Role of diffusion weighted imaging in head and neck lesions

Introduction:

Diffusion weighted imaging (DWI) was commonly used for detecting acute stroke in patients. Currently its extracranial applications are on the rise. Currently DWI is being used for evaluation of brain tumors, head and neck lesions and several other cancers of the body. Technical advances in MRI imaging have managed to overcome the limitations of DWI. DWI uses software to generate images using diffusion of water molecules to generate contrast in MR images. It maps the diffusion process of molecules, mainly water in biological tissues. Because of its noninvasive nature it is becoming popular.

In DWI the intensity of each image element (voxel) reflects the estimate of the rate of water diffusion at that location. Since mobility of water is driven by thermal agitation and is highly dependent on cellular environment, it is very sensitive in estimating early changes after a stroke than traditional T1 / T2 weighted MR image sequences.

Some of the uses of DWI include:

1. Tissue characterization for primary tumors and nodal metastasis

2. Differentiation of suppurative from necrotic metastatic adenopathy

3. In predicting and monitoring treatment response of various tumors of head and neck

4. Differentiating recurrent tumor from post therapeutic changes

5. In staging non-small cell lung cancer.

The acquisition time for DWI is rather short and hence can be incorporated easily into routine Head & Neck MRI protocols.

Disadvantages of currently available imaging modalities:

Currently MR sequences of T1 weighted and fluid sensitive sequences like short-tau inversion recovery (STIR) or fat suppressed T2 weighted images in two / three planes are commonly used for diagnostic purposes. Intravenous gadolinium is also administered as a contrast agent. These images however provide only morphological details without any information on the biological behavior.

DWI is a noninvasive method used to measure the Brownian motion of water molecules in the adjacent microscopic environment. DWI is created by applying diffusion sensitizing gradients to T2 weighted images. B values are used to determine the parameters of sensitizing agents. A b value of 0 is similar to T2 weighted image. With increasing b values, images become more sensitive to the detection of restricted diffusion. Signal loss due to dephasing due to the movement of water molecules between two opposing gradients is directly proportional to the degree of movement of water molecules and the b value. The imaging sequences are repeated with different b values and apparent diffusion coefficient maps are obtained. This quantifies the observed signal loss. A higher number of images sets with different b values results into more accurate determination of apparent diffusion coefficient calculation. The only down side to this technique of procuring images under different b values is that the scanning duration increases. This increased scanning time is a deterrent for routine scans.

An area of restricted diffusion appears as a bright signal on DWI and a dark signal on the ADC map. The classic example being highly cellular tumors such as high-grade malignancies and small cells tumors like lymphoma and neuroblastoma.

Increased restricted diffusion in high grade malignancies is also related to high nucleo-cytoplasmic ratio in rapidly proliferating tumors as there is higher restriction of water molecules in the nucleus in comparison to cytoplasm. The central necrotic portion of abscesses is related to restricted water motion due to high viscosity pus due to the presence of inflammatory cells, bacteria, proteins and cellular debris resulting in diffusion restriction in the central portion.

Problems encountered in head and neck imaging:

1. Head and neck tissues are heterogenous in nature causing difficulties in interpreting various structures

2. Changing geometric shape of structures of the neck again poses strong challenges during image interpretation

3. Presence of bony structures can cause images to have low signal-to-noise ratios

4. Presence of metal implants and dental fillings can cause image artifacts

Steps taken to improve image accuracy in head and neck imaging:

1. Use of antisusceptibility pads in order to reduce image distortion especially when higher b values are used. Scanning using higher b values increases the amount of image distortion.

2. Use of high transmitter bandwidth reduces image distortion

3. Obtaining DWI before an Intravenous contrast administration to avoid perfusion effects

4. Choosing a range of b values between 0-1000 generates good curve for ADC calculation.

Clinical uses of DWI in head and neck imaging:

Distinguishing benign from malignant lesions.

Malignant tumors are more likely to have high cellularity, less intracellular space, increased nucleus to cytoplasmic ratio, and higher numbers of intracellular organelles and macromolecules when compared to that of benign lesions. These properties of malignant lesions show pronounced diffusion restriction when compared to that of benign lesions. As with all other investigations there is exceptions to this general feature. This could lead to false negative and false positive diagnosis. Examples of false positive include a completely benign process like an abscess which may show diffusion restriction in the central portion. Benign masses such as Warthin’s tumor will show restricted diffusion due to high cellularity. False negative examples include well differentiated malignant tumors that have lower rates of cellular proliferation when compared to that of poorly differentiated / undifferentiated high-grade tumors.

Low grade versus high grade malignant tumors.

Well differentiated tumors are characterized by less mitotic activity resulting in low nucleus cytoplasmic ratio, relatively low cellularity and bigger size of cells as compared to that of poorly differentiated high-grade malignancies. The degree of restricted diffusion would be higher for high grade poorly differentiated tumors than low grade well differentiated malignant lesions.

Benign versus malignant lymph nodes.

Lymph node enlargement could be nonspecific and is seen both benign and malignant process. Benign varieties can be subdivided into benign reactive and infective. Malignant lymphadenopathy may be due to different malignancies. Pathologically more inflammatory cells and follicular hyperplasia are features of reactive lymph nodes. Its general architecture is maintained including the intact fatty hilum. Infective nodes often have a liquified center harboring pus. Malignant nodes are characterized by disorganized architecture usually nodal enlargement and occasional necrosis.

On imaging the common appearance of benign adenopathy is enlarged nodes with intact fatty hilum, smooth outline and normal appearance of surrounding fat. Infective nodes can be of normal size / enlarged with heterogenicity and central abscess formation. Surrounding fat planes are likely to be infiltrated. Malignant adenopathy can be really deceptive in imaging. It can look completely normal, enlarged and heterogenous with irregular margins and surrounding fat infiltration. Conventional MRI detects only gross morphological variables such as size, margins and to a limited extent internal architecture. DWI plays a vital role in being sensitive to pathological abnormalities. Benign reactive nodes will not show restricted diffusion, where as malignant nodes will show variable degree of diffusion restriction. Using conventional MRI alone it would be really difficult to differentiate malignant lymphadenitis with central necrosis from infective adenitis with abscess formation. Infected nodes with central abscess would show restricted diffusion centrally due to presence of inflammatory cells, bacteria, proteins and cellular debris. A necrotic malignant node would demonstrate peripheral rim of restricted diffusion due to high cellularity around a necrotic area without restricted diffusion.

Orbital Lesions.

Lesions involving orbit are rather varied in nature which include inflammatory, infectious, vascular, benign lesions and malignant lesions. Among the inflammatory group inflammatory pseudotumor which could be confused with that of lymphoma. Orbital cellulitis can also present clinically similar picture to that of inflammatory pseudotumor. CT and MRI imaging is really non-specific in majority of these situations. DWI is really promising in differentiating between inflammatory pseudo tumor and lymphoma. Similarly, it plays a vital role in differentiating benign from malignant orbital lesions.

Salivary gland lesions.

Neoplasm of salivary glands constitute nearly 3% of all head and neck tumors. 1% of head and neck malignancies arise from salivary glands. Except for Warthin’s tumor all other tumors can be differentiated as benign / malignant using DWI.

Lymphoma versus squamous cell carcinoma.

Squamous cell carcinomas are the most common of all head and neck malignancies followed by lymphomas. As management of both these lesions are radically different it would be better if imaging could provide some inkling about the diagnosis. Imaging supplemented with DWI may be a powerful tool as to which type of malignancy that is being dealt with. Lymphomas are more cellular than squamous cell carcinoma hence would show greater diffusion restriction and hence less ADC because of relative paucity of cytoplasm and extracellular space.

Necrotic versus viable part of tumor.

Malignant tumors are not homogenous always. It is common for the tumor to have a spectrum of cellular composition areas of high cellularity and areas of necrosis. The necrotic area could be very small to be detected by routine MR imaging. These necrotic areas are frequent following chemoradiation. DWI is sensitive to differentiate between necrotic and non-necrotic areas of tumor. In cases of post therapy follow up it would be useful to document tumor viability and extent of necrosis as a response to therapy.

Skull lesions.

DWI is helpful in differentiating malignant from benign skull lesions. It is also helpful in differentiating residual / recurrent cholesteatoma from normal post-operative granulation tissue.

DWI in pediatric head and neck masses.

Most common orbital malignancies in pediatric age group are retinoblastoma, rhabdomyosarcoma and optic nerve glioma. Rhabdomyosarcoma is the most common pediatric soft tissue malignant tumor with orbits and sinuses being involved commonly. In conventional imaging it would look similar to inflammatory cellulitis and hemangioma. This is where DWI has a crucial role to play.

Prediction of response to radiation and chemotherapy.

Radiation and chemotherapy are the most important options in treatment of head and neck malignancies. Even if tumor is surgically resectable, still chemoradiation is required to control future recurrences. In advanced lesions chemoradiation could be the only treatment option available. Response of tumors to chemoradiation can be predicted. Tumors having higher pre-therapy restricted diffusion / lower ADC respond better than those that show lack of diffusion restriction / higher ADC. This is understandable because chemoradiation can effectively kill actively dividing cells. The amount of actively dividing cells can be predicted by law levels of DWI seen in these lesions.