Clinical diagnosis
Case 114
【Progress】She was admitted to our hospital and scheduled to get rehabilitation.
【Discussion】
There are two transverse relaxations: T2 relaxation and T2* relaxation in decay of transverse magnetization caused by using a radiofrequency pulse. MR signal of T2 relaxation is acquired to purely get proton-proton relaxation by using 180° pulse which is called spin-echo imaging. Meanwhile, MR signal of T2* relaxation is acquired by eliminating 180° pulse which is called gradient-echo imaging. T2* relaxation includes proton-proton relaxation + magnetic field inhomogeneity. T2* relaxation time is shorter than T2 relaxation time (which represents the decay of the signal by 1/e, 37%). Because T2* relaxation includes proton-proton relaxation + magnetic field inhomogeneity, the contrast of T2* weighted images is similar as the contrast of T2 weighted images which are also susceptible to the materials of deoxyhemoglobin in tiny veins, air-tissue interface, metallic implants.
Diffusion weighted imaging (DWI) is conducted by that motion probing gradients (MPG) pulses are applied between 180° RF pulse to a spin echo-echo planner imaging (EPI) which made it possible to obtain high-speedy MR slices in the time of 50-100 msec. Diffusion b0 means the image before MPG and Diffusion b1000 means the image after MPG with b1000. The difference between diffusion b1000 and diffusion b2000 implies the higher b value brings about the prominent contrast, compared to the surroundings: the lesion with the diffusion repression more markedly stands out in diffusion b2000 than b1000 since the surrounding with the diffusion ability drops the signal intensity more significantly in b2000 than in b1000. Then, DWI is exploited to find out the diffusion-repressing lesion. The diffusion data can be presented as signal intensity or as an image map of the apparent diffusion coefficient (ADC). Calculation of the ADC requires 2 or more acquisitions with different diffusion weightings (1). Namely, an ADC equals to the slope of the curve using the acquired signal intensity values of b0 and b1000 or b2000(ADC = Loge Sb/Sb0)(2). An ADC map is drawn using the slope of the acquired signal intensity values of b0 and b1000 or b2000
Although DWI b0 is obtained by EPI technique and T2* is obtained by gradient echo technique, both are the same contrast as T2 weighted imaging. In the clinical situation, the image with DWI b0 is corresponded to the images with T2 or T2* with fat suppression.
In our case, DWI b1000 showed bright signal intensity and low values of ADC, indicating the lesion with diffusion repressing and T2*WI and DWI b0 showed bright image inside the lesion and low signal image in the margin, indicating the margin of the lesion included the materials causing magnetic field inhomogeneity.
Brain hemorrhage is known to evolve phases to hyperacute, acute (1-3 days), early subacute, late subacute and chronic. Hyper acute hematoma with < 24 hours after onset indicates oxyhemoglobin intra-red blood cell, acute hematoma with 1-3 days after onset indicate deoxyhemoglobin intra-red blood cell, early subacute with 4-7 days indicate methemoglobin intra-red blood cell, late subacute with 7-14 days indicate methemoglobin extra-red blood cell and chronic with > 14 days indicate hemosiderin extra-red blood cell.
Oxyhemoglobin is diamagnetic, while deoxyhemoglobin and methemoglobin are paramagnetic and hemosiderin is super paramagnetic. T2WI and T2*WI show the similar signal intensity in all phases of hematoma, but magnetic field inhomogeneity caused by paramagnetic is enhanced by T2*WI. When hemosiderin is phagocyted by microglia or macrophages and moved to the margin, T2*WI showed low signal intensity in the margin. T2WI and T2* show low signal intensity in the hematoma with swollen red blood cells within 7 days after onset and high signal intensity in the hematoma after hemolysis. Meanwhile, T1WI showed iso-intensity and/or hyperintensity signals at the hemorrhage in the hyper acute to late subacute within one month. In our case, T2* and T2WI showed high signal intensity inside the lesion with low signal intensity in the margin.
DWI is affected by the effects of T2WI + diffusion repressing. Because the signal intensity is basically affected by T2WI (T2 shine through), the presence or absence whether diffusion suppressing lesion includes, had better make sure that ADC values lowers or not.
In our case, T2WI and DWI showed high signal intensity and ADC map revealed low values, implying the lesion with diffusion repressing. Although brain hemorrhage from hyperacute to late acute is reported the low values of approximately 0.5, DWI showed variable signal intensity because of the effects of T2WI. Methemoglobin extra-red blood cell also repress diffusion because of proton existing surrounded by large molecules such as abscess and mucin fluids. Then, the hematoma in our case evolves in probably late subacute. In the clinical situation, MRI was conducted at the day of the appearance of clinical symptoms which did not always reflect the hyperacute hematoma. That was probably because that our patient had Alzheimer disease which made it hard and delayed to find out the ischemic symptoms.
【Summary】
We present a ninety one-year-old female with the decrease of consciousness and paresis of the left upper extremity. Brain MRI showed the lesion at the left temporal lobe with high signal intensity on T1WI, T2*WI and DWI , and low values on ADC map, indicating the late subacute hematoma. We should keep in mind that compared with T2WI, T2* relaxation includes proton-proton relaxation + magnetic field inhomogeneity. Meanwhile, DWI includes T2WI + diffusion repressing. ADC map is imperative to clarify whether the lesion truly includes diffusion repressing or T2 shine through. The low signal intensity in the margin represent hemosiderin phagocyted by macrophages or microglia, indicating late subacute or chronic hematoma. The contrast of images with T2* is the same as that with T2WI. Diffusion b0 is considered to be T2* with fat suppression or T2WI with fat suppression.
【References】
1.Aoki S et al. Diffusion MRI 3rd edition. Syujunnsha 2013 (Japanese)
2.Kita M. Personal communication in Fuchu Hospital
2018.7.25