Number 34, 2007
Picture my heart!

Obtaining cardiac images from positron emission tomography, computed tomography, and magnetic resonance imaging: physical principles

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Ronald Boellaard
Department of Nuclear Medicine & PET Research, VU University Medical Center, Amsterdam, The Netherlands
Correspondence: Ronald Boellaard, Department of Nuclear Medicine & PET Research, VU University Medical Center, Amsterdam, The Netherlands.
E-mail: r.boellaard@vumc.nl

Introduction
Positron emission tomography (PET) is a molecular imaging technique used to obtain functional or physiological information on activities such as glucose metabolism, perfusion, and cardiac function. In contrast, computed tomography (CT) and magnetic resonance imaging (MRI) provide structural (anatomical) information and are used to study parameters such as myocardial wall thickness, heart function, wall motion, and stress, or are used for the purposes of angiography. The present paper will provide a short introduction to the physical principles of PET, CT, and MRI.

Positron emission tomography
Positron emission tomography is a molecular imaging technique that measures the distribution of a radioactive tracer in vivo [5]. This radiotracer (or radiopharmacon) is a molecule labeled with a positron-emitting radioactive atom, such as fluorine-18, carbon-11, or oxygen-15. When very small amounts (pico- or nanomoles) of a radiotracer are administered to a patient, the tracer distributes over and within the organs. The radioactive atom of the radiotracer emits positrons, which combine with electrons after traveling a distance up to several millimeters in tissue. Each positron and electron pair is then converted into two photons, each having an energy of 511 keV, and these are emitted in opposite directions. PET image acquisition is based on the simultaneous (coincidence) detection of these two photons.
A PET scanner consists of many photon detectors surrounding the patient. During a PET scan, millions of coincidence detections are collected, providing information about the distribution of the radiotracer in tissues. Figure 1 demonstrates the principle of PET imaging. By making sequential images over time, it is possible to follow the uptake, retention, and washout of the tracer in tissue. This time course of the radiotracer or the distribution of a tracer at a certain period after its administration can be used to obtain functional (or physiological) information such as cardiac wall perfusion or glucose consumption [6–9].

Table I. Examples of some commonly used (cardiac) positron emission tomography radiotracers and their application.

Computed tomography
Computed tomography (CT) or computed axial tomography (CAT) measures the attenuation (absorption and scattering) of an X-ray photon beam through the patient, similar to X-ray imaging. In a CT system, an X-ray beam, generated using an X-ray tube, is rotated around the patient, providing information about photon attenuation through the patient from different angles (Figure 2). By combining measured attenuation profiles from these various angles it is possible to create a cross-sectional image of the distribution of attenuation coefficients. The attenuation coefficient indicates the amount of attenuation per centimeter of tissue, which for CT is expressed in Hounsfield units. Different tissue types (bone, blood, soft tissue, lungs) have different attenuation coefficients, and the measured distribution of these coefficients thus directly provides an anatomical cross-sectional image.

Table II. Examples of cardiac computed tomography (CT) applications and basic information derived from the CT image.

Magnetic resonance imaging
Magnetic resonance imaging makes use of the magnetic moment of protons (situated in the nucleus of each atom) [13]. In MRI, it is mainly the protons of water molecules that give rise to a detectable signal. These protons have a positive charge and they ‘spin’ (rotate). This ‘rotating’ charge causes a magnetic dipole moment; thus protons can be considered as ‘mini-magnets’. When protons are placed in a strong external magnetic field, the magnetic dipoles are aligned (parallel or antiparallel) with the external field (Figure 3). By using a radiofrequency wave, the magnetic dipoles can be ‘flipped’ into a plane transverse to the direction of the magnetic field.

Table III. Some applications of cardiac magnetic resonance imaging.

Structural versus functional (molecular) imaging
As explained above, CT and MRI provide detailed anatomical information, as opposed to PET, which can provide functional or molecular images (Table 4). CT and MRI are not (yet) able to provide quantitative hemodynamic or metabolic information. To assess perfusion quantitatively or to assess metabolic processes, PET is the method of choice. Recently, combined PET/CT systems have become available that allow the simultaneous acquisition of both structural and physiological information [16,17]. Integrated anatomical and functional (molecular) imaging seems very promising, and its potential additional clinical value will be assessed in the near future [16].

Table IV. Physical principles of image generation.

Limitations of current overview
The purpose of this paper was to present a short introduction to the physical principles of image acquisition with PET, CT, and MRI. Sophisticated CT and MRI scanning techniques, such as MRI tagging, have not been addressed and are beyond the scope of this paper. Moreover a short, but not complete, list of some of the main applications of these imaging techniques is given. Interested readers are encouraged to read the review and educational papers listed in the reference list for a more detailed description of the physical principles of the imaging modalities [5,12,13].

Acknowledgements
Hedy Folkersma is thanked for providing illustrations used to make the figures, and Petra Pouwels for reviewing the paper.

REFERENCES