Multislice Spiral Computed Tomography Angiography Research Articles

Application of multi-planar reconstruction technique in endovascular repair of aortic dissection

BACKGROUND Endovascular repair of aortic dissection is an effective method commonly used in the treatment of Stanford type B aortic dissection. Stent placement during the operation was one-time and could not be repeatedly adjusted during the operation. Therefore, it is of great significance for cardiovascular physicians to fully understand the branch status, position, angle, and other information regarding aortic arch dissection before surgery. AIM To provide more references for clinical cardiovascular physicians to develop treatment plans. METHODS Data from 153 patients who underwent endovascular repair of aortic dissection at our hospital between January 2021 and December 2022 were retrospectively collected. All patients underwent multi-slice spiral computed tomography angiography. Based on distinct post-image processing techniques, the patients were categorized into three groups: Multiplanar reconstruction (MPR) (n = 55), volume reconstruction (VR) (n = 46), and maximum intensity projection (MIP) (n = 52). The detection rate of aortic rupture, accuracy of the DeBakey classification, rotation, and tilt angles of the C-arm during the procedure, dispersion after stent release, and the incidence of late complications were recorded and compared. RESULTS The detection rates of interlayer rupture in the MPR and VR groups were significantly higher than that in the MIP group (P < 0.05). The detection rates of DeBakey subtypes I, II, and III in the MPR group were higher than those in the MIP group, and the detection rate of type III in the MPR group was significantly higher than that in the VR group (P < 0.05). There was no statistically significant difference in the detection rates of types I and II compared to the VR group (P > 0.05). The scatter rate of markers and the incidence of complications in the MPR group were significantly lower than those in the VR and MIP groups (P < 0.05). CONCLUSION The application of MPR in the endovascular repair of aortic dissection has improved the detection rate of dissection rupture, the accuracy of anatomical classification, and safety.

Diagnostic value of multi-slice spiral computed tomography angiography for intracranial aneurysm.

To investigate the clinical value of multi-slice spiral computed tomography angiography (MSCTA) in the diagnosis of intracranial aneurysms (ICA). The imaging data of 68 patients suspected of having ICA who were examined in the Department of Radiology of the First Affiliated Hospital of Nanjing Medical University from March 2018 to March 2021 were retrospectively analyzed. MSCTA and digital subtraction angiography (DSA) were performed on each patient, and the MSCTA imaging analysis was compared with DSA. The accuracy of DSA in the diagnosis of ICA was 98.53% (67/68), while the accuracy of MSCTA in the diagnosis of ICA was 97.06% (66/68), with no significant difference in diagnostic accuracy (P > 0.05). There were no significant differences in the diameter of the aneurysm, the width of the aneurysm neck, or the location of the aneurysm in the comparison of the ICA image between DSA and MSCTA (P > 0.05). MSCTA offers high accuracy and has favorable clinical value in the diagnosis of ICA. It is worth popularizing as the recommended examination method in clinical practice.

Application effect of multi-slice spiral CT angiography combined with MRI in the diagnosis of cerebral aneurysm

ABSTRACT To investigate the clinical value of multi-slice spiral computed tomography (CT) angiography (MSCTA) combined with MRI in the diagnosis of cerebral aneurysm. A total of 90 patients with cerebral aneurysms diagnosed by DSA were selected as the subjects of this study. Another 30 patients with cerebral infarction were selected as negative controls (NC). Before diagnosis, all patients underwent comprehensive examination using MSCTA and MRI. The results of the comparison and the clinical data of all patients were retrospectively analyzed. MSCTA and MRI examinations can clearly show the specific location, shape, size and anatomical relationship with surrounding tissues of cerebral aneurysms. MSCTA diagnosed 82 patients and missed or misdiagnosed 8 patients in the 90 patients with cerebral aneurysm. The diagnostic sensitivity and accuracy of MSCTA were 91.1 (82/90) and 89.2 (107/120), respectively. MRI examination diagnosed 87 patients and missed or misdiagnosed 3 patients in the 90 patients with cerebral aneurysm. The diagnostic sensitivity and accuracy of MRI were 96.7 (87/90) and 96.7 (116/120), respectively. The sensitivity and accuracy of MSCTA combined with MRI were 100.0 (90/90) and 99.2 (119/120), respectively. MSCTA combined with MRI can not only display the whole picture of brain tissue, but also display the size, shape and relationship with the parent vessel of the aneurysm. The combination of MSCTA and MRI has high sensitivity and accuracy in diagnosing intracranial aneurysms, which provides a promising diagnostic protocol for patients with aneurysms.

Evaluation of isolated abdominal visceral artery dissection with multi-scale spiral computed tomography: a retrospective case series

BackgroundTo evaluate the role of multi-slice spiral computed tomography (MSCT) angiography in the diagnosis of spontaneous isolated visceral artery dissection (SIVAD).MethodsTwenty-seven patients with abdominal SIVAD were included in the study. The MSCT scans of the patients were subjected to various post-processing techniques to visualize the visceral artery wall. Clinical features including arterial dissection, thrombosis, dissection length, true/false lumen, and complications were recorded.ResultsType I, IIa, and IIb SIVADs were observed in 11, 6, and 10 patients, respectively. Superior mesenteric artery (SMA) dissection was the most common (n = 16), followed by abdominal aortic dissection (n = 6), splenic artery dissection (n = 2), renal artery dissection (n = 2), and splenic artery dissection (n = 1). One patient with SMA dissection suffered small intestine ischemia, 1 with splenic artery dissection had splenic infarction, and 1 patient with left renal artery dissection experienced renal infarction. The false lumen was bigger than the true lumen in 20 patients, with 9 patients having thrombus. The true lumen was bigger than the false lumen in 7 patients.ConclusionsMSCT angiography is a valuable technique in the diagnosis and treatment of patients with SIVAD. Patients with abdominal pain suspected due to SIVAD should be examined with MSCT angiography for early detection of SIVAD.

Research Status of Postmortem MSCT Angiography in Forensic Science

Virtual autopsy is a new technique for investigating the morphological changes of cadaveric tissues and organs by medical imaging technology. It has been widely used in the identification of causes of death. Multislice spiral computed tomography （MSCT） has become a routine inspection method in some identification institutions, although it cannot completely replace traditional autopsy, it plays a key auxiliary or substitute role in the identification of certain abnormal causes of death. Plain MSCT scan cannot visualize cadaveric vessels, but can perform cadaveric angiography to determine vascular injury or disease. This technology has entered a rapid development period in recent years, and has made a considerable progress in contrast agent, perfusion methods and imaging methods. This article summarizes several common cadaveric MSCT angiography methods, such as systemic angiography, angiography through cardiopulmonary resuscitation, targeted angiography, and angiography by cardiac puncture, and analyzes and compares the application prospects.

Usefulness of ultrasound in the management of aortic dissection.

Aortic dissection is an acute large vessel disease characterized by sudden onset, rapid disease progression and high mortality. Color doppler ultrasound analyzed the morphological and hemodynamic status of aortic branches before and after aortic dissection surgery, including carotid, subclavian, renal, celiac, superior mesenteric and iliac arteries. Transthoracic echocardiography, vascular ultrasound, transesophageal echocardiography, intravascular ultrasound and contrast-enhanced ultrasonography are complementary to aortic multi-slice spiral computed tomography angiography for diagnosis, treatment and prognostic evaluation.

Guiding values of multislice spiral computed tomography angiography in laparoscopic D2 radical gastrectomy of local advanced gastric carcinoma.

This study aims to investigate the application values of preoperative multislice spiral computed tomography angiography (MSCTA) in laparoscopic radical resection of gastric carcinoma (GC). A total of 108 GC patients were divided into Groups I (i.e., where preoperative MSCTA was performed; n = 60) and II (i.e., where preoperative MSCTA was not performed; n = 48). Surgery was performed by the same group of surgeons. Seven cases of blood vascular variation were detected in Group I during surgery, whereas four cases were detected in Group II. The operation time for Group I ([207 ± 24] min) was shorter than that for Group II ([260 ± 31] min) (95% confidence interval [95% CI]: -21.543--0.920, P = 0.044). The operation time of patients with gastric vascular anatomic variation in Group I ([189 ± 49] min) was shorter than that of patients in Group II ([257 ± 61] min) (95% CI: -99.68--3.201, P = 0.048). The differences in the number of lymph node dissection, average blood loss, eating time, complication rate, and postoperative hospital stay between the two groups was not statistically significant (P > 0.05). MSCTA before endoscopic radical resection exhibits guiding values in assessing blood vascular variations and shortening operation times.

Relationship of the angle between the A1 and A2 segments of the anterior cerebral artery with formation and rupture of anterior communicating artery aneurysm

PurposeTo measure the angle between A1 and A2 segments of the anterior cerebral artery and analyze the relationship of this angle with the formation and rupture of anterior communicating artery aneurysm (ACoAA). MethodsPatients with ACoAA (n=64) and with non-ACoAA (n=187) randomly chosen were included. The A1-A2 segment angles were measured using multislice spiral computed tomography angiography. The angular dimensions and differences were recorded and compared between the ACoAA and non-ACoAA groups and the ruptured (n=23) and unruptured group (n=41). The A1 segment morphology was divided into predominant and balanced type. The ACoAA aneurysm protrusion direction was divided into five types. ResultsThe incidence of ACoAA was significantly higher in patients with A1 predominance compared to A1 balance (p<0.05). The mean A1-A2 segment angle was significantly smaller in the ACoAA group compared with the non-ACoAA group (p<0.001). There was no significant difference in mean A1-A2 segment angle between ruptured and unruptured groups. There was no significant relationship between aneurysm protrusion rupture and direction. ConclusionsThe formation of ACoAA is more likely when there is A1 segment predominance in the anterior cerebral artery. The A1-A2 angle can help predict the formation of ACoAA but not useful for rupture predicting.

The role of doppler ultrasound and multislice spiral computed tomography angiography in the common form of stomach cancer diagnosis

Aim. To evaluate the results of Doppler ultrasound and multislice computed tomography angiography in patients with gastric cancer and the sensitivity of these diagnostic methods for the detection of the abdomen great vessels lesions.Methods. The study included 32 patients aged 37 to 82 years who had histologically confirmed gastric adenocarcinoma diagnosis. The control group consisted of 30 apparently healthy people of the same age. Transabdominal ultrasonography of the abdominal cavity organs and stomach in gray-scale mode (B-mode before and after the stomach filling with degassed liquid), Doppler ultrasound of the abdomen and the stomach wall vessels, multislice spiral computed tomography angiography were performed in all patients at the preoperative stage. The analysis of the study results and their comparison with intraoperative data were conducted.Results. In patients with gastric cancer a statistically significant increase in peak systolic velocity in the celiac trunk, superior mesenteric artery, left gastric artery and resistive index decrease in these blood vessels (p &lt;0.05) with the presence of atypical vascularization in the affected stomach walls were registered. The data obtained during the multislice spiral computed tomography angiography were analyzed. The sensitivity of Doppler ultrasound in the preoperative detection of abdominal cavity great vessels affection in patients with gastric cancer was 77.8%, multislice spiral computed tomography angiography - 88.9%, the combination of these two methods - 96.3%.Conclusion. The analysis of used diagnostic methods of examination of patients with stomach cancer showed that Doppler ultrasound should be included in the patients examination standard to assess the abdominal cavity great vessels condition, what is important in deciding on the possibility of operative intervention; the sensitivity of the combination of dopplerography and multislice spiral computed tomography angiography in the diagnosis of great vessels affection is 96.3%.

Computed Tomography Angiography in Diagnosis and Treatment of Splenic Artery Aneurysm.

Computed Tomography Angiography in Diagnosis and Treatment of Splenic Artery Aneurysm.

Clinical application of three-dimensional reconstruction and rapid prototyping technology of multislice spiral computed tomography angiography for the repair of ventricular septal defect of tetralogy of Fallot.

Three-dimensional (3D) reconstruction and rapid prototyping technology (RPT) of multislice spiral computed tomography angiography (CTA) was applied to prepare physical models of the heart and ventricular septal defects of tetralogy of Fallot (ToF) patients in order to explore their applications in the diagnosis and treatment of this complex heart disease. CTA data of 35 ToF patients were collected to prepare l:l 3D solid models using digital 3D reconstruction and RPT, and the resultant models were used intraoperatively as reference. The operations of all 35 patients were completed under the guidance of the 3D solid model, without difficulty. Intraoperative findings of the patients were consistent with the morphological and size changes of the 3D solid model, and no significant differences were found between the patches obtained from the 3D solid model and the actual intraoperative measurements (t = 0.83, P = 0.412). 3D reconstruction and RPT of multislice spiral CTA can accurately and intuitively reflect the anatomy of ventricular septal defects in ToF patients, providing the foundation for a solid model of the complex congenital heart.

Application value of multislice spiral computed tomography angiography in the evaluation of renal artery variation in living donor kidney transplantation.

This study aims to investigate the accuracy and value of multislice spiral computed tomography (MSCT) angiography in the evaluation of renal artery variation in living donor kidney transplantation. Two hundred seventy-three kidney transplantation donors underwent preoperative MSCT scanning. Two doctors determined the running direction and variation of the renal artery through joint analysis of the preoperative original MSCT image and the recombination image using the blind reading method, compared the imaging results with the intraoperative results, and evaluated the accuracy and application value of MSCT angiography in the evaluation of renal artery variation in living donor kidney transplantation. CT angiography (CTA) can better show the renal artery and its variation. A total of 52 accessory renal arteries were found in the 273 kidney transplant operations, whereas 55 accessory renal arteries were found in preoperative MSCT. Four accessory renal arteries indicated in the MSCT were not found during the operation, and one accessory renal artery found during the operation was not indicated in the preoperative MSCT. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of MSCT in the diagnosis of accessory renal arteries were 98.1, 98.2, 92.7, 99.5, and 98.2%, respectively. MSCT angiography can sensitively and accurately show the renal artery and its variation in living donor kidney transplantation, and has important clinical value for the formulation of the operative scheme before the transplantation.

Diagnosis and treatment of congenital abdominal aortic aneurysm: a systematic review of reported cases.

BackgroundCongenital abdominal aortic aneurysm (AAA) is distinctly rare in infants and children and carries a high mortality rate. Our objective was to summarize the experience of the diagnosis and treatment in patients with congenital AAA.MethodsReported cases of congenital AAA published prior to November 8, 2014, were identified through PubMed, EMBASE, Web of Science, and reference lists. All selected cases were evaluated for main clinical characteristics.ResultsTwenty-six cases of congenital AAA were identified in the English language literature. Congenital AAA occurred primarily in children under three years old, but it was also found in young adults and fetuses. With regards to the localization, the great majority of congenital AAA was infrarenal AAA. The majority of the AAA patients lacked specific symptoms, and a painless pulsatile abdominal mass was the most common clinical presentation. The diagnosis of AAA was based on ultrasound scanning in twenty-five cases, multi-slice spiral computed tomography angiography (MSCTA) in sixteen cases, and magnetic resonance angiography (MRA) in nine cases. Histopathological analyses were available in seven cases. Seven patients received conservative management. Surgical treatment was performed in seventeen cases, and open repair with an artificial graft was the main surgical intervention. The mortality associated with congenital AAA was high (30.76%). Ruptured aneurysm and renal failure were the main causes of death.ConclusionsGood outcomes can be achieved in children with early identification of congenital AAA and individualized surgical repair with grafts.

Clinical value of multi-slice spiral computed tomography angiography and three-dimensional reconstruction in the diagnosis of double aortic arch.

The aim of this study was to evaluate the clincal value of multi-slice spiral computed tomography (MSCT) in the diagnosis of double aortic arch (DAA) and analyze the associated angiography and 3D reconstruction imaging to increase the accuracy of DAA diagnosis. Clinical and imaging data from 15 patients diagnosed with DAA by MSCT were summarized and compared with the corresponding surgical observations. The DAA diagnosis by MSCT for the 15 patients was confirmed by the surgical observations. A total of 13 cases were categorized as type I (double arches are open), including nine with a larger right arch, two with a larger left arch and two with balanced arches. Two cases were categorized as type II (one atretic arch), characterized by left atretic arch. Among the 15 patients, ultrasound diagnosis succeeded in nine cases and failed in the rest. A single malformation was identified in only two cases, whereas the rest had single or multiple combination(s) of intracardiac and extracardiac malformations, including seven with different levels of tracheobronchial stenosis. MSCT was demonstrated to precisely diagnose DAA complicated by malformation and tracheobronchial stenosis. MSCT is an essential therapeutic strategy and serves as a primary method in DAA diagnosis.

Tumor feeding artery reconstruction with multislice spiral CT in the diagnosis of pelvic tumors of unknown origin

We aimed to compare multislice spiral computed tomography (MSCT) angiography diagnosis with both surgical findings and postoperative pathological results in patients with pelvic tumors of unknown origin. In addition, the diagnostic accuracy of MSCT angiography was compared with that of routine computed tomography for tumor feeding artery volume reconstruction to determine the origin and nature of pelvic tumors. The records of 43 patients with pelvic tumors of unknown origin who underwent MSCT angiography were retrospectively reviewed. Volume reconstructions using add vessel and merge views methods were performed for abdominal and pelvic blood vessels. The tumor origin was identified based on observations of the origin, number, morphology, starting/ending locations, route, and distribution of the tumor feeding arteries. Overall, the mean tumor diameter was 9.8±3.5 cm (range, 4.2-23.5 cm); 11 tumors (25.6%) were cystic in nature; and 32 tumors (74.4%) were either solid/cystic or solid in nature. When considering all MSCT angiography examinations used to predict the nature of the tumor (e.g., malignant or benign), the sensitivity and specificity were 77.3% and 95.2%, respectively. The positive and negative predictive values were 94.4% and 80%, respectively. The overall diagnostic accuracy was 86.05% with an area under the curve of 0.961 (95% confidence interval, 0.913-1.000). MSCT angiography volume reconstruction for pelvic tumor feeding arteries of unknown origin is highly valuable for localization, qualitative diagnosis, and quantitative diagnosis of pelvic tumors.

Clinical significance of hepatic artery variations originating from the superior mesenteric artery in abdominal tumor surgery

Background Hepatic artery variations are frequent clinical occurrences. The aim of this study was to investigate the characteristic course of variant hepatic arteries originating from the superior mesenteric artery for the purpose of providing instructions for abdominal tumor surgery. Methods The course of variant hepatic arteries originating from the superior mesenteric artery was studied in 400 patients with liver cancer confirmed by digital subtraction angiography (DSA) and multi-slice spiral computed tomography angiography (MSCTA), and 86 patients with gastric cancer confirmed by preoperative MSCTA between June 2008 and June 2010 in the First Affiliated Hospital of Guangxi Medical University. Results Hepatic artery variations originating from the superior mesenteric artery were noticed in 49 liver cancer patients and 14 gastric cancer patients (total 63 cases), with a variation rate of 12.96%, including two cases (3.17%) where the hepatic arteries ran along the anterior pancreas, and 61 cases (96.83%) where the hepatic arteries ran along the posterior pancreas. Conclusions Hepatic artery variations originating from the superior mesenteric artery present as two types: the pre-pancreas type and the post-pancreas type with the latter predominating. This finding is of clinical significance in abdominal tumor surgeries where clearance of portal lymph nodes is needed.

Retroperitoneoscopic Renal Pedicle Lymphatic Disconnection for Chyluria in Presence of Complex Renal Vasculature

To evaluate retroperitoneoscopic renal pedicle lymphatic disconnection (RRPLD) for chyluria in the setting of complex renal vasculature and compare those outcomes with the RRPLD outcomes of patients with normal renal vasculature. From December 2002 to December 2011, RRPLD was performed in 14 patients with complex renal vasculature and 64 patients with normal renal vasculature. Preoperative multislice spiral computed tomography angiography for renal vessels was done on 5 patients with complex vasculature. The demographic and perioperative data were collected to assess critical outcomes. The abnormal vasculature was identified using preoperative multislice spiral computed tomography angiography in 5 patients and surgical exploration in 9 patients. RRPLD was successfully completed in all patients without conversion to open surgery or vascular injury. The mean operative time was significantly longer in those with complex renal vasculature than those with normal renal vasculature (105.4 ± 18.7 vs 84.5 ± 15.6 minutes; P = .000). The outcomes were similar in the 2 groups in terms of intraoperative blood loss (P = .060), mean hospital stay (P = .478), and intraoperative complications (P = .660). The occurrence of postoperative gross hematuria was significantly greater in those with complex renal vasculature than in those with normal renal vasculature (4 of 14 vs 2 of 64; P = .008). The event was resolved uneventfully. Although it is technically challenging, RRPLD is feasible and safe for patients in the presence of complex renal vasculature. Preoperative evaluation of the renal vasculature with multislice spiral computed tomography angiography is beneficial for managing abnormal renal vessels.

Is it necessary to dissect the lymph nodes around an abnormal hepatic artery in D2 lymphadenectomy for gastric cancer?

Surgical resection remains the principal treatment for advanced gastric cancer. However, one of the difficulties in radical D2 lymphadenectomy is the identification of the vascular abnormalities around the stomach, especially abnormal hepatic artery. The hepatic artery arising from the superior mesenteric artery shows great variations, and whether or not the lymphatic tissues around the abnormal artery should be dissected is still controversial. Eighty-six gastric cancer patients who underwent radical D2 lymphadenectomy by the same surgeon in our department in the past 2 years were included in this study. All patients underwent multislice spiral computed tomography angiography for preoperative evaluation of the abnormal hepatic artery arising from the superior mesenteric artery. Postoperative immunohistochemistry of the lymphoid tissues around the abnormal hepatic artery was performed using CK20 and CEA antibodies to determine the occurrence of lymph node micrometastasis around the abnormal artery. In our study, we found 14 cases with an abnormal hepatic artery arising from the superior mesenteric artery. The hepatic artery ran in front of the pancreas in 1 case and behind the pancreas in 13 cases. Immunohistochemical analyses revealed no micrometastasis in the lymphoid tissues surrounding the abnormal artery. Abnormal hepatic arteries arising from the superior mesenteric artery can be classified into pre-pancreas and post-pancreas types. Lymph nodes around the abnormal artery may not need to be dissected in radical D2 lymphadenectomy.

Clinical significance of aberrant hepatic artery arising from superior mesenteric artery in liver cancer and gastric cancer operations

Objective To analyze the clinical significance of aberrant hepatic artery which originated from superior mesenteric artery in liver cancer and gastric cancer operations.Method Four hundred liver cancer patients who underwent digital subtraction angiography (DSA) and multislice spiral computed tomography angiography (MSCTA) and 86 gastric cancer patients who underwent MSCTA between June 2008 and June 2010 and operated at the First Affiliated Hospital of Guangxi Medical University were included in this study.Preoperatively,the origins of the aberrant hepatic arteries from superior mesenteric arteries were detected by medical imaging.Postoperatively,immunohistochemistry of lymphoid tissues around the aberrant hepatic arteries of the gastric cancer patients were performed using recombinant human cytokeratin 20 (CK20) and carcino-embryonic antigen (CEA) to determine the incidence of lymph node metastases around the aberrant arteries.Results Among 486 patients,49 patients with liver cancer and 14 patients with gastric cancer had an aberrant hepatic artery which originated from the superior mesenteric artery.The rate was 12.96％ (63/486).The hepatic artery ran in front of the pancreas in 2 patients (3.17％) and behind the pancreas in 61 patients (96.83％).Immunohistochemical analyses of CK20 and CEA were negative which revealed no metastases in the lymphoid tissues surrounding the aberrant arteries.Conclusions Aberrant hepatic artery originated from the superior mesenteric artery can be classified into the pre-pancreas type and postpancreas type.The majority of aberrant hepatic artery belonged to the post-pancreas type.The clinical significance of aberrant hepatic artery is that the hepatic hilar lymph nodes should be dissected in liver cancer and gastric cancer operations. Key words: Aberrant; Hepatic artery; Tumor; Operation

Automatic Transfer Function Specification for Visual Emphasis of Coronary Artery Plaque

Abstract Cardiovascular imaging with current multislice spiral computed tomography (MSCT) technology enables a non‐invasive evaluation of the coronary arteries. Contrast‐enhanced MSCT angiography with high spatial resolution allows for a segmentation of the coronary artery tree. We present an automatically adapted transfer function (TF) specification to highlight pathologic changes of the vessel wall based on the segmentation result of the coronary artery tree. The TFs are combined with common visualization techniques, such as multiplanar reformation and direct volume rendering for the evaluation of coronary arteries in MSCT image data. The presented TF‐based mapping of CT values in Hounsfield Units (HU) to color and opacity leads to a different color coding for different plaque types. To account for varying HU values of the vessel lumen caused by the contrast medium, the TFs are adapted to each dataset by local histogram analysis. We describe an informal evaluation with three board‐certified radiologists which indicates that the represented visualizations guide the user's attention to pathologic changes of the vessel wall as well as provide an overview about spatial variations.