The Basics of MRI

Chapter 1

INTRODUCTION

NMRI or MRI ?

Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The technique was called magnetic resonance imaging rather than nuclear magnetic resonance imaging (NMRI) because of the negative connotations associated with the word nuclear in the late 1970's. MRI started out as a tomographic imaging technique, that is it produced an image of the NMR signal in a thin slice through the human body. MRI has advanced beyond a tomographic imaging technique to a volume imaging technique. This package presents a comprehensive picture of the basic principles of MRI.

Before beginning a study of the science of MRI, it will be helpful to reflect on the brief history of MRI. Felix Bloch and Edward Purcell, both of whom were awarded the Nobel Prize in 1952, discovered the magnetic resonance phenomenon independently in 1946. In the period between 1950 and 1970, NMR was developed and used for chemical and physical molecular analysis. In 1971 Raymond Damadian showed that the nuclear magnetic relaxation times of tissues and tumors differed, thus motivating scientists to consider magnetic resonance for the detection of disease. In 1973 the x-ray-based computerized tomography (CT) was introduced by Hounsfield. This date is important to the MRI timeline because it showed hospitals were willing to spend large amounts of money for medical imaging hardware. Magnetic resonance imaging was first demonstrated on small test tube samples that same year by Paul Lauterbur. He used a back projection technique similar to that used in CT. In 1975 Richard Ernst proposed magnetic resonance imaging using phase and frequency encoding, and the Fourier Transform. This technique is the basis of current MRI techniques. A few years later, in 1977, Raymond Damadian demonstrated MRI of the whole body. In this same year, Peter Mansfield developed the echo-planar imaging (EPI) technique. This technique will be developed in later years to produce images at video rates (30 ms / image). Edelstein and coworkers demonstrated imaging of the body using Ernst's technique in 1980. A single image could be acquired in approximately five minutes by this technique. By 1986, the imaging time was reduced to about five seconds, without sacrificing too much image quality. The same year people were developing the NMR microscope, which allowed approximately 10 mm resolution on approximately one cm samples. In 1987 echo-planar imaging was used to perform real-time movie imaging of a single cardiac cycle. In this same year Charles Dumoulin was perfecting magnetic resonance angiography (MRA), which allowed imaging of flowing blood without the use of contrast agents. In 1991, Richard Ernst was rewarded for his achievements in pulsed Fourier Transform NMR and MRI with the Nobel Prize in Chemistry. In 1993 functional MRI (fMRI) was developed. This technique allows the mapping of the function of the various regions of the human brain. Six years earlier many clinicians thought echo-planar imaging's primary applications was to be in real-time cardiac imaging. The development of fMRI opened up a new application for EPI in mapping the regions of the brain responsible for thought and motor control. In 1994, researchers at the State University of New York at Stony Brook and Princeton University demonstrated the imaging of hyperpolarized 129Xe gas for respiration studies. MRI is clearly a young, but growing science.

Tomographic Imaging

This computer based teaching package will provide you with an understanding of the principles of MRI form both the microscopic, macroscopic, and imaging system perspective. Let's begin with a pictorial introduction to some basic MRI. Magnetic resonance started out as a tomographic imaging modality for producing NMR images of a slice though the human body. Each slice had a thickness (Thk). This form of imaging is in some respects equivalent to cutting off the anatomy above the slice and below the slice. The slice is said to be composed of several volume elements or voxels. The volume of a voxel is approximately 3 mm3. The magnetic resonance image is composed of several picture elements called pixels. The intensity of a pixel is proportional to the NMR signal intensity of the contents of the corresponding volume element or voxel of the object being imaged.

Magnetic resonance imaging is based on the absorption and emission of energy in the radio frequency range of the electromagnetic spectrum. It is clear from the attenuation spectrum of the human body why x-rays are used, but why did it take so long to develop imaging with radio waves, especially with health concerns associated with ionizing radiation such as x-rays? Many scientists were taught that you can not image objects smaller than the wavelength of the energy being used to image. MRI gets around this limitation by producing images based on spatial variations in the phase and frequency of the radio frequency energy being absorbed and emitted by the imaged object.

Microscopic Property Responsible for MRI

The human body is primarily fat and water. Fat and water have many hydrogen atoms which make the human body approximately 63% hydrogen atoms. Hydrogen nuclei have an NMR signal. For these reasons magnetic resonance imaging primarily images the NMR signal from the hydrogen nuclei. Each voxel of an image of the human body contains one or more tissues. For example here is a voxel with one tissue inside. Zooming in on the voxel reveals cells. Within each cell there are water molecules. Here are some of the water molecules. Each water molecule has one oxygen and two hydrogen atoms. If we zoom into one of the hydrogens past the electron cloud we see a nucleus comprised of a single proton. The proton possesses a property called spin which:

  1. can be thought of as a small magnetic field, and
  2. will cause the nucleus to produce an NMR signal.

Not all nuclei possess the property called spin. A list of these nuclei will be presented in Chapter 3 on spin physics.

Problems

  1. What is the abbreviation for slice thickness in MRI? 

  2. What is the smallest information element of a picture? 

  3. The tomographic image plane contains many small volume elements called? 

  4. The two transmission windows for electromagnetic energy through the human body are in which regions of the spectrum? 

  5. Most MRI looks at NMR signals from what nuclei?  

  6. Who first demonstrated MRI using the backprojection technique?  

  7. Who first demonstrated MRI using the Fourier technique?  

  8. Who showed that the nuclear relaxation times of healthy tissues differed for tumors, and later produced a whole body MRI?  

  9. What two types of molecules are responsible for the signal in MRI?  

  10. What is meant by a tomographic image?  

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