MDCT is one of the most used imaging modalities for the imaging of the hepatobilliary system and pancreas.

2. Preliminaries
 Consent
 Patient Preparation;
 Positioning;

3. Introduction
 With MDCT multiphasic examinations of the liver and pancreas can be performed without
compromises with regard to spatial or temporal resolution.
 However, an adequate examination technique is still critical for sensitive detection and specific
characterization of lesions.
 The criteria for the characterization of lesions are derived from their behavior and degree of
contrast agent enhancement in the different vascular phases of CT.
 For a valid characterization usually at least two different phases are mandatory, so that changes of
enhancement over time (e.g., wash-in and wash-out of contrast agent) can be appreciated.

4. Basic Concepts for Liver Imaging
 The conspicuity of a liver lesion depends on the attenuation difference between the lesion and the
normal liver.
 On a non enhanced CT-scan (NECT) liver tumors usually are not visible, because the inherent contrast
between tumor tissue and the surrounding liver parenchyma is too low.
 Only a minority of tumors contain calcifications, cystic components, fat or hemorrage and will be
detected on a NECT.
 So i.v. contrast is needed to increase the conspicuity of lesions.

5. Basic Concepts for Liver Imaging
 When we give i.v. contrast, it is important to
understand, that there is a dual blood supply to
the liver.
 Normal parenchyma is supplied for 80% by the
portal vein and only for 20% by the hepatic artery,
so it will enhance in the portal venous phase.
 This difference in blood supply results in different
enhancement patterns between liver tumors and
normal liver parenchyma in the various phases of
contrast enhancement

6. Small Hepatocellular carcinoma in
cirrhotic liver not visible on NECT
(left), clearly visible in arterial phase
(middle) and not visible in portal
venous phase (right)

7. Arterial Vs Parenchymal Enhancement
 Enhancement of the arterial phase is dependent on the:
 contrast medium injection rate,
 injection duration, and
 the time of the scan performed relative to the contrast bolus.
 Arterial opacification can primarily be controlled by the:
 iodine administration rate, which is further dependent on the flow rate and the
concentration of medium administered.
 It is important that the injection duration be longer than the scanning time to ensure strong
vascular enhancement by the recirculation of contrast.

8. Arterial Vs Parenchymal Enhancement
 On the other hand, the parenchymal enhancement is
 independent of the injection flow rate and
 depends on the total volume (dose) of contrast administered.
 Thus, to obtain optimal liver parenchymal enhancement, a sufficient volume of contrast medium is
required approximately 120–150 cc of 370 mgI contrast agent.
 The iodine dose is directly proportional to the
 contrast volume administered and/or
 the iodine concentration of the contrast medium.

9. Arterial Vs Parenchymal Enhancement

11. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 According to the different enhancement curves of the hepatic artery, portal vein, and hepatic
parenchyma, four phases can be distinguished:
1. Early arterial phase (EAP)
2. Late arterial phase (LAP)
3. Portal venous phase (PVP)
4. Equilibrium phase (EQP)

12. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 Early arterial phase (EAP)
 appears 20–25 s after administration of contrast material when there is conspicuous enhancement
in the hepatic arteries compared with almost no enhancement of liver parenchyma or
hypervascular lesions.
 typically provides the least information for imaging the liver, since the contrast media at that time
has accumulated neither in hypervascular liver lesions nor in liver parenchyma.
 this phase is well suited for CT angiography when used to evaluate the anatomical configuration of
hepatic arteries prior to liver transplantation, hepatic tumor resection, or arterial
chemoembolization.

13. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 Early arterial phase (EAP)
 For optimum timing for EAP scanning, an automated triggering system can be used.
 The scanner is typically set at the top of the liver with the trigger placed in the descending
thoracic aorta.
 When the trigger, reaches a predefined attenuation (typically 90–100 HU), the scan begins for the
EAP.

14. First-pass acquisition shows
isolated enhancement of
hepatic artery at hepatic hilum.

15. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 Late arterial phase (LAP)
 appears at about 30–35 s following initiation of contrast material administration.
 using the automated triggering technique, to avoid the EAP, an additional 8- to 10-s delay is
required.
 The LAP is also referred to as the portal vein inflow phase, since the portal vein is already starting
to enhance during this phase.
 The hepatic arterial systems as well as prominent neovasculature of hypervascular hepatic
neoplasms continue to enhance during the LAP, while there is only minimal enhancement of hepatic
parenchyma.

16. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 Late arterial phase (LAP)/ portal vein inflow phase
 At this point, there is a maximum attenuation difference between hypervascular liver lesions and
the surrounding liver parenchyma.
 Thus, LAP is the optimal phase for detecting hypervascular neoplasms of the liver.
 acquisition of the LAP together with the PVP is considered sufficient for detection of HCC with
MDCT.

17. enhancement of both hepatic
artery and portal vein at hepatic
hilum.

18. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 Portal venous phase (PVP) or hepatic venous phase
 appears at about 60–70 s following initiation of a contrast media bolus, when the enhancement of liver
parenchyma reaches its peak and the portal vein and hepatic veins are well enhanced.
 For accurate timing of the PVP in a single-phase exam, automated scanning technology instead of a
fixed time delay is preferred.
 The trigger is placed in liver parenchyma, and when attenuation reaches a predefined threshold (50–70
HU), the table is moved to the top of the liver and the diagnostic scan initiated.
 For a dual-phase exam, there is a fixed time delay of 40 s following the end of the LAP.

19. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 Portal venous phase (PVP) or hepatic venous phase
 Hypovascular tumors are optimally detected during the PVP when enhancement of liver parenchyma is
maximal and there is the greatest liver-to-lesion attenuation difference.
 For detection of these tumors, a single scan during the PVP is sufficient, since there is no further
advantage performing unenhanced or arterial-phase imaging.
 The PVP is also the appropriate phase for visualization and evaluation of intrahepatic bile ducts, when
there is the greatest difference of attenuation between the maximally enhanced liver parenchyma and
the hypoattenuating intraductal bile

20. shows enhancement of main portal vein
and its branches in hepatic parenchyma
and enhancement of hepatic veins (arrows)

22. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 Equilibrium phase (EQP), or interstitial phase
 appears at approximately 3 min postinjection, when there is an increased diffusion of contrast
media into liver parenchyma and attenuation difference between parenchyma and vessels is
minimal.
 Washout of the contrast material in different liver lesions may vary vastly depending on their
histological nature.

23. Different Phases of Hepatic Vascular and Parenchymal
Enhancement
 Equilibrium phase (EQP), or interstitial phase
 One clear indication for acquiring images during the EQP includes intrahepatic cholangiocarcinoma.
This tumor may accumulate the contrast agent and show a delayed washout compared with
surrounding liver parenchyma. This delay causes hyperattenuating lesions.
 By comparison, HCC may show a faster washout during the EQP relative to the surrounding liver
parenchyma, representing a hypoattenuating mass.

24.  Following an intravenous bolus of contrast
material, the hepatic artery enhances first at
approximately 15 s and reaches peak
attenuation at approximately 30 s. After the
contrast medium returns from the splanchnic
system, the portal vein starts to enhance at
around 30 s. Enhancement of liver
parenchyma begins later, reaching a plateau
at 60–70 s. The plateau may last up to 20–30
s. Finally, there is the equilibrium phase
(EQP) (3 min and later), which occurs when
the amount of contrast material in the intra-
and extravascular extracellular space is
essentially the same.

25. Multiphase Imaging of Liver
 In the arterial phase hypervascular tumors will enhance via the hepatic artery, when normal
liver parenchyma does not yet enhances, because contrast is not yet in the portal venous
system. These hypervascular tumors will be visible as hyperdense lesions in a relatively
hypodense liver. However when the surrounding liver parenchyma starts to enhance in the
portal venous phase, these hypervascular lesion may become obscured.
 In the portal venous phase hypovascular tumors are detected, when the normal liver
parenchyma enhances maximally. These hypovascular tumors will be visible as hypodense
lesions in a relatively hyperdense liver.
 In the equilibrium phase, tumors become visible, that either loose their contrast slower than
normal liver, or wash out their contrast faster than normal liver parenchyma. These lesions will
become either relatively hyperdense or hypodense to the normal liver.

27. Detection of a lesion depends on difference in attenuation between liver and lesion.
LEFT: Arterial phase showing hypervascular FNH MIDDLE: Portal venous phase
showing hypovascular metastasis RIGHT: equilibrium phase showing relatively dense
cholangiocarcinoma

28. Unenhanced Phase
 There are selected cases in which an unenhanced CT scan of the liver is helpful and recommended.
Reasonable clinical indications for a non-contrast hepatic CT include:
 Depiction of acute hemorrhage of the liver
 Delineation of siderotic nodules
 Detection and characterization of hepatic calcification (e.g., calcified metastases,, hydatid cysts)
 Evaluation of parenchymal liver diseases (e.g., fatty infiltration, hepatic cirrhosis,
hemochromatosis)
 Follow-up CT scan after embolization of hypervascular liver lesions.

29. Arterial phase imaging
 Optimal timing and speed of contrast injection are very important for good arterial phase imaging.
 Hypervascular tumors will enhance optimally at 35 sec after contrast injection (late arterial
phase).
 This time is needed for the contrast to get from the peripheral vein to the hepatic artery and to
diffuse into the liver tumor.

30.  two phases of arterial imaging
at 18 and 35 seconds. In the
early arterial phase we nicely
see the arteries, but we only
see some irregular
enhancement within the liver.
In the late arterial phase we
can clearly identify multiple
tumor masses.
Arterial phase imaging

31.  Timing of scanning is important, but almost as important is speed of contrast injection.
 For arterial phase imaging the best results are with an injection rate of 5ml/sec.
 There are two reasons for this better enhancement:
 at 5ml/sec there will be more contrast delivered to the liver when you start scanning and
 this contrast arrives in a higher concentration.
Arterial phase imaging

32. Arterial phase imaging
 patient with cirrhosis examined
after contrast injection at
2.5ml/sec and at 5ml/sec.
At 5ml/sec there is far better
contrast enhancement and better
tumor detection.

37. Portal Venous phase
 Portal venous phase imaging works on the opposite idea.
We image the liver when it is loaded with contrast through the portal vein to detect
hypovascular tumors
 The best moment to start scanning is at about 75 seconds, so this is a late portal venous phase,
because enhancement of the portal vein already starts at 35 sec in the late arterial phase.
 This late portal venous phase is also called the hepatic phase because there already must be
enhancement of the hepatic veins. If we do not seen enhancement of the hepatic veins, it’s too
early.
 If we only do portal venous imaging, for instance if we are only looking for hypovascular
metastases in colorectal cancer, fast contrast injection is not needed, because in this phase the
total amount of contrast is more important and 3ml/sec will be sufficient.

38. Portal Venous phase
 Hypovascular metastases
seen as hypodense lesions
in the late portal venous
phase.

41. Equilibrium Phase
 The equilibrium phase is when contrast is moving away from the liver and the liver starts to
decrease in density.
 This phase begins at about 3-4 minutes after contrast injection and imaging is best done at 10
minutes after contrast injection.
 This phase can be valuable if you're looking for:
 fast tumor washout in hypervascular tumors like HCC or retention of contrast in the blood pool
as in hemangiomas, or
 the retention of contrast in fibrous tissue in capsules (HCC) or scar tissue (FNH,
Cholangiocarcinoma).

42. Equilibrium Phase

43. Equilibrium Phase
Relative hyperdense lesions in the delayed phase
 Fibrous tissue that's well organized and dense is
very slow to let iodine in.
 Once contrast gets in however, it is equally slow
to get back out in the equilibrium phase.
 So when the normal liver parenchyma washes
out, the fibrous components of a tumor will look
brighter than the background liver tissue.
 Cholangiocarcinoma may have a fibrous stroma
and in the delayed phase it may be the only time
when you see the tumor (figure).
Small cholangiocarcinoma not visible in
portal venous phase (left), but seen as
relative hyperdense lesion in the delayed
phase (right).

44. Equilibrium Phase
Relative hypodense lesions in the delayed phase
 the importance of the delayed phase in a cirrhotic
patient with an HCC is demonstrated.
 Notice that you do not see the tumor on the
nonenhanced scan and also not in the portal venous
phase. This is often the case and demonstrates the
importance of the arterial phase.
 Now the issue at hand is in small enhancing lesions in a
cirrhotic liver whether it is a benign lesion like a
regenerating nodule or a HCC.
 In the delayed phase we see that the tumor is washed
out more than the surrounding liver parenchyma.
HCC in a cirrhotic liver. Notice fast wash out in
equilibrium phase compared to surrounding liver
parenchyma.

45. Equilibrium Phase
Relative hypodense lesions in the delayed phase
 Benign lesions typically will not show this kind of wash
out.
 For instance a FNH or adenoma will show fast
enhancement in the arterial phase, become isodense in
the portal venous phase, but it will stay isodense with
liver in the equilibrium phase.
 These benign tumors do not have enough neoplastic
neovascularity to have a fast wash out.
 Especially in cirrhotic patients you have to rely heavily
on this delayed phase to differentiate benign little
enhancing lesions from small HCC's.
HCC in a cirrhotic liver. Notice fast wash out in
equilibrium phase compared to surrounding liver
parenchyma.

51. Tailored CT protocol
 We have to adapt our protocol to the type of scanner, the speed of contrast injection and to the
kind of patient that we are examining.
 If we have a single slice scanner, it will take about 20 seconds to scan the liver.
 For late arterial phase imaging 35 sec is the optimal time, so we start at about 25 seconds and
end at about 45 seconds.
 However if we have a 64-slice scanner, it’s possible able to examine the whole liver in 4 seconds.
So we start scanning at about 33 seconds, which is much later.
 In aterial phase imaging the time window is narrow, since we have only limited time before the
surrounding liver will start to enhance and obscure a hypervascular lesion.

52. Tailored CT protocol
 For portal venous phase imaging it is different
 Here we don't want to be too early, because we want to load the liver with contrast and it takes time
for contrast to get from the portal vein into the liver parenchyma.
 Besides we have more time, because the delayed or equilibrium phase starts at about 3-4 minutes.
 So we start at 75 seconds with whatever scanner we have.
 Only when we inject with high speed at 5ml/sec we may start earlier at about 65-70 seconds.

53. Tailored CT protocol

54. Contrast Volume
 Reduced volumes of contrast injection are not favored for liver imaging due to concerns about image
quality.
 Also, the volume of contrast to be injected varies depending on the iodine concentration in the contrast
medium.
 Usually in cases of MDCT liver imaging, 120–150 cc of 300 mgI/ml of nonionic contrast is injected at a
rate of 4 cc/s.
 On the other hand, if 370 mgI/ml is used, only 80–100 cc would be required, but this needs to be
balanced with a slightly higher injection rate of 4–5 cc/s.

55. Contrast Volume
 Thus, with use of higher or lower iodine concentration contrast media, appropriate adjustments in
injection rate and contrast volume are needed
 For most applications, 38–44 g of iodine is recommended
 Total iodine doses less than 30 g are also not recommended, as the duration and magnitude of hepatic
enhancement will decrease, resulting in a lower detection rate of focal liver lesions.

56. Rationale for High-Concentration Contrast Medium
 The use of high iodine concentration contrast medium has gained importance in patients with
decreased cardiac output, obesity, in conditions such as cirrhosis of the liver or portal vein
thrombosis,
 maximum hepatic enhancement in obese patients is significantly lower than in those who are
lighter in weight.
 This could be attributed to the decreased level of perfusion of the liver in obese patients

57. Rationale for High-Concentration Contrast Medium
 in cases of liver cirrhosis, due to decreased portal perfusion, the peak contrast enhancement
in liver is late, and usually, the plateau of contrast enhancement occurs in the late portal
phase.
 The injection of contrast medium with standard iodine concentration could increase the
possibility of missing hypovascular metastases in heavy patients or in patients with cirrhosis
or chronic hepatitis.
 the lesion-to-liver contrast can be improved when high iodine concentration contrast medium
is used

59. Liver MDCT: Siemens Definition Edge 128 CT scanner (Siemens Medical Solutions, Malvern, PA, USA).
Intravenous (IV) contrast: 370 mg/mL.
Weight-based dose: <60kg: 80 mL, 60-90kg: 90 mL, >90kg: 120 mL.
IV gauge: 18 gauge.
IV contrast rate: 4 mL/s.
Oral contrast: water.
Bolus tracking: 150 HU of abdominal aorta measured at the level of the mid liver, monitored delay of 10 seconds and scan delay
of 15 seconds for arterial phase.

61. Dual-Energy Computed Tomography
 Imaging at a lower energy of 80 kVp can improve hypervascular lesion conspicuity as well
as reduce the effective radiation dose, and the amount of iodinated contrast required
for an examination.
 The disadvantage of using a lower kVp is that image noise increases due to attenuation
of the x-ray beam by the patient, which is exacerbated in larger patients.

62. Hepatocellular carcinoma (A, B, arrow) shown during
the late arterial phase during dual-energy acquisition
is more conspicuous on the output of the 80-kV
image (A) compared with 140-kV image (B). Both the
portal phase image at 120 kV (C) and the prior late
arterial phase at a single-energy 120-kV examination
(D) show the tumor, but with decreased conspicuity
compared with the 80-kV image. Note increased
noise in the 80-kV examination (A).

63. Radiation protection/dose
 Dose reduction techniques: automatic exposure control (mA) and iterative slice reconstruction.
 Expected DRL:
 CTDIvol per sequence (mGy) – 14
 DLP per complete examination (mGy cm) – 910

65. The Biliary System
 MDCT is not generally considered to be a first-line imaging technique for patients with suspected
biliary pathology
 The intrahepatic bile ducts, which are linear structures accompanying the portal vein and hepatic
arterial branches, can be best visualized during the PVP when there is an optimal attenuation
difference between hypodense bile ducts and the adjacent enhanced vessels and parenchyma
 role of MDCT in the evaluation of different biliary pathologies includes:
 Cholangiocarcinoma
 Primary Sclerosing Cholangitis
 Acute and Chronic Cholecystitis
 Gallbladder Carcinoma

66. Early primary sclerosing cholangitis during the unenhanced state (a), LAP (b), PVP (c). Note the scattered
intrahepatic ductal dilatation

67. Multidetector computed tomography (MDCT) image
of the gallbladder in a patient with acute cholecystitis

68. Pancreas CT

69. Concepts in Pancreatic Imaging
 Detection of lesions within the pancreas on CT depends largely on the enhancement pattern of
the lesion and the alteration in contour of the normal pancreas.
 Before initiation of contrast-enhanced MDCT of the pancreas, administration of negative oral
contrast medium is performed to distend the stomach and duodenum, which facilitates detection
of abnormalities in the pancreatic bed.
 The use of negative oral contrast medium has an added advantage in that it does not mask
radiopaque stones in the common bile duct, and it may aid in the evaluation of gastric and
duodenal wall lesions
 The administration of oral contrast material is necessary to improve delineation of the pancreatic
head against the duodenum.

70. Concepts in Pancreatic Imaging
 Administration of a positive oral contrast material may sometime impair the delineation of the
pancreatic head if dilution of the ingested contrast material by retained fluid in the stomach yields
the same CT density as the pancreatic head enhanced by intravenous contrast material.
 Thus, Richter et al. (1998) developed the concept of hydro-CT of the pancreas: Patients are to drink
at least 1,000 ml of water prior to the examination.
 Filling of the stomach and duodenum with water, which acts as a negative contrast agent, allows
excellent delineation of the pancreatic head, which improves detection of pancreatic tumors
adjacent to the duodenum
 When initial scans fail to differentiate the margins of the pancreas from the duodenum, the patient
is often given additional oral contrast material and additional slices are obtained with the patient
lying in a right decubitus positions

71. Indications
Typical indications include an evaluation of the following:
 jaundice
 evaluation of pancreatic tumors and/or cystic lesions
 acute or chronic pancreatitis
 complications of pancreatic diseases
 unclear findings on ultrasound or CT abdomen
 pancreatic interventions (e.g. CT-guided biopsy, drainage)

73. Standard CT for Pancreatic Tumor Evaluation
 A pancreas-specific protocol for pancreatic cancer typically utilizes a thin-section, multiphase
technique, with either two phase or four phase scans.
 Two phase scan protocol is more commonly used than four phase protocols, and include pancreatic
phase (also known as the late arterial phase, 35–50 s after the start of contrast injection) and portal
venous phase images (55– 70 s after the start of contrast injection).
 Pancreatic phase images show peak pancreatic parenchymal enhancement, and therefore provide the
best lesion to pancreas contrast, and can be useful in identifying vascular involvement by pancreatic
cancer.
 Portal phase images are helpful to assess the extent of the venous involvement as the portomesenteric
venous system is well opacified and to identify possible liver metastases.

74. Standard CT for Pancreatic Tumor Evaluation
 Four phase scans include precontrast images and early arterial phase (CT angiography phase, 17–25 s
after the start of contrast injection) in addition to pancreatic phase and portal venous phase images.
 Noncontrast images can be helpful in identifying pancreatic calcifications, ductoliths, and biliary
stones, and therefore, generally, there is no diagnostic need for noncontrast scans in patients with
suspected pancreatic tumors unless there is coexistent chronic pancreatitis.
 In addition, early arterial phase imaging is good for evaluation of vascular anatomy

76. Pancreatic multidetector computed
tomography angiography: Siemens
Definition Edge 128 CT scanner.
Intravenous (IV) contrast: 370 mg/mL.
Weight-based dose: <60kg: 80 mL, 60-
90kg: 90 mL, >90kg: 120 mL.
IV gauge: 18–20 gauge.
IV contrast rate: 4 mL/s.
Oral contrast: water.

77. SPLIT BOLUS
 Recently, utilization of split-bolus injection in conjunction with spectral CT has been shown to
improve pancreatic tumor conspicuity and reduce radiation dose by combing the pancreatic and
portal venous phases into one scan.
 This is done by initially administering 100 mL iodinated intravenous contrast before the CT for the
portal venous phase and then injecting an additional 40 mL of contrast 35 seconds later to enhance
the pancreatic phase.
 Bolus tracking initiates scanning 15 seconds after the abdominal aorta reaches an attenuation of 280
HU. Images are then reconstructed at 60 and 77 keV. This scanning protocol offers marked reduction
in radiation dose and higher tumor conspicuity with the 60 keV compared with a standard pancreatic
protocol CT.

78. Post-process of MDCT for pancreatic
neck cancer. (a) Axial CT scan that
obtained during pancreatic phase
shows a 2.5 cm, ill-defined
hypovascular mass (arrow) in the
pancreas neck. (b) Curved
multiplanar reconstruction (MPR)
image along the pancreatic duct
demonstrates dilatation of the
upstream pancreatic duct
(arrowheads), and decreased
enhancement of parenchymal due to
associated pancreatitis. (c) Oblique
coronal MPR image, mass encases
the gastroduodenal artery (arrow)
while preserving fat plane between
CHA (open arrow) and mass. (d) On
coronal maximum intensity projection
(MIP) image, the main portal vein
and proximal superior mesenteric
vein show a long luminal narrowing
(arrow) over 2.5 cm due to tumor
involvement

79. Pancreas cancer on dual-energy CT.
(a–c) Dual-energy CT examination
composed of (a) virtual noncontrast,
(b) virtual monoenergetic spectral
image (VMS, reconstruction on 50
KeV monoenergy level), (c) iodine
map images. Compare to (d)
conventional 120 kVp pancreatic
phase image, an ill-defined
hypovascular mass (arrow) in
pancreas body is more clearly
delineated on (b) VMS and (c) iodine
map image

86. Pancreatic carcinoma is best imaged at 35 sec p.i.
Liver metastases are best imaged at 70 sec p.i.

87. Reference
 Hammerstingl R, Vogl T. Abdominal MDCT: protocols and contrast considerations. European
Radiology Supplements. 2005;15(S5):e78-e90.
 Price M, Patino M, Sahani D. Computed Tomography Angiography of the Hepatic, Pancreatic,
and Splenic Circulation. Radiologic Clinics of North America. 2016;54(1):55-70.
 Ji H, McTavish J, Mortele K, Wiesner W, Ros P. Hepatic Imaging with Multidetector CT.
RadioGraphics. 2001;21(suppl_1):S71-S80.
 https://radiologyassistant.nl/abdomen/liver/characterisation-of-liver-masses
 https://radiologyassistant.nl/more/ct-protocols/ct-contrast-injection-and-protocols
 https://www.youtube.com/watch?v=z_M3oQytmGY

88. Reference