Cases & Papers

INTRODUCTION

Spatial and Temporal Image Correlation (STIC) has become a widely accepted technique in 2nd and 3rd trimester fetal echocardiography since its introduction in 2003. Performing a STIC acquisition of a fetal heart involves acquiring one slow 3D sweep of approximately 10 seconds duration across the fetal thorax. During this sweep the fetal heart beats approximately 20-28 times. The machine detects the location and timing of each systolic beat and calculates the heart rate. Then the system determines the time frame between each beat which allows rearranging of the B-mode frames into a new order depending on their temporal event within the heart cycle. Since the machine knows the length of the sweep and the heart rate it can calculate the location of each peak systolic frame and other points in the cardiaccycle and combine the information in it with all the other frames of the corresponding times. Because many frames at the exact time reference are averaged together the temporal resolution compares to a high frame rate B-mode image. The rearranging results in a fi nal product of one heart cycle replayed in a continuous cine loop. Since each frame represents a volume of information, the image can be reconstructed in different planes for the examination of fetal cardiac structures from many angles¹.

The benefits of this technique with the addition of color Doppler were initially described in 2004^2,3. For utilization with early 2D fetal echocardiography the addition of color Doppler is necessary in order to exclude such conditions as valvular lesions and septal defects⁴.

When a small structure, such as the fetal heart in the first trimester, needs to be captured into a volume, the angle of acquisition needs to be set according to the size of the object. We describe this technique as targeted acquisition. When the volume sweep angle is larger than the structure being examined it causes redundant information to be included in the volume and the volume itself to be of reduced resolution. Using a targeted acquisition technique, the smaller the object the faster the time of acquisition. When the time of acquisition is short then the chance of the occurrence of motion artifacts is minimal. This is extremely important in an active first trimester fetus. In the following white paper we describe a technique of targeted volume acquisition which results in high quality STIC color imaging.

Obtaining high quality first trimester STIC volumes requires a high level of practical and theoretical education in early fetal echocardiography together with 2nd and 3rd trimester STIC acquisitions. Also, ultrasound systems must be equipped with state of the art processing and appropriate transducers to produce the images. On GE Voluson systems, the 4-8MHz volume transducer is capable of producing high quality 2D, color fetal heart and STIC images⁵. Because satisfactory 2D imaging is not always a guarantee of good volume imaging, it is also extremely important to be familiar with modern image enhancers and filters which reduce speckle artifacts so as to not risk rewriting them into the volume dataset.

Authors:

A. NOCUN
Medical Practice
IPL Agnieszka Nocun,
Krakow, Poland

M. WIECHEC
Centre for Fetal Care,
Imperial College Healthcare,
London, UK