CT
Pulmonary CT angiography protocols have been evolving over the years for evaluating pulmonary embolus [
10]. Adequate contrast opacification is critical for diagnostic quality, which depends upon patient weight, cardiac output, scan duration and contrast delivery protocol [
11,
12]. Arterial enhancement depends on the amount of contrast delivered per unit of time (injection flow rate) and the injection duration, measured in seconds [
13]. Suggested minimal luminal attenuation to see all acute and chronic pulmonary venous emboli (PE) is 93 and 211 HU respectively [
14]. On a 64-detector CT, a mean pulmonary artery opacification of 250 HU could be achieved with 1.2 ml/kg of 350 mg I/ml injected at 4 ml/s [
11]. Iodine flow rate of 1.6 g I/s has been suggested as optimal to reach the pulmonary artery enhancement of 300 HU [
15]. The scan duration depends upon the scanner (16, 64, dual source, dual source high pitch, 256, 320 slice multidetector [MD] CT), which on a high pitch scanner this may be less than 2 seconds [
16]. With a faster scanner, contrast volume can also be decreased by using a higher iodine concentration [
12].
For CTA, a region of interest can be placed in the main pulmonary artery and a timing bolus or bolus tracking can be utilised to determine the time it takes for intravenously injected contrast to reach the pulmonary arteries [
17]. Either of these techniques results in homogenous opacification and diagnostic image quality [
18]. Contrast flow rate of at least 3 ml/s is associated with lower frequency of insufficient contrast enhancement during chest CT [
19]. Flow rate of more than 4 ml/s using an 18-G cannula has been suggested for PE exams [
20,
21] A lower volume of contrast and iodine dose can be administered using a higher concentration (350 mg iodine/ml vs 300 mg/ml) [
22]. Wu et al. [
23] have described a low contrast dose (30 ml) pulmonary 64-detector CT angiography technique without compromising diagnostic image quality. The duration of contrast administration is calculated as scan duration plus additional few seconds (6–8 s). This delay accounts for the interval between the scan trigger and the start of acquisition [
12].
When evaluating for Fontan circulation, Park et al. [
24] found that a 3-min delay time from the time of injection to be optimal for enhancement of the pulmonary arteries, irrespective of the intravenous route used for administration. Bolus tracking demonstrated a high failure rate in providing homogenous enhancement of the Fontan circulation and of the pulmonary arteries.
For all pulmonary CT angiography studies, a caudocranial direction of acquisition is recommended as it reduces the chances of having respiratory motion related artefacts [
14]. At our institution, in-patients with normal (Stage 1, glomerular filtration rate [GFR mL/min/1.73 m2] = 90+) and mildly reduced renal function (Stage 2, GFR = 60–89) and no contraindication to CT contrast agent, contrast volume is determined from patient height, weight, age, sex, heart rate and scan duration using vendor-specified protocol (MEDRAD) with a timing bolus (test bolus of 20 ml contrast and 50 ml saline at 4 ml/s to find the time to peak in the main pulmonary artery is used to determine the scan delay, scan delay = time to peak in pulmonary artery + 9 s) [
25]. The maximum allowed injection flow rate is 6 ml/s. In patients with moderately impaired renal function (stage 3 A, GFR = 45–60), bolus tracking with 75 ml of contrast at 4–5 ml/s is used. In patients with moderately reduced renal function (Stage 3 B, GFR = 30–44) 30 ml of contrast with bolus tracking from SVC, preferably on the 256 slice MDCT is used. Any contrast injection is avoided in patients with GFR less than 29 unless they are on haemodialysis. CT angiography protocol used at our institution is presented in Table
1.
Table 1
Pulmonary CT angiography protocol used at our institution
Congenital | Power or hand injection 3 ml/s, 50 ml contrast (300 mg I/ml), no saline chaser | Small = 80, medium = 100, large = 120 80–120 | Tube current modulation | Axial: 3 × 2 mm, 2 × 1 mm Coronal: 3 × 2 Axial MIPS: 8 mm | 25 s delay, Complete thorax |
Pulmonary embolism | Dual head power injector 4–5 ml/s (350 mg/ml), + 50 ml saline chaser | 80–140 | Tube current modulation | Axial: 3 × 2 mm, 2 × 1 mm Coronal: 3×2 Axial MIPS: 8 mm | Weight-based contrast, Bolus track or timing bolus, Minimal post threshold delay |
Pulmonary hypertension | Power or hand injection 2–3 ml/s, no saline chaser | 80–140 | Tube current modulation | Axial: 1 × 0.5 mm, 3 × 2 Coronal: 3 × 2 Axial MIPS: 8 mm | Low kVp, 50–75 ml contrast, Additional expiratory scans, HRCT recons |
Pregnant patient | Dual head power injector 4–5 ml/s, + 50 ml saline chaser | 80–100 | Tube current modulation | Axial: 3 × 2 mm Coronal: 3 × 2 Axial MIPS: 8 mm | Low kVp, max. 75 ml contrast, Z-axis coverage: Aortic arch - diaphragm |
Renal dysfunction | Dual head power injector 3–4 ml/s, + 50 ml saline chaser | 80–120 | Tube current modulation | Axial: 3 × 2 mm Coronal: 3 × 2 Axial MIPS: 8 mm | 30–75 ml contrast, preferably on 256 MDCT, Trigger from SVC |
MR
MR imaging for the diagnosis of pulmonary artery disease can be performed using high-field MR scanners (>1.5 T) [
26]. It is indicated when cardiac function and flow needs to be evaluated, such as congenital heart disease, calculating intra/extra-cardiac shunts, right ventricle strain in PE and pulmonary hypertension. Non-contrast sequences used include a bright blood steady state free precession (SSFP), T2-weighted inversion recovery and T1 GRE (gradient echo). Post-contrast MR angiography is performed with extracellular gadolinium contrast agent injected at 0.1–0.2 mmol/kg. When evaluating for PE, a combination of MR angiography GRE and SSFP images have the highest sensitivity [
27]. MR is the imaging modality of choice for evaluating the right ventricle size and function [
28]. Contrast-enhanced MR angiography with gadolinium-based MRI contrast agent, using both high–spatial-resolution and high–temporal-resolution protocols (high–spatial-resolution contrast-enhanced MR angiography and time-resolved contrast-enhanced MR angiography), is an excellent non-invasive imaging tool for the evaluation of surgical cavopulmonary connections [
29]. Pulmonary MR angiography should be considered as an alternative to CT angiography when iodine contrast injection or radiation is a significant matter [
30]. It has been proposed that electrocardiograph (ECG)-gated and respiratory navigator-gated MR angiography at 3 T using a blood-pool contrast agent at 0.3 mmol/kg can deliver better image quality and vessel sharpness [
31]. Although, gadolinium-based contrast agents are not recommended in patients with a GFR less than 30 or acute renal failure in patients with hepatorenal syndrome unless essential due to risk for nephrogenic systemic sclerosis [
32]. Pulmonary MR angiography protocol used at our institution is presented in Table
2.
Table 2
Pulmonary MR angiography protocol used at our institution on a 1.5-T magnet
Non-contrast | SSFP | Axial, coronal, ventricle short axis | 4/0 | 1.4/3.4 | 45 | 200 × 160 | 350–420 | 125 | 0.75 | Morphology, ventricle function |
T1 | Axial, short axis | 6/0 | 42 | 90 | 256 | 38 | 62.5 | 1 | Morphology, characterise mass lesions, oedema |
T2 | Axial, short axis | 6/0 | 41/1,791 | 90 | 256 × 256 | 350 | 62.5 | 1 | |
Phase Contrast | Perpendicular to pulmonary flow | 8 | 2.7/5.6 | 25 | 192 × 128 | 350 | 31.25 | 1 | Quantify pulmonary flow volume, peak-mean velocity, regurgitation |
Contrast-enhanced MR angiography | MR angiography | Coronal | 2.0 | 1.4/3.9 | 30 | 224 × 224 | 320–420 | 62.5 | .5 | Luminal assessment |
Time resolved | Coronal | 2.6 | 1.2/3.2 | 38 | 256 × 192 | 40 | 62.5 | 0.5–0.75 | |
3D GRE | Axial | 4/–2 | 1.9/3.9 | 12 | 320 × 160 | 320–420 | 83.3 | 0.75 | |
Delayed enhanced | Axial, short axis | 8/0 | 1.3/5.3 | 20 | 224 × 192 | 35 | 22.7 | 1 | Thrombus, vessel wall, inflammation/scar |
PET-CT
F-18 fluorodeoxyglucose (FDG) PET/CT is useful in identifying a pulmonary artery lesion as malignant if the luminal lesion has high FDG uptake [
33] and is useful in preoperative evaluation [
34]. It is also very useful in identifying active vasculitis in patients with pulmonary vasculitis such as Takayasu’s arteritis [
35] and monitoring response to immunosuppressive treatment [
36]. At our institution, a PET-CT for these indications is combined with a contrast-enhanced CT angiography of pulmonary arteries to better depict the vascular anatomy rather than a non-contrast CT for attenuation correction.