Diffusion weighted image

1. 확산 영상의 원리

1) 정의 : 인체 내 물 분자들의 self-diffusion 정도를 영상화 함.
2) 물 분자의 확산 계수 ( D : diffusion coefficient )
의미 : 물 분자의 확산 정도를 수치로 나타냄.
D = d2 / 2t ( d = 일정 시간 t 동안 물 분자가 움직인 거리 )
3) Restricted-diffusion & anisotropic-diffusion
생체 내 물 분자는 cell membrane 또는 거대 단백질 분자 등에 의해 움직임의 제약이 있으며, 주위 구조물에 의해 어느 한 방향으로 잘 움직이게 된다.
->전자는 제한 확산(restricted-diffusion), 후자는 비등방성 확산(anisotropic-diffusion) 임.
** 또한 조직 자체의 움직임, 미세 혈관 내 혈류, 환자의 움직임 등에 영향을 받음
 ADC(apparent diffusion coefficient : 현성 확산 계수 ) 위 요소의 영향이 포함됨.

** 확산 강조 영상 : 확산이 크거나 빠른 조직은 검게, 확산이 작거나 느린 조직은 고신호
** ADA 영상 : 확산이 잘 되는 조직이 고신호 강도

2. 확산 영상의 기법
일반적으로 Stejskal-Tanner 기법 이용
2 번의 확산 민감 경사 펄스 (diffusion-sensitizing gradient pulse)를 180o 고주파 양쪽에 대칭적으로 주면서 영상을 얻는 기법.


① 정지된 스핀 : 앞에 삽입된 경사자계에 의해 dephasing -> 뒤에 삽입된 경사자계에 의해 rephrasing ∴ 위상차 소실
② 확산 현상이 있는 스핀 : 한쌍의 확산 강조 경사자계가 걸리는 동안 이동 -> rephrasing 이 불완전. ∴ 신호 감소

신호 강도 (S) = Soe-Db
So : 확산 강조 경사자계가 걸리지 않은 영상의 신호강도
b value = b factor : 확산 강조 경사자계의 세기( s/cm2 or s/mm2 )
임상적으로 300 – 1000 s/mm 정도


확산 민감 경사 자장을 다르게 하여, 즉 b value 를 달리 적용하여 얻은 영상을 감산
 ADC map or ADC image
 T1 , T2 (T2 shine through effect ) , 양성자 밀도 효과가 모두 배제되고, 어떤 조직 내 물 분자의 확산 정도에 의해 영상 대조도 결정 !
 확산 계수가 높으면 고 신호강도/ 확산 계수가 낮으면 저 신호강도

③ Trace or tensor imaging
x , y ,z 세 방향 이상의 방향으로 모두 확산 강조 경사자계 걸어 확산 강조 영상 얻기( diffusion tensor imaging)
x , y , z 확산 영상의 평균값 구해 방향성 배제, 크기만 영상화 (diffusion trace imaging)
=> Anisotropy effect 최소화 가능

3. 임상적용
1) 급성 뇌허혈증 발견 : 6시간 이내 초급성 병변 쉽게 발견 가능ㅇ
: 혈류 차단-> cytotoxic edema 초래
-> DWI 에서 뇌 실질보다 고신호 강도, ADC map 에서 저 신호강도

=> Rt. MCA infartion 에서 ADA 상 hypointense, DWI 상 hyperintense lesion

=> Lt.MCA occlusion 에서 DWI, PWI, MRA
Large D/P mismatch

2) 급성 뇌허혈증과 만성 뇌경색 감별
3) Arachnoid cyst 와 epidermoid 감별
- Arachnoid cyst
: 내부에 높은 확산 계수를 가진 CSF 성분 있으므로 DWI 에서 저신호강도 , ADC map 에서 고 신호강도
http://rad.usuhs.mil/medpix/medpix_image.html?mode=quiz&imid=26575&quiz=no&page=&th=&map=#pic
- Epidermoid
: 내부에 고형성분 -> DWI에서 고 신호강도, ADC map 에서 저 신호강도(= 뇌실질 정도)


Epidermoid cyst 의 DWI : high signal -> T2 shine through effect 에 의한 것으로 생각
4) 탈수초화 질환 또는 dysmyelinating disease 특성 파악
5) 뇌종양과 뇌농양의 감별
뇌 농양


Noncontrast CT 와 DWI ( 뇌 농양 )

 

Diffusion-Weighted Imaging

      Diffusion imaging makes use of the variability of “Brownian motion” of water molecules in brain tissue. Brownian motion refers to the random movement of molecules. Water molecules are in constant motion, and the rate of movement or diffusion depends on the kinetic energy of the molecules and is temperature dependent. In biological tissues, diffusion is not truly random because tissue has structure. Cell membranes, vascular structures, and axon cylinders, for example, limit or restrict the amount of diffusion. Also, chemical interactions of water and macromolecules affect diffusion properties. Therefore, in the brain, water diffusion is referred to as “apparent diffusion.”

      To obtain diffusion-weighted images, a pair of strong gradient pulses are added to the pulse sequence. The first pulse dephases the spins, and the second pulse rephases the spins if no net movement occurs. If net movement of spins occurs between the gradient pulses, signal attenuation occurs. The degree of attenuation depends on the magnitude of molecular translation and diffusion weighting. The amount of diffusion weighting is determined by the strength of the diffusion gradients, the duration of the gradients, and the time between the gradient pulses.

      Diffusion imaging is performed optimally on a high-field (1.5 T) echo-planar system, but it can be accomplished with a turboSTEAM sequence on systems with conventional gradients.

      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. A low ADC corresponds to high signal intensity (restricted diffusion), and a high ADC to low signal intensity on diffusion-weighted images.

      In the setting of acute cerebral ischemia, if the cerebral blood flow is lowered to 10 ml/100gm/min, the cell membrane ion pump fails and excess sodium enters the cell, which is followed by a net movement of water from the extracellular to intracellular compartment and cytotoxic edema. Diffusion of the intracellular water molecules is restricted by the cell membranes. The restricted diffusion results in a decreased ADC and increased signal intensity on diffusion-weighted images. Severe ischemia can lower the ADC by as much as 56% of normal tissue at 6 hours.

                              Bykowski J, Schellinger PD, Warach S: Diffusion and perfusion MRI, in Edelman, Hesselink, Zlatkin &Crues, eds., Clinical Magnetic Resonance Imaging, 3^rd edition, Saunders-Elsevier, Philadelphia, 2006, pp1538-70.

 

      In patients who present with symptoms of cerebral ischemia, diffusion-weighted images are very helpful to identify any area of acute ischemia and to separate the acute infarction from old strokes and other chronic changes in the brain. Only the acute infarcts appear hyperintense on the diffusion

images. Subacute and chronic infarcts, vasogenic edema, the punctate and confluent changes of deep white matter ischemia, and dilated VR spaces are not bright.

                              Schaefer PW, Ozsunar Y, He J, et al. Assessing tissue viability with MR diffusion and perfusion imaging.AJNR 24:436-43, 2003.

Bacterial abscesses may exhibit restricted diffusion due to thick cellular debri within the central cavity. Other diseases of the brain, such as non-bacterial infections, neoplasia, contusions, and demyelinating diseases, are not associated with cytotoxic edema, and therefore as a rule, they are not hyperintense on the diffusion images. One exception is epidermoid tumors, which have restricted diffusion due to the waxy consistency of their contents. Also, the central portions of some primary and secondary brain tumors may exhibit restriction diffusion as they outgrow their blood supply and become ischemic. Occasionally, an acute MS plaque may be mildly hyperintense with diffusion weighting.

      Lesions with prolonged T2 relaxation times are commonly mildly hyperintense on diffusion-weighted images. This phenomenon of “T2 shine-through” can easily be distinguished from true restricted diffusion on the ADC map. Only true restricted diffusion is low signal on the ADC map.