Imaging pancreatobiliary ductal system with optical coherence tomography: A review

Table 1 Imaging methods for diagnosis of bile duct strictures n (%)

Techniques	SEN (%)	SPEC	PPV	NPV	Accuracy
BC/FNA[2,11,23]	30 (30-60)	95 (90-100)	100 (90-100)	28 (28-50)	48 (30-50)
Forceps biopsy[2,11,23]	43 (40-70)	90 (90-100)	95 (90-100)	31 (30-50)	48 (30-70)
BC + FNA + biopsy[2,11,23]	62 (60-75)	90 (90-100)	96 (90-100)	39 (35-60)	55 (45-75)
ERCP/MRCP[9,17,32,50-52]	70 (67-90)	75 (70-80)	80 (68-90)	88 (70-95)	70 (50-80)
ERCP-BC/BX[9,11,33,38]	43 (36-60)	80 (75-100)	95 (94-100)	90 (56-100)	70 (60-75)
EUS[17,32,33,47,53]	80 (70-100)	80 (75-100)	80 (76-100)	80 (54-90)	80 (78-90)
EUS-FNA[9,23]	85 (80-100)	95 (90-100)	95 (95-100)	80 (60-90)	85 (80-90)
IDUS[32,38]	90 (85-100)	85 (80-100)	85 (80-100)	90 (80-100)	90 (83-90)
IDUS + ERCP/biopsy[32,33,38,54]	91 (90-100)	93 (90-100)	94 (84-100)	90 (84-95)	92 (90-100)
OCT[1,2]	79 (75-90)	69 (65-90 )	75 (70-90)	73 (70-90)	74 (70-85)
OCT-BC/BX[2]	84 (80-90)	69 (70-90)	76 (70-90 )	78 (70-100)	77 (70-90)

True positive (TP) and true negative (TN) represent the accurate diagnosis of biliary and non-biliary strictures respectively; False positive (FP) reflects the incorrect diagnosis of non-malignancy, whereas, false negative (FN) reflects incorrect diagnosis of the benign strictures; Sensitivity, specificity, positive predictive values and negative predictive values were calculated as Ref. [54]. BC: Brush cytology; BX: Forceps biopsy; FNA: Fine-needle aspiration; ERCP: Endoscopic retrograde cholangiopancheatography; MRCP: Magnetic resonance cholangiopancreatography; EUS-FNA: Endoscopic ultrasound-guided FNA biopsy; IDUS: Intraductal ultrasonography; OCT: Optical coherence tomography; SEN: TP (TP + FN); SPEC: TN/(TN + FP); PPV: TP/(TP + FP); NPV: TN/(TN + FN).

Table 2 Comparison of various imaging modalities

Imaging modality	PTC	ERCP	MRCP	US/HFUS/EUS/IDUS	CT	OCT
Projection/ tomograph	Projection	Projection	Projection or tomographic	Tomographic	Tomographic	Projection or tomographic
Resolution	1-2 mm	1-2 mm	Fairly poor 3-5 mm	US/EUS 100-250 μmHFUS/IDUS 50-100 μm	300-500 μmμCT: 3-125 μm	Fairly high1-10 μm
Imaging depth	1-5 mm	5-60 mm	Entire biliary tree	US/EUS: 5-10 cmHFUS/IDUS: 1-3 cm	Entire biliary tree	1-3 mm
Tissue sampling	+ +	+ + +	-	US +EUS + + +	+	-
Portability	-	+	-	US + + +EUS + +	-	+ +
Therapy	+ + +	+ + +	-	US -EUS +	-	+
System cost	+ +	+ + + +	+ + +	US - EUS + +	+ +	+ +
Operator dependence	High	High	Low	Very high	Low	Low
Staging of malignancy	-	-	+ +	US +EUS + + +	+ + +	-
Safety	-	+	+ + +	+ +	+ +	+++
Experiment duration	2-4 h	30-120 min	10-30 min	20-40 min	15-30 s	5-10 min
Complications	+ + +Risk (5%-10%) of Infection, bleeding and bile leaks	+ +Risk (< 5%) Bleeding, perforationpancreatitis cholangitis	-Claustrophobia in some patients	+Risk (1%) of failure rate, bleeding and perforation	-Rare allergic reaction (< 1%) to iodinated agents	-No complication
Comments pros	+ Diagnosis and therapeutic (treatment) procedure	+ Diagnosis and treatment procedure	Non-invasive+ No ionizing radiation+ Relatively operator -independent	Usually non-invasive (sedation)+ Diagnosis tool combined with tissue and/or lesion sampling	Non-invasive+ Faster method+ High resolution+ Operator-independent	Non-invasive+ No ionizing radiation+ High resolution+ Faster method+ Operator-independent
Cons	Invasive ionizing radiationOperator- dependent	InvasiveIonizing radiationOperator dependent	Expensive-poor resolutionSolely diagnostic methodMotion sensitiveclaustrophobia	Operator dependentHighly motion sensitiveThermal effects and cavitations	Ionizing radiationSolely diagnostic method	Low imaging depth 3 mmMotion sensitive

PTC: Percutaneous transhepatic cholangiography; ERCP: Endoscopic retrograde cholangiopancheatography; MRCP: Magnetic resonance cholangiopancreatography; US: Ultrasound; EUS: Endoscopic ultrasound; HFUS: High frequency ultrasound (> 10 MHz); IDUS: Intraductal ultrasonography; CT: Computed tomography; OCT: Optical coherence tomography.

Table 3 Comparison of different types of optical coherence tomography systems

Parameters	TD-OCT	SD-OCT	SS-OCT/OFDI
Mechanism	Interference signals are detected as a function of optical time delay between obj. and ref. arm.	Interference signals are detected with a camera as a function of optical frequency	Spectral fringes are mapped to time domain by use of a swept laser and are measured with a detector as a function of time
Major components	Broadband laser, optical delay line and a detector	Broadband laser, spectrometer and camera	Tunable laser, digitizer and a balanced detector
Spectrum	800 nm, 1000 nm, 1300 nm	800 nm, 1000 nm, 1300 nm	800 nm, 1000 nm, 1300 nm
Imaging depth	1-3 mm	1-3 mm	1-3 mm
Resolution	≥ 10 μm	1-10 μm	1-10 μm
Imaging speed (axial scan rate)	Slow ( ≤ 5 kHz)	Fast (20-150 kHz)	Fairly fast (20-400 kHz)
SNR	Low	High	High
Image quality	Moderate	Fairly high	High
Sensitivity	Low (70-90 dB)	High (85-105 dB)	High (≥ 100 dB)
Phase stability	Low	High	Moderate
Portability	Yes	Yes	Yes
System cost	Low	High	Moderate

SNR: Signal-to-noise ratio; dB: Decibel; TD-OCT: Time domain OCT; SD-OCT: Spectral-domain OCT; ODFI: Optical frequency domain imaging; SS-OCT: Swept source OCT.