Imaging techniques

Modern medicine continues to rely on imaging to aid in diagnosis and intraoperative monitoring. Beginning with the discovery of X-rays, imaging has revolutionized medicine and made modern procedures possible.

In this section we will review a few of the most popular imaging techniques, and just touch on the physics that makes them work. Knowledge of these basics will help you better comprehend and interpret medical imaging in general.

X-ray imaging

In an X-ray source, high-energy electrons crash into a metal target. The sudden deceleration of the electrons produces electromagnetic waves that can penetrate soft tissue. An X-ray image is the "shadow" cast by the patient's internal structures onto an X-ray detector; the more dense the structure, the greater the absorption of the X-rays. Thus X-rays are a common diagnostic protocol for bone diseases.

Initially, photographic film was used to detect the X-rays, but current machines use solid-state detectors that require much lower dosage levels.

Computed tomography (CT)

A CT image is also created using X-rays, but now the emitter-detector system is revolving around the patient, imaging "slices" of the body.

This produces data about the patient's transverse density profile taken from many different angles. A powerful computer program then analyzes these data slices and reconstructs a computerized three- dimensional image of the patient.

Here we see a commercial CT scanner in a clinical setting with a patient in the bore.

This is an anterior X-ray of the patient. The following images were reconsructed from the transverse CT scans (think viewing a bread loaf, slice by slice). Note the metal screws and plates fixating L4-L5 and L5-S1. A mild pre-existing scoliosis is also apparent.

The transverse scans begin cephalid and progress caudally. NOTE: the view is the inferior aspect, or looking "up" towards the head.

Magnetic resonance imaging

MRI is a fairly recent technique that does a much better job of differentiating the body's soft tissues than other previous imaging techniques. Although an MRI machine resembles a CT scanner, MRI does not use X-ray radiation.

Inside the housing, there is a large coil of superconducting wire. On installation, electric current is run through the coil until a strong magnetic field is produced in the open core. Then the current source is disconnected and the superconducting coil will produce the magnetic field forever using zero additional power. A 3T MRI machine in clinical setting. T is for Tesla, a measure of magnetic field strength. The Earth's magnetic field is 0.00005T.

Sound too good to be true? The catch is that the coil must be kept VERY cold. The coil is  immersed in liquid helium (-270°F).  To help keep the helium cold, an insulated container of liquid nitrogen (-196°F) encloses the liquid helium and the coil.

The theory of operation is inherently very complex, but here is the quick version: Each type of tissue contains a different density of water, H2O. Each molecule of this water contains two hydrogen atoms, each of which has a single proton as its nucleus. A proton has a quantum property that makes it act like a tiny magnet. (animation)

Inside the MRI machine, the tiny proton magnets align with the machine's magnetic field. If you could flick the proton with your finger, it would vibrate at a specific frequency and gradually realign with the external field, all the while producing an electro-magnetic signal. The electromagnetic signal is produced by this relaxation of the tiny vibrating proton magnets.

In reality, the machine pulses the protons with an electromagnetic wave instead of a finger flick, and the relaxation signals from the protons are detected by another set of coils. These signals are processed by a very sophisticated computer program to reconstruct the 3D distribution of water in the body.

The computer creates image "slices" of the body that clearly differentiate tissue by the relative amount of water in the tissue. Actually, there are TWO different relaxation signals called T1 and T2.  In water T1 and T2 are identical, but in tissue T2 is faster. Thus you may see "T1 weighted" or "T2 weighted" related to MRI images.

These realxation time differences can be used to enhance image contrast.  T1 better differentiates between gray and white matter in the central nervous system. T2 is favored for other tissue and cerebrospinal fluid flow.

Discography

A discogram is an enhanced X-ray or CT examination of the intervertebral disks. It is used to evaluate the integrity of the disk. A needle is inserted into the nucleus pulposus. A radio-opaque dye (contrast agent) is then injected to make the disk's image more distinct. This can help determine whether the nucleus pulposus has herniated and the extent of the herniation.

If applying pressure with the injection fluid produces an increase in the symptomatic pain, this is more evidence that the pain is related to a herniated disk. The injection fluid also contains antibiotics as a preventative against infection.

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