云南师范大学信息学院 系列讲座 3 医学成像 杨扬

内容 CT CTA 核磁共振成像 fMRI 超声波 PET（正电子发射断层成像） SPECT（单光子发射计算机断层成像） Hybrid（混合成像）

X-ray Computed Tomography

构成硬件

发展

发展

发展

发展

成像原理 衰减图 正弦投影 CT 图像

衰减图

衰减图

正弦投影图

Example

CT图像

CT成像例子

Summary：CT成像流程

运用范围 硬组织，心脏等 静态成像 动态成像

CTA：CT Angiography（血管造影）

构成硬件

CTA的利弊 Benefits CTA can be used to examine blood vessels in many key areas of the body. This method displays the anatomical detail of blood vessels more precisely than magnetic resonance imaging (MRI) or ultrasound. Risks There is a risk of an allergic reaction (过敏反应). CTA should be avoided in patients with kidney disease （肾病）or severe diabetes（糖尿病）, because x-ray contrast material can further harm kidney function. If a large amount of x-ray contrast material leaks out under the skin where the IV is placed, skin damage can result. Compared with other imaging modalities, CTA is associated with a significant dose of ionizing radiation （高放射）.

运用范围 头部血管 心脏

核磁共振成像

构成硬件 A magnet which produces a very powerful uniform magnetic field. Gradient Magnets (梯度磁场)which are much lower in strength. Equipment to transmit radio frequency (RF:射频). A very powerful computer system, which translates the signals transmitted by the coils.

成像原理： 氢原子 Biological tissues are predominantly 12C, 16O, 1H, and 14N Essentially all MRI is hydrogen (proton) imaging

成像原理：Spin （旋转） m There is electric charge on the surface of the proton, thus creating a small current loop and generating magnetic moment m. The proton also has mass which generates anangular momentum J when it is spinning. J + + + Thus proton “magnet” differs from the magnetic bar in that it also possesses angular momentum caused by spinning.

成像原理：平衡

成像原理：共振 The gradient magnets are rapidly turned on and off which alters the main magnetic field. The pulse directed to a specific area of the body causes the protons to absorb energy and spin in different direction, which is known as resonance Frequency (Hz) of energy absorption depends on strength of external magnetic field.

成像原理：射频 与 能量吸收 The MRI machine applies radio frequency (RF：射频) pulse that is specific to hydrogen. The RF pulses are applied through a coil that is specific to the part of the body being scanned.

成像原理：能量释放与成像 Imaging: Releasing energy： When the RF pulse is turned off the hydrogen protons slowly return to their natural alignment within the magnetic field and release their excess stored energy. This is known as relaxation. Releasing energy： Released as heat Exchanged and absorbed by other protons Released as Radio Waves.

成像原理: Summary 1) 把目标放入主磁场中。 2) 打开Gradient Coil改变主磁场，产生核磁共振。 3) 通过RF Coil发射射频，H质子吸收波能。 4) 关闭Gradient Coil恢复主磁场，H质子旋转恢复平衡，释放出RF 信号。 5) 通过接受到的RF信号成像。

运用范围 软组织 静态成像 动态成像（但是效果不好）

fMRI（Functional MRI）

定义 Functional magnetic resonance imaging or functional MRI (fMRI) is an MRI procedure that measures brain activity by detecting associated changes in blood flow

成像原理：BOLD BOLD: Blood Oxygenation Level Dependent Deoxyhemoglobin (dHb：脱氧血红蛋白) has different resonance frequency than water dHb acts as endogenous contrast agent dHb in blood vessel creates frequency offset in surrounding tissue (approx as dipole pattern)

成像原理：BOLD Signal Very indirect measure of activity (via hemodynamic response to neural activity)! Complicated dynamics lead to reduction in [dHb] during activation (active research area)

运用范围 大脑血流变化

超声波

构成硬件

运用范围 Musculoskeletal （骨骼肌） Cardiac （心脏） Surgical （外科手术） Abdominal （腹部） OB / GYN （胎儿）

种类：A Mode A超是一种幅度调制型，是国内早期最普及最基本的一类超声诊断仪，目前已基本淘汰。

种类：M Mode M型显示体内各层组织对于体表(探头)的距离随时间变化的曲线，是反映一维的空间结构，因M型超声多用来探测心脏，故常称为M型超声心动图，目前一般作为二维彩色多普勒超声心动图仪的一种显示模式设置于仪器上。

种类：B Mode Presents the spatial changes in echoes in real time. Typically a “pie-shaped” display in which the sensor is at the top.

种类：多普勒 彩色多普勒又称二维多普勒，它把所得的血流信息经相位检测、自相关处理、彩色灰阶编码，把平均血流速度资料以彩色显示，并将其组合，叠加显示在B型灰阶图像上。

成像原理：波的特性 The difference in the speed of sound at tissue boundaries is a fundamental cause of contrast in an ultrasound image

成像原理：波的特性 Higher frequency sound has shorter wavelength Ultrasound wavelength determines the spatial resolution achievable along the direction of the beam However, the depth of beam penetration is reduced at high frequency and increased at low frequencies For thick body parts (abdomen), a lower frequency ultrasound wave is used (3.5 to 5 MHz) to image structures at significant depth For small body parts or organs (thyroid, breast), a higher frequency is employed (7.5 to 10 MHz)

成像原理 A pulse is propagated and its reflection is received, both by the transducer. Key assumption: Sound waves have a nearly constant velocity echo time maps to depth. Acoustic Impedance Z：is equal to density of the material times speed of sound in the material in which ultrasound travels。 Z = *c  = density (kg/m3) and c = speed of sound (m/sec)

运用范围 软组织 血流 动态

PET

构成硬件

成像原理：放射性显像剂

成像原理：放射性显像剂 18-F-FDG most common nuclide Other PET Radionuclides 11C 82Rb

成像原理：湮灭反应 e+ e- Positron scatters in tissue losing energy Positron emission 511 keV -ray e- 511 keV -ray Annihilation

成像原理：伽马照相机 Detector Block LSO PROCESSING ELECTRONICS

成像原理：Coincidence

成像原理：正弦投影 A sinogram is formed by placing the projection for the angle 0 at the top of a matrix and the other angles in ascending order. EXACT, ART, Accel: 192 x 192 elements. HR+: 288 x 288 elements Data is collected as the integral of the activity along parallel rays for each angle from 0 to 180 degrees.

成像原理：Summary

运用范围 肿瘤 神经 心脏 药物反应 肌肉

SPECT

构成硬件

构成硬件

成像原理：放射性显像剂

成像原理：放射性显像剂 Target 放射性显像剂 Bone scan 99mTc Myocardial perfusion scan Sestamibi parathyroid scan Brain scan Neuroendocrine or neurological tumor scan Iodine-123 or Iodine 131 White cell scan indium-111 & 99mTc

运用范围 心肌灌注 大脑血流

Hybrid

种类 PET/CT SPECT/CT PET/MRI

发展

目的

PET/CT

SPECT/CT

PET/MRI

PET/MRI

其他混合成像