الفهرس 
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Abstract
A neck mass is a common finding and can present a difficult diagnostic challenge. Neck masses can be divided into three main categories: tumour and tumour like lesions, cystic and inflammatory lesions. A variety of imaging techniques (like US, FNAUS, CT and MR) can help in characterization of neck masses, but their performance remains, however, unsatisfactory. Diffusion-weighted imaging (DWI) is a noninvasive MRI technique that provides image contrast dependent on the molecular motion of water. Therefore, DWI provides in characterization of different tissues and lesions.
The developments of echo-planar imaging (EPI), high gradient amplitudes, multichannel coils, and parallel imaging have been instrumental in extending the applications of DWI and increasing its use for the evaluation of extracranial diseases. The extent of translational diffusion of molecules measured in the human body is referred to as the apparent diffusion coefficient (ADC). The ADC is expected to vary according to the cellular density of the lesion. Diffusion weighted imaging is successfully used in detection of tumours. Tumours are frequently more cellular than the tissue from which they originate, thus appear as high signal intensity (restricted diffusion) at DWI. Metastases appear as high-signal-intensity foci at DWI.
The Diffusion weighted image can be used in differentiation between benign and malignant lesions. The mean ADC value of benign solid lesions is significantly higher than that of malignant tumors. The differences in ADCs may reflect distinct differences in the histopathologic features of the benign and malignant tumors.
Malignant tumours treatment modalities can produce edema, fibrous inflammatory reaction, and scarring of the adjacent normal soft tissues. On routine MR imaging, residual or recurrent lesions and treatment-induced changes show similar imaging characteristics and are, therefore, difficult to distinguish.
By using the Diffusion weighted MRI, the mean ADC values for residual or recurrent tumors are significantly lower than that for posttreatment changes. Residual or recurrent lesions appeared as areas of low signal intensity on the ADC map, while, post-treatment changes appeared as areas of high signal intensity.
Also diffusion weighted imaging is helpful for predicting treatment response, as cellular tumor with low baseline pretreatment ADC values respond better to chemotherapy or radiation treatment than tumors that exhibit high pretreatment ADC values.
Diffusion-weighted MRI with ADC map can help the differentiation of non-necrotic malignant from benign lymph node, delineation of the solid viable part of the lymph node and even allows detecting small adenopathies.
Metastatic and lymphomatous nodes appear hyperintense on DWI and hypointense on ADC maps; adversely inflammatory nodes are hypointense on DWI and hyperintense on ADC maps. The addition of diffusion-weighted MRI to routine MRI provides additional useful physiological and functional information regarding characterization of the cervical lymph nodes.
The MRI criteria for metastatic lymph node include enlarged size of minimal axial diameter of 8 to 9 mm in level II and 7 to 8 mm for the rest of neck, shape tend to be spherical, extracapsular tumor spread and abnormality of the internal architecture.
In Diffusion weighted imaging, the presence of necrotic areasgreatly affected the ADCs of metastatic nodes, resulting in large variability in ADC values in the affected node.
All MRI criteria for nodal metastasis detection are helpful for characterization of nodal metastasis. However, ADC values and abnormal signal intensity on T1- and T2-weighted images are the most reliable and significantly correlated to metastatic nodes.
The Diffusion weighted MRI can differentiate between nodal Squamous cell carcinoma and lymphoma. The ADC values of lymphoma were significantly lower than those of SCC.
DWI even differentiates between different grades of SCC. The ADC values were significantly greater in the well and moderately differentiated squamous cell carcinomas than in the poorly differentiated squamous cell carcinomas.
Also DWI is useful in soft tissue tumours differentiation, in where the ADC of myxoid tumours is significantly higher than that of nonmyxoid tumors, because of the high mucin and low collagen contents of mexoid tumours.
DWI also differentiates paraganglioma from other soft tissue tumours of paraganglioma like criteria, in where the mean ADC of paraganglioma is higher than the paraganglioma like tumours this might be due to the vascular architecture of paraganglioma.
Diffusion weighted MRI with ADC map can be used in differentiation between cystic and solid lesions, as the ADC value of cysts is generally markedly higher than that of benign and malignant tumours. It can also differentiate between different types of cysts and between cysts and neoplastic or inflammatory necrotic lesions depend on its contents of protein. The higher protein component in the fluid will increase the viscosity and decrease the water proton mobility.
So the mean ADC of the benign cystic lesions is markedly higher than both infammatory and tumoural necrosis and the ADC of the tumoral necrotic lesions is markedly higher than that of infective necrotic lesions (abscess and lymphadenitis), because in cystic or necrotic tumors, the amount of the inflammatory cells and debris is much less than the abscess cavity.
In conclusion:
Diffusion weighted Magnetic resonance imaging proved to have a useful role in diagnosis of neck masses, where it can be used in differentiation between benign and malignant solid and cystic lesions, predicting and monitoring malignant tumours treatment and used successfully in detection and staging of nodal metastasis and differentiating it from non specific lymphadenopathy.
It also helped in tissue characterization and differentiation between benign cystic lesions, inflammatory and malignant necrosis. So it can be valuable as a routine MRI sequence especially in evaluation of undiagnosed neck mass or follow up studies for malignant lesions.