Optical frequency domain imaging (OFDI) [1
], also known as swept-source OCT [2
], is a high-resolution (~10 µm), cross-sectional, fiber-optic imaging method that is capable of measuring tissue microstructure, birefringence [3
], and blood flow [5
]. The most important feature of OFDI, however, is its very fast image acquisition speed, which enables wide-field imaging studies in vivo
]. Since the interferometric ranging signal in OFDI is collected in the Fourier domain, high-speeds can be achieved while maintaining sufficient detection sensitivity [2
]. With the advent of rapid-scanning wavelength-swept lasers [12
], the speed of clinically-viable OFDI systems is currently limited by digital acquisition and storage capabilities. The relationship between imaging speed and the required digital throughput is determined by several factors, but the minimum necessary sampling rate is generally given by fA
is the A-line rate and N
is the number of points per A-line. N
is given by 2*Δλ/δλ and Δλ and
δλ are the wavelength sweep range and instantaneous line-width of the laser, respectively. In addition, polarization diversity or polarization-sensitivity is highly desirable for robust clinical systems and doubles the required digital throughput.
In order to preserve the inherent dynamic range of OFDI, systems have typically utilized 12, 14 or 16 bit-depth digitizers. A typical polarization-diverse system, based on an 100 kHz repetition rate laser, capable of generating 195 images per second (512 A-lines per image), would therefore require a data throughput rate of 819.2 MB/s at 2048 points per A-line and assuming each sample is transferred as a two byte (16 bit) word. Clinical imaging with such a system may generate total data volumes in excess of 100 GB per patient and ten’s of terabytes per study. Lowering the bit depth of acquisition would be a simple strategy for reducing data rates and volumes while also making it possible to utilize a broader range of fast digital acquisition electronics.
We have therefore investigated whether 8-bit sampling can be used without inhibiting image quality. In order to reduce the data transfer demand, lower bit-depth data acquisition boards (DAQ) could be used. However, the tradeoffs between sensitivity, dynamic range, and bit-depth have not been thoroughly investigated, making it difficult to justify and or evaluate reduced bit-depth systems. Prior analyses of the sensitivity of Fourier Domain OCT systems typically assume the system is designed such that DAQ noise terms can be ignored [2
], or have explicitly stated that the quantization noise is minimized by the choice of detector gain [12
]. Previously it has been suggested that high bit-depth DAQ boards are required for imaging through scattering tissue with high dynamic range [16
]. Some groups have used 8-bit digitization for faster acquisition and lower cost and achieved a 52 dB dynamic range [17
]. Huber et. al.
also used an 8-bit osilliscope at 5GS/s in order to compare 8 and 14 bit-depth images [18
]. They achieved an image contrast of 37 dB in the 8 bit image, but a formal noise analysis was not presented. Here, we present a formal noise analysis of an OFDI system including the effects of bit-depth on signal quantization noise. We digitize OFDI signals at various bit-depths to analyze the effect on sensitivity and dynamic range, and compare these results with a theoretical model of OFDI that includes quantization noise. Our results show that a true 8-bit data acquisition system can achieve high system sensitivity and dynamic range with only a minimal drop in the signal-to-noise ratio. In
-vivo images of a human coronary demonstrate no significant differences between images acquired at 8- and 14-bits suggesting that 8-bit DAQ boards can be used to increase imaging speeds in clinical OFDI systems.