the gas inlet and a dry pump of the scroll or multistage roots principle is often used. The ions are accelerated into one or several analysis chambers. These chambers require high and, in some cases, even ultra high vacuum to reach the detector system. In most cases so-called multi-inlet or split-flow turbomolecular pumps are used. Rather than several discrete TMPs, the variants have several pumping ports for the different vacuum chambers of the instrument which saves cost and space. The design of these instruments is very compact to save footprint, as you can see in the inside view in Image 2.
Image 2: Mass Spectrometer inside view (Thermo Scientific Orbitrap TM Exploris TM).
Image 3: Mass Spectrometer ion path vacuum layout (Thermo Scientific Orbitrap TM Exploris TM)
Image 4: Mass Spectrometer complete instrument (Thermo Scientific Orbitrap TM Exploris TM)
Image 5 and 6: examples of special multi-stage TMPs for mass spectrometry (source: Edwards, Leybold)
Another instrument used for medical analysis and research is the Electron Microscope. Due to the relative long wavelength of light (380 - 780 nm) the resolution of light microscopes is limited to approximately 500 nm. Most bacteria are 1 - 10 um in size but the average size of viruses is 100 - 200 nm (Corona virus: 160 nm), so they cannot be seen with a light microscope. The resolution of electron microscopes can be as low as 0.5 nm! Electron microscopes use the wavelength of high energy electrons which is 1 - 100 nm.
Image 7: schematic of an electron microscope
To ensure the electrons pass from the electron gun through the lenses and the specimen without being scattered by air molecules the complete instrument must be under vacuum. To ensure a mean free path of a few meters a vacuum better than 10-04 mbar is needed, rising to 10-06 mbar in larger instruments. Vibration insulation is important to avoid the small structures in specimens becoming blurred. The first electron microscopes (1932, Ernst Ruska, Physics Nobel Price 1986) used oil diffusion pumps. Today the classic vacuum system is a turbomolecular pump (mechanical or magnetic bearing), a vibration insulator plus a 2-stage rotary vane pump or a dry scroll pump. For cost saving in most instruments there is no gate valve on top of the TMP - the specimens are inserted by venting the complete vacuum system. This requires fast acceleration and deceleration of the TMP.
In this blog series, we have shown that vacuum technology is present in many fields of medical research, treatment and diagnostics. Vacuum technology here underlies high safety and regulatory standards and maintenance schedules. In many cases the operators are not aware that they use vacuum and probably now understand the sound coming from the inside of the instrument.
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