Medical Imaging 101 pt 1

My circled friends keep asking me to post a tutorial about what each modality can and can’t do. So today for ScienceSunday , here is part 1 of what I plan to be a series of medical imaging tutorials. I’ll start out by listing the main players and then follow up each week with a detailed post about each modality. Some of them require a contrast agent or exogenous component. A contrast agent is a compound that is given either orally or intravenously to enhance the image.

Here’s the list: x-ray CT, MRI, PET, SPECT, Ultrasound, and optical. I’ll throw in EPRI as a bonus even though it is experimental (relatively speaking).

X-ray computed tomography (CT) uses a series of 2D x-ray images. Computer algorithms (hence the computed part) reconstruct the 2D projections (as they are called) into 3D images. The method is called Filtered-back projection (FBP) reconstruction. CT is best for visualizing bone but can be used for soft-tissue imaging in humans, especially with a contrast agent. It can have high spatial resolution but suffers from poor sensitivity.

Magnetic Resonance Imaging (MRI) MRI is the imaging version of nuclear magnetic resonance (NMR). Although MRI can image different nuclei, most MRI are tuned for protons. Those protons are predominantly from water. A large magnetic field and magnetic gradients are used to generate frequency data, called k-space. Unlike CT a simple Fourier transform is used to reconstruct the image from the frequency data. MRI is great for soft-tissue imaging. Bone, having very little fluid, is dark, i.e., without signal, in MRI. There are functional images that can be generated from MRI with and without contrast agents. It has high relative spatial resolution and moderate sensitivity.

Positron Emission Tomography (PET) requires a radionuclide. It detects pairs of gamma rays. Most pre- and clinical PET scanners include a CT scanner for anatomic reference. Its main use is with 18-F labeled fludeoxy glucose, which is an analog of glucose. FDG gets trapped in highly metabolically active cells. Therefore, PET is good at detecting tumors as they are relatively more metabolically active relative to say muscle. The raw data is somewhat in between CT and MRI. Coincidence events, i.e., the two gamma rays, are detected. The reconstruction groups these coincidence events into projections similar to CT but unlike CT the project cannot be used alone to visualize anything. The projections are called sinograms and they are used to generate 3D images. It has high sensitivity but relatively low spatial resolution.

Single Photon Emission Computed Tomography (SPECT) is another technique that requires a radionuclide. For those familiar with autoradiography, it uses similar radioisotopes. Unlike PET, SPECT tracers are most often a radioisotope linked to a ligand. Like PET, SPECT scanners often have a CT on board, for the same reason mentioned. Like CT, SPECT acquires a series of 2D projections, that are reconstructed into a 3D image. PET images two gamma rays and infers the position of the radionuclide. SPECT images the radionuclides directly. SPECT has even lower resolution than PET but has equal and sometimes better sensitivity.

Ultrasound (US) is an imaging technique that uses sound waves with a frequency above the human audible range, hence ultra. Most are probably familiar with US due to its use during pregnancy. As you can image, it is good for imaging soft-tissue. It’s real strengths are portability, relatively low cost, and no radiation. It does have relatively poor sensitivity, poor spatial resolution, and poor contrast. Gas bubbles can be used for enhanced contrast. Also, US can be used for therapy via thermal ablation.

Optical imaging in medical imaging is either fluorescence (FI) or bioluminescence imaging (BLI). A near-field image (think standard photograph) is used for anatomic reference. Some newer systems are starting to use CT with optical. BLI relies on light emitting organisms. The most common technique is to genetically engineer an organism/cell to be bioluminescent via introduction of luciferase (think firefly). FL requires tagging a ligand with a fluorophore. Optical imaging is typically 2D but there are some systems that are giving either 3D or “synthisized” 3D. Optical imaging has high sensitivity and relatively poor resolution. Its strength is relatively low cost, small size, and ability to visualize cell signaling.

Electron Paramagnetic Resonance Imaging (EPRI) is like NMR but detects unpaired electrons instead of nuclei (protons). It requires an exogenous contrast agent. It has a long history in spectroscopy like NMR but is relatively new as an imaging modality. It uses a relatively low magnetic field and gradients. Unlike MRI, it typically collects projections, 2D sinograms like PET, and FBP to reconstruct. It’s strength is the quantitative physiological environments that it can image, e.g., oxygen, pH, etc. It has relatively low resolution (on the order of PET) and high sensitivity.

Images below

The first image is a visual example of the machines and images they can produce. The second image helps understand the scale of what medicine is interested in. The third image is a graphic for sensitivity vs. spatial resolution. The final image is a table comparing the techniques.

Edit Next week will be a detailed post on CT, followed by MRI, and so on. Please post questions and they will be answered either here or in the appropriate subsequent post.

Edit 2 I’m only interested in 3D imaging. If people are really interested in 2D techniques like fluoroscopy or x-ray, radiographs, let me know. I’ll consider discussing them.

#CHMedicalImagingSeries

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ScienceSunday curated by Allison Sekuler Rajini Rao and Robby Bowles and guest curator (me, Chad Haney )