Techniques for diffusion-weighted imaging of bone marrow
Introduction
In the last decades magnetic resonance imaging (MRI) has become an indispensable tool for clinical diagnosis. A major role in its success has been played by its highly non-invasive character and its unique capability to differentiate soft tissues. Tissues can be distinguished in MRI on the basis of their different structural and chemical properties, which determine their behavior in the presence of a magnetic field. For example, in MRI different tissues can be differentiated from each other due to their different magnetization relaxation times.
Other contrast mechanisms in MRI are also possible as well. Whenever tissue-dependent changes in the signal intensity are induced, a contrast mechanism can be established. Of course, the usefulness of a particular mechanism depends on its reliability and capability to diagnose diseases whose detection or diagnosis are difficult with the standard techniques. One method which has been shown to be of an exceptional diagnostic value is diffusion-weighted imaging (DWI). This technique is based on the tissue-depending signal attenuation caused by incoherent thermal motion of water molecules (Brownian motion). Changes of the motion of water molecules in biological tissue can be detected in various pathologic conditions. Initially, DWI was successfully applied in disorders of the brain. The first confirmation of its diagnostic possibilities was in the assessment of acute stroke, which can be detected by DWI within minutes after its onset [1], [2].
More recently, DWI has revealed great potential in diagnosing of bone marrow diseases. This technique has been proven to be a highly useful method for the differential diagnosis of acute osteoporotic and neoplastic vertebral compression fractures [3], [4]. DWI can distinguish between both types of fractures since the microscopic structure is altered in both cases in a different way. DWI of benign fractures shows iso- or hypointensity in comparison to normal bone marrow because of a higher water mobility in bone marrow edema. Conversely, compression fractures due to malignant tumors lead to hyperintensity related to the high cellularity of tumor tissue. Other applications of DWI of the bone marrow include the detection of infectious diseases [5], and the monitoring of therapy in patients with metastatic infiltration [6].
Unfortunately, the application of DWI to bone marrow is greatly prone to artifacts. For example, spin echo sequences are normally affected by motion artifacts mostly caused by physiological motions of the patient, such as cardiac and respiratory motion and the pulsation of the cerebrospinal fluid (CSF). These movements can result in severe ghosting artifacts, so that the diagnostic value is no longer given in most of the cases. Therefore, in order to preserve the image quality extra correction techniques or even the development of new sequences are required. One way to eliminate motion artifacts is to use a single-shot sequence, whose very short acquisition time normally underlies the time scale of human physiological motions. However, these sequences are affected by other artifacts, which can also limit their applicability to DWI, and sometimes make the redesign the sequences inevitable.
In this article the basic principles of DWI are summarized, and the most common sequences for DWI of the bone marrow are reviewed in detail. Special attention is paid to the explanation of the most common artifacts of each sequence type and to possible solutions how these artifacts might be overcome.
Section snippets
Physical principles of DWI
MRI in vivo is usually based on the coherent precession of the proton spins of water in an external magnetic field due to the abundance of hydrogen atoms in living organisms. An important point concerning the formation of the MRI signal is that by the presence of magnetic field gradients random movements of molecules containing hydrogen produce a partial annihilation of the precession coherence of the spins (so-called de-phasing), and thus a decrease of the signal intensity results.
Thermal
Diffusion-weighted sequences for bone marrow imaging
The great versatility and simplicity of the Stejskal-Tanner method made it possible to adapt numerous MRI sequences to DWI. However, the addition of extra diffusion gradients is not straightforward for all sequence types, and therefore is usually implemented for each sequence type in a different way. Furthermore, the presence of such gradients is normally associated with some undesirable effects that seriously degrade the image quality. The most common are the so-called motion artifacts, which
Conclusions
In this article we describe the different sequences that have been used for DWI of the bone marrow so far. The problems of each sequence type is analyzed carefully and their possible solutions are explained. Although the standard diffusion-weighted SE and TSE pulse sequences are seriously affected by motion artifacts, images suitable for clinical use can be obtained either by adding a correction technique such as the navigator echo, or by using new developed SE-based sequence (such as the k
Acknowledgment
The authors would like to thank Dr. Sonja Buhmann for providing the MR images of Fig. 4 and for critical reading of the manuscript. This work was supported by the Deutsche Forschungsgemeinschaft (DFG), Grant No. BA-2089(1-3).
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