Imaging diagnosis
Case 239
【Progress】
He was introduced to the hospital where orthopedic surgeon and plastic surgeon could serve together.
【Discussion】
Diffusion weighted imaging (DWI) is created by the idea of how much ability of water molecular diffusion in tissue is preserved. DWI is made based on T2WI because the signal intensity of T1WI is low and motion probing gradient (MPG) is given twice at before and after 180 pulse on T2WI.
MPG is given whether water molecular diffusion is preserved or not. When water molecular diffusion is preserved such as normal tissue, signal intensity by spin echo usually drops. However, in case that water molecular diffusion of fresh infarction or dens tumor is disordered, signal intensity does not drop so much, where signal intensity remains and can be emphasized on T2WI with modulating window/level. Diffusion b0 means the image before motion probing gradient (MPG) and Diffusion b1000 means the image after MPG with b1000. The numerical value of signal intensity of each tumor is useless because signal intensity of b0, b1000 and b2000 are obtained by minute induced current and tuned by multiply with more than thousands (1). Then, signal intensity values are unreliable. ADC (apparent diffusion coefficient) which implies the slope of the curve, is more reliable for evaluating diffusion repression of water molecule. Calculation of the ADC requires 2 or more acquisitions with different diffusion weightings (2). Namely, ADC of each tissue equals to the slope of the curve using the acquired signal intensity values of b0 and b1000 or b2000 (ADC = Loge Sb/Sb0)(1). An ADC map is drawn using the slope data of each pixel (1).
The differentiation of soft tissue tumor whether malignant or benign is crucial on MRI. DWIMRI with ADC values is an important indicator for the differentiation. When the lesion is shown high signal intensity on DWIMRI, ADC map and values are obtained. When the lesion is shown low signal intensity on DWIMRI, ADC values are meaningless because Sbo indicates to as close as zero.
For differentiation of malignant soft tissue tumor from malignant tumor, it is reported that with accuracy of around 80%, the cutoff values for the differentiation is 1.24 : mean ADC value of benign soft tissue tumor is 1.53, more than 1.3 while mean ADC value of malignant soft tissue tumor is 0.80 below 1.10 (3).
As the soft tissue tumor of foot and ankle, benign soft tumors include plantar fibroma, fibromatosis, angioleiomyoma, hemangioma, soft tissue chondroma, peripheral nerve sheath tumor (neurinoma, neurofibroma), giant cell tumor of tendon sheath and pigmented villous-nodular synovitis. Meanwhile, malignant soft tissue tumors include dermatofibrosarcoma protuberans, synovial sarcoma, rhabdomyosarcoma, extraosseous Ewing tumor, malignant lymphoma, and squamous cell carcinoma (4, 5).
However, epidermoid cyst is more often experienced than soft tissue tumor. There is a pitfall to use ADC value for differentiation between benign and malignant tumors because the mean ADC values of both epidermoid cyst and abscess are nearly 0.80 (6, 7), apt to misdiagnose it as malignancy. Keratin in epidermoid cyst and pus effusion in abscess interrupt diffusion of water molecule as well as malignant compact tumor.
In our case, imaging diagnosis is plantar fibroma. ADC value of the plantar fibroma is 1.35, indicating a benign tumor. Plantar fibroma is formed from a small tear and occupied by fibrous tissue, embedded in the plantar fascia. It can occur in both feet. It can be painful as it grows. It happens in middled aged man. Our patients felt a nodule in foot arch in both feet and becoming painful when walking.
【Summary】
We presented a forty five-year-old male suffering from gradually painful nodules when walking at both feet arch. Foot MRI with T1WI and T2WI depict the lesion with low signal intensity and iso-high signal intensity, respectively. DWIMRI depict high signal intensity and ADC values are 1.35.
It is borne in mind that ADC values are useful for the differentiation between benign soft tumor and malignant one. With accuracy of around 80%, the cutoff values for the differentiation is 1.24 : mean ADC value of benign soft tissue tumor is 1.53, more than 1.3 while mean ADC value of malignant soft tissue tumor is 0.80 below 1.10. However, there exists a pitfall: ADC values of both epidermoid cyst and abscess are nearly 0.80, apt to misdiagnose it as malignancy.
【References】
1.Kita M. Personal communication in Fuchu Hospital
2.Aoki S et al. Diffusion MRI 3rd edition. Syujunsha 2013 (Japanese)
3.Hassanien OA, et al. Diffusion weighted MRI of soft tissue masses: Can measurement of ADC value help in the differentiation between benign and malignant lesions? The Egyptian Journal of Radiology and Nuclear Medicine. 2018; 49: 681-688
4.Hughes P, et al. MRI imaging of soft tissue tumours of the foot and ankle. Insights into Imaging. 2019; 10: 60-70
5.Chou LB, et al. Tumors of the foot and ankle: experience with 153 cases. Foot Ankle Int 2009; 30:836–841
6.Suzuki C, et al. Apparent Diffusion Coefficient of Subcutaneous Epidermal Cysts in the Head and Neck: Comparison With Intracranial Epidermoid Cysts. Academic Radiology. 2007; 14: 1020-1028.
7.Neubauer H, et al. Diffusion-weighted MRI of abscess formations in children and young adults. World J Pediatr. 2012;8:229-34
2021.7.30