Diffusion weighted imaging of bone marrow pathologies
Introduction
Systemic and focal diseases frequently affect bone marrow resulting in marrow alterations, which were prior to the availability of magnetic resonance imaging (MRI) difficult to diagnose. The introduction of MRI revolutionized bone marrow imaging because this technique provides images with excellent soft tissue contrast [1], [2], [3].
In adults the bone marrow is in general composed of fatty tissue, which appears hyperintense on T1 and T2 weighted images and occurs hypointense on MR-sequences with fat-satturation [4], [5]. In case of trauma, tumor or infection infiltration, replacement and depletion of fatty bone marrow takes place resulting in intermediate to hypointense signals on T1-weighted images and hyperintense signals on STIR (short-tau inversion recovery) and T2 weighted images with fat-saturation. These changes obscure the distinct appearance of fatty bone marrow and serve as early indicators of pathology, which makes this imaging technique a very sensitive diagnostic tool [1], [6], [7], [8], [9].
In spite of its high sensitivity, MRI is of only limited specificity in the evaluation of bone marrow alterations, because routine MR-sequences are in general not suitable to differentiate between their causes. The limited specificity of MRI in patients with bone marrow alterations requires additional, sometimes invasive diagnostic steps to obtain accurate diagnosis, which is mandatory to apply adequate therapy [5], [9], [10], [11], [12].
Diffusion weighted imaging (DWI) is an imaging technique which probes the structure of a biologic tissue [13], [14], [15], [16], [17], [18]. The contrast achieved with this technique is considered to be tissue specific and can be used for non-invasive tissue characterization. Thus, DWI seems to have the potential to increase the specificity of MRI in characterizing bone marrow alterations.
In this article, we will focus on the potential of DWI in evaluating bone marrow pathologies, we will discuss its strengths and its pitfalls, and we will give an overview of recent reports in this field.
Section snippets
Diffusion weighted imaging
DWI is a non-invasive imaging technique, which is suitable for probing the physical structure of a biologic tissue at a microscopic level well below the typical millimeter-scale resolution of MRI [18], [19], [20], [21]. DWI exploits the random, translational motion of water protons in a biologic tissue, which reflects the tissue specific diffusion capacity and can be used for tissue characterization.
Brownian motion generates random translational motion of water protons within a tissue. This
Diffusion weighted sequences for evaluating bone marrow
DWI is a well known diagnostic tool to evaluate CNS pathologies. For many years the use of DWI was limited to the brain because the image quality of DWI of non-CNS-tissue was inferior. Physiologic motion and the challenging magnetic environment outside the brain made it difficult to achieve DWI with sufficient image quality within a reasonable acquisition time. Within the last years advances in the software and hardware-sector of MRI improved image quality considerably and led to several
DWI—bone marrow
Healthy vertebral marrow contains 20–70% fat with an estimated 7% increase per decade of life [50], [51], [52], [53], [54]. The diffusion capacity of fat is reported to be low resulting in decreased ADC values in healthy vertebrae [55]. Reported values are 0.15–0.59 mm2/s, which are considerably lower compared to other benign tissues, such as muscle (ADC: (1.2–1.7) × 10−3 mm2/s) or liver ((1.38–1.87) × 10−3 mm2/s) [9], [39], [40], [45], [56], [57]. These low ADC values are not compatible with the
DWI—malignancy-trauma
With the infiltration of tumor cells the lipid component of fatty bone marrow will diminish resulting in a natural contrast on T1-weighted images. Although this contrast is very sensitive its specificity is limited, because benign edematous processes, such as infection or trauma may produce similar signal intensity changes on routine MRI, which may cause diagnostic problems [25], [26]. However, accurate differentiation between a benign edematous process and a malignant infiltration is essential
DWI—therapy monitoring
The cellular structure of a tumor is considered to be an indicator of tumor aggressiveness, and to influence the response to tumor therapy [62]. Currently, these features can only be evaluated from invasively obtained specimen [63]. DWI is considered to be a potential tool for monitoring treatment effects by exploiting the structural differences of biologic tissue. DWI depicts differences in diffusion and in membrane integrity between viable and necrotic tumor and thus, may be used to monitor
DWI—infection
The usefulness of DWI in the evaluation of osteomyelitis is still a matter of controversy. Several authors reported that in tuberculous spondylodiscitis restricted diffusion is present, resulting in increased signal intensities on DWI [66], [67]. Consecutively, low ADC values ((0.94–1.02) × 10−3 mm2/s) were reported, mimicking malignancy [39], [68], [69]). Pui et al. reported that the ADC values of pyogenic spondylodiscitis are not significantly different than the ADCs from tuberculous
Conclusion
DWI has proven to be a powerful tool for evaluating bone marrow infiltration. Quantitative diffusion measurements seem to be highly specific and very sensitive to differentiate between malignant and benign skeletal processes by probing the microstructure of a biologic tissue. Changes of the microstructure result in alterations of the diffusivity of a tissue, which in turn may provide non-invasively, acquired information for therapy monitoring. However, one has to take into account that in
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