BACKGROUND ON DWI -
Diffusion is the net movement of molecules or atoms from a region of high concentration with high chemical potential to a region of low concentration with low chemical potential.
Diffusion-Weighted Imaging (DWI) is used to evaluate the random, thermally-induced, microscopic water motion within a tissue, well known as Brownian motion.
DWI is a powerful tool in the head and neck MR Imaging because it provides information about tissue microarchitecture (fig. 1), which is very important for the detection, evaluation and characterization of the lesions (e.g. neoplasm, inflammation).
Due to its speed, chemical fat suppressed single-shot echo-planar imaging (SS-EPI) technique is typically used to acquire clinical diffusion-weighted images. Nevertheless, SS-EPI STIR technique is the preferred choice on head and neck DW Imaging, because it offers robust fat suppression and reduces the chemical shift artifacts and image distortions (fig. 2).
Fig. 1 Diffusion in normal and cancer tissue. Cancer tissue is characterized by hypercellularity, which causes diffusion restriction. DWI can help in the detection, evaluation and differentiation of the tissues and lesions.
Fig. 2 illustrates the limitations of the conventional chemical fat suppressed SS-EPI DWI technique in challenging body regions, such as head and neck. Chemical fat suppressed SS-EPI DWI with b800 (left), SS-EPI DWI STIR with b800 (middle), and 18F-FDG PET/CT imaging on a patient with tongue cancer (blue circle). Left image suffers from severe geometric distortions, chemical shift artifacts and heterogeneous fat suppression. Image dataset was acquired at 1.5 Tesla. The TI value for DWI STIR acquisition was 180 Msec.
CLINICAL APPLICATIONS -
DW Imaging allows the detection and characterization of head and neck masses in adults (fig. 3) and children, it is helpful for the differentiation between recurrent tumor and post-therapeutic changes and it can be used for cancer staging. Moreover, the quantitative analysis of the ADC map (which is expressed in mm2/s), can be very useful in the accurate characterization of lesions and differential diagnosis between benign and malignant tumors.
Finally, a number of studies have shown that DW Imaging is mostly a complementary technique at the primary site for tumor diagnosis in the head and neck, but it is a very promising technique for the characterization and differentiation between neoplastic and non-neoplastic lymph nodes (fig. 4).
Fig. 3 MR Imaging of the head and neck on a female patient with left parotid gland tumor. DWI is a powerful tool in head and neck MR Imaging because it provides information about tissue biology and microarchitecture, assisting in detection, evaluation and characterization of the lesions. Image dataset was acquired at 1.5 Tesla.
Fig. 4 61-year-old male patient with tongue cancer. RS-EPI DWI provides excellent detection and characterization of the lymph nodes, both in axial and sagittal plane. Normal lymph nodes are demonstrated in this particular case. In addition, susceptibility artifacts and heterogeneous fat suppression are shown here, due to the presence of dental restoration material. Image dataset was acquired at 1.5 Tesla. Images courtesy of Bac Nguyen.
LIMITATIONS AND NEWER TECHNIQUES -
Single-shot echo-planar imaging (SS-EPI) is the technique typically used to acquire clinical diffusion-weighted images. However, performing SS-EPI DWI acquisitions in the head and neck area is particular difficult, because it is a very heterogeneous region, containing a variety of tissues that include fat, muscle, air, glandular tissue and bone. For this reason, standard chemical fat suppressed SS-EPI suffers from susceptibility artifacts that cause geometric distortions, signal intensity dropouts, signal heterogeneity, ghosting and image blurring and make DWI difficult to be interpreted. SS-EPI STIR technique is a good alternative, especially on old scanners with non-updated software, however, this technique cannot be used for ADC map quantification because it suffers from T1 contamination (STIR is an inversion recovery technique).
Multi-shot Read-out Segmented Echo-Planar Imaging (RS-EPI) and Echo-Planar Imaging with reduced Field-Of-View (rFOV-EPI) are new, revolutionary techniques for obtaining high-quality, distortion-minimized diffusion-weighted images of challenging body regions, such as the head and neck area, in clinically acceptable acquisition times (fig. 5).
RS-EPI uses the same diffusion preparation as conventional SS-EPI, however, the k-space trajectory is divided into multiple segments in the readout direction, which allows the reduction of the echo time (TE) and inter-echo spacing (ESP), increasing the image quality. Reduced-FOV EPI method uses 2D dynamic excitation pulses in phase encoding (PE) and slice-select (SS) directions, in order to achieve selective FOV imaging. This implementation decreases the required number of k-space lines in the PE direction, enabling high resolution DW Imaging with reduced image distortions and susceptibility artifacts, and without wrap-around artifacts.
Fig. 5 73-year-old male patient with right parotid gland tumor. Coronal RS-EPI DWI b1000 provides higher image quality with less distortions compared to the SS-EPI DWI b1000 technique. The size and the shape of the tumor are well defined on RS-EPI images. Image dataset was acquired at 3.0 Tesla. Images courtesy of Bac Nguyen.
Diffusion-weighted MR Imaging in the head and neck area has a wide variety of clinical applications with a special focus on oncology.
Standard chemical fat suppressed SS-EPI technique is very difficult to be used in this region due to magnetic field inhomogeneities. SS-EPI STIR technique should be used in this anatomical region, especially on old systems with non-updated software.
RS-EPI and rFOV-EPI are new techniques which provide outstanding balance between imaging speed and quality compared to SS-EPI and make feasible the high resolution diffusion-weighted MR Imaging of the head and neck.
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